Monday, January 12, 2015

Weather and its characteristics - A brief description

Hello,
Here is a bit of weather info...






Weather


THE TROPOSPHERE


The troposphere is from the surface up to altitudes of 20,000 to 60,000 feet. Almost all of the weather exists in the troposphere. The boundary where the troposphere ends and the stratosphere begins is called the tropopause. It is the place where temperatures stop getting lower and start getting higher again. This is important because it gives us an altitude or ceiling that thunderstorms will be somewhat limited to. It also can show us extreme and intense power of thunder cells that grows thousands of feet beyond the tropopause due to inertia and speed of its growth.


WIND


 Wind is caused by pressure differences. Pressure differences are simply caused by unequal heating of the Earth’s surface. Any given geographic region receives about the same amount of sunlight from above. Some parts – such as a forested area or wet, grassy field – will not heat up as rapidly as other areas such as a dry, plowed field. This is because all materials / surfaces have a properties that allow quick absorbtion and dissipation of heat and energy.  The scientific term is known as specific heat. Specific heat refers to the specific amount of heat that you have to add to a substance to cause it to change its temperature. Water, for instance, has a very high specific heat. Dry land has very low specific heat. Due to water’s high specific heat, it resists changing temperature. It will eventually cool off and then freeze, but it will take it longer than dry land. Therefore, the presence of large bodies of water tends to exert a powerful moderating influence on the local climate. Water moderates weather. Sand or dirt, especially when it’s dark and dry, has a relatively low specific heat. It doesn’t take much energy, sunlight, to make it extremely hot. It also cools off very rapidly. This is why the Sahara desert can be searing at noon but frigid at night. A moist or shady section of the landscape, such as a marsh or swamp, receives the same amount of sunlight, but, unlike the parking lot or the dry, dusty field, it does not get very hot very fast. Now ponder this question:  Is the atmosphere heated from above or below? Although it may seem counterintuitive, the atmosphere is actually heated from below. Air is invisible and sunlight passes through to the first opaque surface. This surface could be a cloud and the sunlight would reflect back into space or it could be land or water. When sunlight warms the ground, it then transfers heat to the air. The air directly above a hot surface, such as a parking lot or a dry, dusty field, will get hot much quicker than the air directly over a cool surface such as a marsh or swamp.  This differential heating, causes pressure differences which then cause air currents. An example: During the day, the sunlight heats the land. The ocean, which has high specific heat, receives the same amount of sunlight but remains relatively cool. So the air over the land is heated. It becomes less dense. It exerts less pressure and rises. The air over the water remains relatively cool and dense. It exerts more pressure and sinks. The sinking, high-pressure air over the ocean flows ashore. This is the sea breeze you feel. You feel it whenever you go to the beach. At night, the entire process reverses. The land, which has low specific heat, loses its heat very quickly by radiating it away into space. The ocean, which has high specific heat, retains its heat and stays at a relatively constant temperature. Now the air over the water is warmer and less dense; the air over the land is cooler and  more dense. The sinking, high-pressure air over the land flows out to sea. This is called a land breeze. Air always wants to flow from high pressure areas into low pressure areas. On a weather chart, isobars are lines of constant pressure. They form concentric rings around high and low pressure areas. The closer together the lines, the stronger the pressure gradient force. The farther apart the lines, the weaker the pressure gradient. The strength of the wind is generally proportional to the pressure gradient force. If the pressure gradient changes dramatically over a small distance , the result will be a strong wind. At ground level, the wind will flow directly from high to low pressure. At altitude the wind tries to do the same thing but the Earth plays a trick on the wind. The rotation constantly changes the target the wind is flowing to. This very changing target cause the wind to continuously change course. Ultimately, the wind just circles counterclockwise / inward to a low pressure and clockwise / outward from a high pressure system. These directions are true for the Northern hemisphere. The Southern hemisphere is opposite rotations, but the inward / outward flows are the same. This bending of the wind has the name,  Coriolis force. This force unexplained is a mystery to many pilots. But now you know it’s just a matter of the wind chasing a moving target; simple. High-pressure systems generally bring fair weather. You may visualize a low-pressure system as a huge vortex of air which is risingconverging and rotating counterclockwise (in the Northern Hemisphere). Low-pressure systems generally bring poor weather. The ultimate example of a low-pressure system is a hurricane. On a much smaller scale, a tornado is an extremely small but also extremely strong low-pressure system.


AIR MASSES

An air mass is a giant section of the troposphere which has relatively uniform properties of temperature and moisture. For example, to the north of that low-pressure system , you might have a cooler, drier air mass. The boundary between two air masses is called a front. When warm air overtakes cool air it’s called a warm front. When cool air overtakes warm air it’s called a cold front. The direction of movement of the boundary, then, is what makes this determination. When the boundary does not move it is called a stationary front. When one front overrides another one it is called an occluded front. A lot of bad weather, but certainly not all, is often associated with frontal activity. It is quite common to have a front with no bad weather associated with it at all. Some of the worst weather in the world can be found in squall lines. A squall line is a narrow and often mostly unbroken band of steady-state thunderstorms usually found ahead of a fast-moving cold front. Squall lines often contain violent wind shear, including microbursts, hail, tornadoes, torrential rain and frequent lightning. Squall lines happen because a fast-moving cold front can generate vertical displacement ahead of the actual front. Squall lines are extremely hazardous and should be avoided at all costs. One good thing about a squall line is that it passes quickly. One smart tactic for dealing with a squall line is to land, tie the airplane down securely, go inside the FBO, have a cup of coffee and wait for the line of severe weather to pass. Never attempt to penetrate a squall line. Looking at a profile view of a front you can see that it has a slope. The slope of a cold front is usually much steeper than the slope of a warm front. The one thing that will always occur with frontal passage is a wind change. The wind may increase, decrease or switch direction but it will always change. Other things, such as relative humidity, barometric pressure and temperature may change as well, but not always. Windshear may occur on the backside of a cold front, after frontal passage. Windshear may occur just before a warm front passes through. See the illustration to better understand the dynamics.  Be very concerned about the potential for wind shear above the airport in respect to an approaching front or frontal passage.



WEATHER SEQUENCE- Warm fronts

The sequence starts with high level cirrus clouds. They gradually spread out eventually touching each other. You can confirm the process is starting with a simple observation. Stand with your back to the wind and observe if the cirrus clouds are spreading out to your right. If so, this is a good indicator that the sequence is in motion.  Continuing… the cirrus clouds will completely join forming a milky veil which will flatten out and become featureless. This layer base will gradually descend becoming alto stratus type clouds. The sun will slowly be blotted out as the clouds thicken. The bases will continue to lower until light precip such as rain or snow starts to fall. Eventually the cloud bases will lower to 1,000 feet above ground level. These thick, dark clouds producing precipitation are nimbostratus. After a few hours, these clouds will rain themselves out.  After this happens the cloud will transform into a relatively thin, low level stratus layer.  The low layer will start to break up becoming stratocumulus and continuing to become individual, fair weather cumulus clouds. The beginning of the sequence is marked by a temperature increase and overall this process is very slow from one day to several days for the entire cycle. Here is a list in order of the cloud types as they progress through the sequence:

Spreading Cirrus - Alto Stratus – Nimbostratus – Stratus – Stratocumulus – Cumulus.

Don’t forget to consider observing the spreading cirrus direction with your back to the wind as a key identifier to the beginning of this process. It will be to the right.



WEATHER SEQUENCE- Cold fronts

Again the initial clue to the beginning process will be the observation of high level cirrus clouds. With your back to the wind, these high level cirrus clouds will spread out to the left rather than the right as noted in the warm front process. The cold front sequence starts with the appearance of alto cumulus clouds or possible cirrocumulus type clouds. These cirrocumulus clouds are also called a “mackerel” sky because they resemble the scales of the fish. As the temperature at the surface drops, cumulus congestus will build and possibly dramatically grow into cumulonimbus clouds. If these clouds do build, they typically rain themselves out quickly and leave behind possible dangerous high winds and wind shear.  Patches of alto stratus may remain and possibly high level wispy cirrus clouds. Ironically, standing again with your back to the wind, these cirrus clouds will still be observed to be blown to the left.  Here is a list in order of the cloud types as they progress through the sequence:

Alto cumulus or cirrocumulus – cumulus congestus – cumulonimbus – alto stratus - cirrus

Don’t forget to consider observing the spreading cirrus direction with your back to the wind as a key identifier to the beginning of this process. It will be to the left. 


THUNDERSTORMS

Cumulonimbus or Thunderstorms, can be the source of some of our most severe and damaging weather. Thunderstorms can spawn a multitude of weather phenomenom such as squall lines, tornadoes, gust fronts and windshear. It has been estimated that one of these single clouds have the equivalent power of 10 Hiroshima sized bombs. Cumulonimbus clouds are born of cumulus congestus. Seen from a distance, congestus is said to have changed to cumulonimbus when parts of its upper region begin to lose their sharp edges due to the cloud droplets freezing into ice crystals. Thunder, lightning and hail also identify cumulonimbus. I note this as a caution to expect cumulonimbus when you spot cumulus congestus.

Thunderstorms may be related to frontal activity or they may exist independent of frontal activity. Thunderstorms can even create their own small-scale fronts, called gust fronts.

All thunderstorms, by definition, contain lightning. Lighting is caused when static electrical charges build up in the atmosphere due to agitated water droplets. The friction of their rubbing together generates an electrical potential that eventually will discharge when the charge becomes large enough. The sudden discharge (release) of these massive static charges is lightning. Lighting can travel from cloud to cloud or from cloud to ground. Although it very often originates within thunderclouds, lighting can come from clear air. As pilots, we should seek to understand them, predict them and most of all avoid them! Generally speaking, the more frequent the lightning the more intense the storm. This is one of the clues you can use in flight to determine how dangerous a particular storm cell is and by how much of a margin you need to avoid it. Constant, heavy lighting suggests that it may be a severe cell. No lightning suggests that it may be no more than a gentle rain shower. Another excellent indicator is cloud tops. The higher the tops, the stronger the storm. A storm with a 10,000-foot top is going to be less dangerous than a storm with a 45,000- foot top. The presence of an anvil structure, called an incus, at the top of a cumulonimbus cloud often indicates a powerful and hazardous storm. The intensity of precipitation is a useful gauge as well. The heavier the precipitation, the stronger the storm. Finally, the speed of cell movement can be a very reliable gauge of its intensity. A fast moving cell is almost always a strong one. It is worth mentioning here that the only thing radar shows you is liquid precipitation. It does not pick up snow very well unless it is a very wet snow. This, by itself, does not always tell you everything you need to know. It is absolutely possible to have severe turbulence in clear air.  So radar, while very helpful, does not tell the whole story. Do not rely too heavily on radar alone. In order for a thunderstorm to develop, three “ingredients” must exist. They are: 1) atmospheric moisture, 2) unstable air and 3) a source of lift. The first factor, moisture, is mostly self-explanatory: water vapor must be present in the air. The second and third require some further discussion. Let’s begin with the concept of atmospheric stability.

STABILITY AND INSTABILITY

The atmosphere, as I explained, is heated from below. Thus, on a typical day, the air is warmer down near the surface than it is up high above the surface.  When the upper air is very cold and the air down near the ground is very hot as compared to the standard lapse rate, the atmosphere is extremely unstable. Stability is measured and determined based on the lapse rate. The average lapse rate is about 2 degrees Celsius, about 4.5 degrees Fahrenheit, per 1,000 feet, meaning that the air gets about 2ยบ C cooler for every 1,000 feet you climb.  Let’s consider an example. At the surface, sea level, we have 24 degrees Celsius and at 10,000 feet we have a temperature of -6 degrees Celsius. This is a temperature drop of 30 degrees Celsius over 10,000 feet which equates to 3 degrees Celsius per 1,000 feet. The standard lapse rate is 2 degrees per 1,000 feet. As you can see, an average drop of 3 degrees is quite significant.  Even a small vertical disturbance will result in a powerful upward movement of air and strong turbulence. When the temperature only decreases slightly or remains more or less constant with increasing altitude, on the other hand, the atmosphere will be fairly stable and will resist any disturbance. And finally, when the temperature actually increases with altitude it’s called an inversion. This is an extremely stable condition. An inversion is the ultimate manifestation of atmospheric stability. The air will tend to be smooth above the inversion. An inversion is often visible as a distinct haze layer where particulate matter and pollutants are trapped. Smoke from a fire, for instance, will rise until it hits the inversion layer and then will spread out in a thin, flat, horizontal layer as if contained under a sheet of glass. Stability is measured and determined based on the lapse rate. If the lapse rate is lower than average the air is stable. If the lapse rate is higher than average the air is unstable. Many useful, practical things can easily be predicted by looking at the lapse rate. On a stable day you can expect smooth air, poor visibility due to haze and smog, stratus-type clouds  and a steady, continuous kind of precipitation (if  precipitation occurs). Conversely, on an unstable day you can expect bumpy air, good visibility due to active mixing of atmospheric layers, cumulus-type clouds (again, if there is enough moisture in the atmosphere for clouds to form) and an intermittent, showery kind of precipitation (if precipitation occurs).  You can forecast quite a bit by just by looking at the lapse rate. Specifically with regard to thunderstorms (assuming you have atmospheric moisture and a source of lift), a steep lapse rate means there will be frequent, powerful and widespread thunderstorms. A modest lapse rate means there will be limited thunderstorms – they will tend to be small, weak and widely scattered, easy to circumnavigate. A flat or negative lapse rate means there will be no convective activity at all. Thunderstorms cannot exist in very stable air.

LIFT

Let’s talk about the third, a source of lift. The most common sources of lift are thermalsridge liftconvergence lift and wave lift. Thermals, technically known as convective currents, are produced when the air directly above a warmer surface breaks away and begins to rise as a bubble. This bubble is very similar to the old lava lamps if you are familiar. Thermals will form when there is a difference in air temperature. Contrary to the very common misconception, thermals do not actually require the air to be hot in order to form. Thermals can and do occur in sub-freezing temperatures. If an airport surface temperature is 0 degrees Celsius, for instance, and the surface temperature of the surrounding countryside is –5 degrees Celsius then a thermal is likely to develop over the field. I was once amazed to learn of a glider club in Jackson Wyoming that actively flew gliders in the winter and stayed aloft on thermals in sub-freezing temperatures. Thermals only go as high as the dew point altitude. The dew point, as all pilots know, is the temperature at which water vapor condenses. Cumulus clouds often form as a “cap” on top of a thermal. Thermals exist below the cloud base. They do not exist above cloud base. If you are looking for smooth air on an unstable day, try flying above cloud base. Air that is at the dew point is said to be saturated. So let’s talk briefly about how a cumulus cloud forms and why it “caps” a thermal and why that same cumulus could potentially become a full blown thunderstorm. First we must understand “latent heat” and how it relates to evaporation of water. When water evaporates, it absorbs heat and becomes water vapor. Water vapor is invisible but contains extra heat. This is the reason why sweating or getting wet cools you in the Summer time as the evaporation occurs. All around us we have invisible water vapor holding heat. When the water vapor condenses into microscopic water droplets, which is what clouds are made of, the latent heat is released into the adjacent air. The air becomes slightly warmer, giving the thermal ability to continue to rise. The reason a fair weather cumulus and a cumulonimbus thunderhead differ is not only the instability on the air, but most importantly how much water vapor is contained in the rising parcel of air. Water vapor is a kind of fuel and on a dry day with low relative humidity, the cloud just runs out of steam when the temperature and dew point merge effectively running the cloud out of gas. Ridge lift, technically known as orographic lift, is developed when wind is deflected upward by a slope. When the prevailing horizontal air currents strike the side of a relatively wide, flat mountainside, for example, they become more vertical as they are forced to go up and over the mountain rather than around it. Ridge lift can also be used by glider pilots. Convergence lift is developed when two air masses collide. In Florida, for instance, the Atlantic Ocean sea breeze front moves inland from the east coast while the Gulf of Mexico sea breeze front moves inland from the west coast. In the afternoon these fronts often collide with each other, producing a steady vertical upward movement of air. And finally, wave lift, the least common of the four types, is developed when wind of about 40 knots or more blows nearly perpendicular to a broad, tall, well-defined ridgeline and the air near the top of the ridgeline is stable. This creates a series of standing waves downwind of the ridgeline. This is very similar to the way standing waves form downstream of a rock in a swift-flowing river. These atmospheric waves reach all the way up into the flight levels, impacting even airline traffic. Although the mountain waves themselves are invisible, standing lenticular clouds often mark their presence. A violent and dangerous horizontal vortex called the rotor exists close to the surface underneath mountain waves. Sometimes the rotor kicks up dust and debris, but it may also be invisible. Because the rotor may contain severe to extreme turbulence, low-level flights beneath  a mountain wave should be avoided or conducted with great caution.

THE LIFE CYCLE OF AN AIR-MASS THUNDERSTORM

An air-mass thunderstorm (as opposed to a frontal or squall-line thunderstorm) has three phases in its life cycle: cumulusmature and dissipating. During the cumulus phase, the developing storm contains primarily updrafts  and existing water vapor fuels its growth. The cumulus phase is like an accelerating chain reaction: condensing water vapor releases latent heat into the adjacent air, which drives the cumulus cloud to billow higher, which causes more moisture to condense, which releases more latent heat and so on until the cloud has grown to a towering height. The mature phase is reached when rain begins at the surface. The cumulus cloud has become a cumulonimbus cloud – a thunderstorm. During the mature phase, the storm contains a violent mix of updrafts and downdrafts. The mature phase is the most dangerous. During the dissipating phase, the storm is “raining itself out” and contains mostly downdrafts. Although it remains dangerous until it is gone completely, the greatest hazard is over. Thunderstorms should always be avoided if possible.  Severe thunderstorms should be avoided by at least 20 miles. Hail can be thrown many miles laterally from a cell, and outflow turbulence can extend far beyond the cloud itself. The anvil will point downwind and the most likely place to find hail will be under the anvil. If thunderstorms are embedded  which means they are concealed within widespread instrument meteorological conditions, then the entire area should be avoided unless you have  airborne weather radar. When flying VFR, simply maintain visual separation from storm cells. Never try to out-climb a building cumulonimbus cloud, sorry Wendy, didn’t mean to scare you that day.  Go around it instead on the upwind side unless very little wind exists. Passing a thunderhead on the right will give you a tailwind due to the counterclockwise circulation around the cell. Be forewarned, any type of significant wind will toss out hail on the downwind side and it this happens to be the right side as well you could encounter hail.  Thunderstorms can erupt very quickly and spread very rapidly. They can grow at a rate that far exceeds your airplane’s performance.   If you do end up accidentally penetrating a thunderstorm, fly straight ahead and do everything you can to keep the wings level. Accept large variations in airspeed and altitude but try to maintain a zero bank angle. Reduce the power to get below maneuvering speed but be prepared to deal with climbs and descents of thousands of feet and possible massive airspeed increases and decreases. Do not try to force the airplane to maintain a certain altitude or a certain airspeed – that would require extreme pitch and throttle changes likely to overstress the airframe. Use the manufacturer’s recommended turbulence penetration configuration. There are two important reasons you want to try to keep the wings level: first, your greatest danger is structural failure caused by severe or extreme turbulence. Any increase in bank angle increases the load factor, which may already be excessive. And second, your next greatest danger is entering an unrecoverable unusual attitude. Keeping the wings level will help to prevent this. Again, that is the only thing you should attempt to hold constant is your zero-bank attitude – allow all other things to fluctuate wildly, and they will. You should also tighten down your seat belt as much as possible. Be sure to fully secure any items which could otherwise become projectiles. Turn up the interior lights to maximum intensity. Remember that lightning can leave you momentarily blinded. If ATC does not cooperate by giving you the headings you need, this may even include declaring an emergency. If a heading change is necessary to prevent you from entering a cell, that definitely constitutes an immediate threat to the safety of flight. If you are professional in making your requests, the controllers usually acknowledge and approve your request. For example, a request may sound like this… “Atlanta Center, Colt 59Z requests 10 degrees left to avoid a build up”. Do try to think ahead and give them a reasonable opportunity to reconcile your needs with their IFR separation requirements. Sudden, last-minute demands are more difficult for controllers to work with. This is particularly true when the frequency is congested with lots of pilots requesting heading and altitude changes.

THE JET STREAM

Why do we need to know anything about the jet stream if we fly at low altitudes? Because  even though it’s way up close to the tropopause, the jet stream drives directly or indirectly most of the weather in the troposphere. What is a jet stream? The Earth revolves on its axis, Its atmosphere revolves with it, or tries to. The atmosphere spins fastest at the equator and barely spins at all near the poles. The velocity difference makes for a defined boundary in which the jet stream lives. Some air falls behind, creating a buildup of pressure. The result is a jet stream – a river in the sky, a band of wind 100 to 400 miles wide, 1 to 3 miles thick and thousands of miles long flowing west to east at speeds between 150 to 300 MPH. It’s the atmosphere’s way of equalizing pressure differences at high altitude. The jet stream is dynamic and complex and exists farther south in the winter. It weakens and retreats north in the summer. The jet stream acts as a barrier wall which blocks cold air masses, keeping them contained on the stream’s north side. Thus, when the jet stream troughs, or dips down, it allows those cold air masses to penetrate further south. And when a ridge in the jet stream carries it up towards Canada, it can help to keep those cold air masses mostly out of the continental U.S. The transition between temperatures on each side of the jet is often very abrupt, as you can easily see when you overlay a map of temperatures with the position of the jet stream. Another way the jet stream influences weather is by either reinforcing or disrupting the circulation around high and low pressure systems. When the cyclonic rotation of a low pressure system moves with the jet stream, it can become very strong. When it moves against the jet stream, however, it tends to break down. That often makes the difference between a powerful storm system and a minor weather event. When the weather forecaster talks about “upper level support”, or the lack of it, he or she is referring to whether the jet stream is working with the rotation (to strengthen it) or against the rotation (to weaken it.)

I. Basic Definitions and Explanations

In general, there are two kinds of in-flight icing: structural (airframe) icing and induction icing, both of which can be extremely hazardous.

A. Induction Icing

In airplanes with fuel-injected engines induction icing refers to ice which forms in an air intake and/or an air filter, blocking the flow of air to the engine. This may happen after flying through heavy rain, for instance, and then into a sub-freezing layer of air. The wet air filter could suddenly become completely frozen, reducing its permeability to near zero. Carburetor ice is also a form of induction icing.

B. Structural Icing

Structural icing, sometimes synonymously called “airframe” icing, refers to ice which forms on the wings, propeller blades and other surfaces of the airplane. It increases weight, changes important profiles and drag while decreasing lift and thrust. Too much of it will cause a partial or total loss of control.

1. Types of Icing

The FAA currently lists three types of airframe icing with respect to composition: Clearrime and mixed. It also lists four levels with respect to intensity or rate of accumulation: tracelightmoderate and severe. Finally, it lists four types with respect to the nature of the ice formation: intercycleknown vs. detectedresidual and runback.

- Clear Ice

Glaze ice (sometimes also referred to as “clear” ice) forms when larger water droplets strike the airframe and freeze slowly as they spread, leaving a heavy, translucent, relatively smooth and often somewhat lumpy coating. Clear ice may sometimes be more difficult to see than rime or mixed; it might appear, from certain angles and under certain lighting conditions, as if the wing is simply very shiny or wet. It is often associated with cumulus clouds (which contain larger water droplets) and turbulent air (because an unstable atmosphere is generally associated with more vertical cloud development).

- Rime Ice

Rime ice forms when smaller water droplets strike the airframe and freeze almost instantly, leaving a rough, granular, milky, porous, brittle surface. It is usually fairly easy to see. It is often associated with stratus clouds and smoother air. Rime ice typically forms along the  leading edge of wings, stabilizers and protrusions where the air divides flowing over the top of the structure  and air that flows under the bottom of the structure.

- Mixed Ice

Mixed ice is a combination of clear and rime ice. Because icing conditions are seldom perfectly consistent as they are in the laboratory, this type is common.  Most ice encountered in the real world is mixed.

Levels of Icing

Clear ice is the most dangerous. Clear ice adds more weight and causes a greater distortion of the wing shape, but rime ice causes a greater separation of airflow.  Generally though, rime ice does not accumulate in vast quantitys like clear ice can.  In icing tests, a razor-thin layer of simulated rime ice with the consistency of fine-grain sandpaper caused an extreme loss of aerodynamic efficiency on many higher-performance airfoils. If you were to fly into area where significant rime ice was accumulating, this could potentially be more dangerous than clear ice. The rate of accumulation is the single most important factor in determining how dangerous ice is.

1) Fast-accumulating rime ice is more dangerous than slow-accumulating clear ice.

2) Fast-accumulating clear ice is more dangerous than slow-accumulating rime ice.

The FAA currently lists four levels of in-flight ice: tracelightmoderate and severe.

- Trace

Ice is perceptible, but the use of de-ice or anti-ice equipment is not necessary unless the condition is sustained over a long period of time (an hour or more).

- Light

The use of de-ice or anti-ice equipment is occasionally or intermittently necessary.

- Moderate

The use of de-ice or anti-ice equipment is continuously necessary and a diversion and/or altitude change becomes highly advisable.

- Severe

Even with the continuous use of de-ice or anti-ice equipment, ice is still accumulating on the airplane. An immediate escape is necessary to prevent a disaster. Never forget: Icing can go from trace to severe almost instantly and without warning!Strictly from a legal standpoint (specifically §91.527 or §135.227), you may fly in widespread or localized trace, light, moderate or even severe reported or forecast icing conditions as long as you are in an airplane which is specifically equipped and/or FAA-approved for it.

- Other FAA Definitions

“Intercycle Ice” is ice that accumulates on a protected surface (such as the booted leading edge of a wing or stabilizer) between actuation cycles. This term applies to de-ice systems rather than anti-ice systems. This is rare.

“Residual Ice” is ice which remains attached to a protected surface immediately after the actuation of a de-icing system. Not all ice will be fully and cleanly shed after each actuation cycle; some ice will often remain. It may take multiple cycles to get rid of it all, and in active icing conditions, between intercycle ice and residual ice the wings and stabilizers may never be 100% ice-free.

“Runback Ice” is ice which forms from the freezing (or, in some cases, refreezing) of water leaving a protected surface and flowing – “running back” – onto the unprotected surfaces. Runback ice is very dangerous because once it is present, there is not much you can do to get rid of it, other than flying into above-freezing air! It generally forms on trailing edges and then creeps forward. It is often associated with higher angles of attack – i.e., lower airspeeds and higher flap deflections.

“Treat ice like smoke in the cockpit. Do something about it!”

C. What Are Icing Conditions?

In order for induction or structural icing to occur in flight or on the ground, two factors must come into play. They are:

1) visible moisture in any form

and

2) an airplane surface temperature at or below freezing.

“Visible moisture” includes clouds, fog, mist, drizzle, rain or even wet snow. Notice that the outside air temperature does not have to be right at freezing. It can be well below . . . or even slightly above. Airframe ice can occur when the outside air temperature is slightly above freezing because the airframe itself can be coldsoaked to below freezing. This may happen after leaving a colder layer of air and entering a warmer one. Consider an airplane that has been flying around in -15° C air for an hour and a half. If it descends into a layer of air that is, say, 2° C, it will not be instantly immune to ice even though the OAT gauge indicates an above-freezing temperature. A cold-soaked airframe may take several minutes or even many minutes to reach the temperature of the surrounding air.  Enough time for a deadly amount of ice to form. A cold-soaked airplane is also vulnerable to ground ice. If you were flying through air that was fifteen degrees below zero for an hour and a half, then when you land at an airport where the surface air temperature is slightly above freezing and mist or drizzle is present ice may form on the airplane while it is parked on the ramp even though ice is not forming anywhere else.  De-icing may be necessary unless you will be on the ground long enough for the airplane to warm up to the temperature of the surrounding air. Airframe ice can also occur when the temperature is well below freezing because supercooled large water droplets (SLD) may be present in the atmosphere. Water can exist in the liquid state even at temperatures far below 0 degrees Celsius because it has the interesting (and dangerous) molecular characteristic that it does not freeze unless it has something to disturb it and/or something on which to crystallize: a nucleus such as a particle of dust or salt. SLD are just what they sound like – large droplets of liquid water at temperatures well below freezing. If you fly through a cloud of them, you could go from having no ice to having so much ice that the airplane is uncontrollable in a matter of only a few seconds. If you fly through a cloud of SLD, you can rapidly become coated with a very large amount of ice, perhaps so much that performance and control will be totally lost. Icing reported or forecast as “severe” often involves SLD. While it is true that icing may be less common when it is that cold, there are never any guarantees. Ice, even severe ice, has been reported in extremely cold conditions. SLD have been created in a laboratory environment at temperatures below -40 degrees Celsius. The bottom line to keep in mind is that it is never so cold that icing is no longer a possibility.

FPD Fluid

“FPD” Stands for “Freezing Point Depressant.” FPD fluids are de-icing or anti-icing fluids, designed to prevent the

formation of ice and/or remove existing ice from an aircraft. The FAA recognizes four types:

Type I Fluids are unthickened fluids. A Type I fluid forms a very thin film over the aircraft surface. Designed for lowspeed aircraft, Type I fluids blow off quickly during the takeoff roll. They are red, red-orange or orange in color.

Type II Fluids are thickened fluids which decrease in viscosity when subjected to shear forces from the relative wind induced during the takeoff roll. They contain a minimum of 50% glycol. Most fluids are clear or pale straw colored.

Type III Fluids are also thickened fluids. They contain a minimum of 50% glycol.

Type IV Fluids are enhanced-performance fluids with characteristics similar to Type II when used in 100% concentration during certain weather conditions. Type IV fluid effectiveness is superior to Type II fluids and holdover time is increased by a significant factor under most conditions.

Ground de-icing is a procedure by which frost, ice, snow, or slush is removed from the aircraft in order to provide clean, uncontaminated aerodynamic surfaces. A heated mixture of Type I fluid and water is generally used as a de-icing fluid. Typically, the FBO sends over a truck and a line service employee who then applies the de-icing fluid. The pilot prepares the airplane, observes the procedure and then conducts the required checks prior to departure.

Ground anti-icing, on the other hand, is a precautionary procedure that provides protection against the formation of frost or ice and the accumulation of snow. This process provides an estimated safe time period (known as the holdover time or HOT) during which the critical surfaces should remain uncontaminated. De-icing and anti-icing operations may be conducted simultaneously.  For example: the aircraft can be cleaned off with warmed Type I fluid, then coated with a cold Type IV fluid. This Type IV fluid is like a gelatinous protective cocoon that protects your aircraft until you depart and transfer anti-icing duties to your aircraft systems. The term “FPD fluid” is used generically to cover fluid applied for either or both purposes. When using only Type I fluids, as should be used only for light general aviation aircraft,  holdover times will be determined by two factors:

1. The outside air temperature

2. The type of weather condition – i.e., wet snow, freezing mist etc.




Avoiding and Escaping From Ice

First, remember that you can often avoid ice simply by staying above, below or between cloud layers. If you aren’t in visible moisture, you won’t pick up ice. This might involve multiple heading changes, but it can be done. Any time that you fly in known icing conditions, you must have an escape route planned before you enter them. When ice is building rapidly and you find your airspeed and altitude suddenly and dramatically decreasing is not the time to be formulating strategy. The age old adage in ice is to “climb” is not the only strategy to consider.  Please think about this,,,. You are flying at 9,000 feet MSL with a freezing level at 8,000 feet MSL, cloud tops at 16,000 feet MSL and an MEA at 2,000 feet MSL. A descent makes much more sense, doesn’t it? You can’t climb out of the ice in this scenario, but you can easily descend out of it. There is no one simple, easy, all-purpose answer. Where escape routes are concerned, sometimes a climb will be required, sometimes a descent will be required and sometimes a change of heading will be required. Heading changes are often overlooked. You can, in some situations, leave icing conditions by changing course. A thorough understanding of the overall weather picture is imperative in order to make a prompt and correct decision. Here are three general guidelines to help you develop an escape plan beforehand.

-When the freezing level is below the MEA, a climb or a heading change

will be the only remaining options (since you can’t descend out of the ice

in that situation).

-When the cloud tops are above your service ceiling, a descent or a

heading change will be the only remaining options (since you can’t climb

out of the ice in that situation).

-When the cloud layer is unbroken and many miles wide, a climb or a

descent will be the only remaining options (since changing heading will

not get you out of the ice in that situation).

Whatever decision you make, you will have to make it fast . . . before the ice narrows your options to one – a rapid descent!

 What Does the Term “Known Ice” Mean?

 “Known ice” can refer to forecast ice OR reported ice.

The only sources of weather information which are considered “official” are those provided by or approved by the FAA, DUATS or the National Weather Service. This includes information received from the FSS over the telephone or radio, as well as ASOS, AWOS, ATIS, HIWAS, TWEB et cetera. It also includes surface analysis charts, significant weather prognostic charts, winds and temperatures aloft tables, METARs, TAFs and area forecasts, along with AIRMETs and SIGMETs and all other government-produced or government-endorsed weather products and services.

Realistically, icing conditions are highly variable, highly unpredictable and highly dynamic. For our purposes,  “Icing Conditions” as any outside air temperature or airplane surface temperature at or below 3 degrees Celsius when any form of visible moisture is present.

Landing With Ice

While many fatal icing accidents result from a loss of control in the midst of a severe ice encounter, many more occur when an accumulation of ice causes an airplane’s performance to be degraded that a normal landing is impossible after the encounter is over. Ice greatly raises an airplane’s stalling speed while greatly reducing its climb rate or even forcing it into a rapid  descent. It may also cause a “tail stall,” in which the horizontal stabilizer, which normally produces a downward force to balance center of gravity and the center of lift, loses its effectiveness and causes the airplane to dive vertically like a dart. Recovery is difficult at best and impossible at worst. If you find yourself with ice on your airplane that you are unable to shed, land at a much higher-than-normal airspeed on the longestwidest runway you can reach. If this means diverting to another airport far away, then do it.  Remember: the airplane’s wings or tail may stall at a very high airspeed – even at speeds approaching cruise.  Any amount of bank angle at all will further increase this stalling speed. You might find yourself forced to land so fast that a runway overrun is unavoidable. This is still better than finding yourself fully stalled on short final at an altitude of 200 feet. A runway overrun is not likely to kill you; a low-altitude full stall almost certainly will. The best solution is prevention, however, so try to avoid the whole ugly situation. Don’t fly into icing conditions in an airplane without ice protection. And if your ice protection fails in icing conditions, immediately declare an emergency and escape.

When you have encountered icing conditions and/or believe that your airplane’s critical surfaces may be contaminated, add at least 15% in knots to your final approach speed and land with only no flaps if equipped.

Ice that accumulates on unprotected surfaces of the airplane such as wingtips, antennae, the prop spinner and so forth, can cause a loss of climb performance of up to 100 FPM. Stall characteristics and low-speed handling will become unpredictable.

Tailplane Icing

Thinner surfaces collect ice more efficiently than thicker surfaces. Your horizontal stabilizer is thinner than your wing. Therefore, if you see any ice at all on your wing you can bet you’ve got more ice on your tail. The horizontal stabilizer is an upside-down wing; it has a slightly negative angle of incidence. The faster you fly, the more of this downward force it generates, lifting the nose – think of the airplane as a see-saw pivoting around its CG. The slower you fly, the less of this downward force it generates, and the natural nose-heaviness of the plane takes over, lowering the nose. (The airplane is nose heavy because the center of lift lies behind the CG ) This is what makes an airplane stable.  But what happens when ice forms on your tail?  Bad things. If the tail stalls, it will be unable to generate the downward force which holds the nose up. The nose will pitch down violently and the airplane will dive into the ground.

A. When are you most in danger from a tail stall?

1) On final approach.

2) At reduced power settings.

3) At higher angles of attack.

4) At higher flap settings.

B. What are the symptoms of a tail stall?

1) Shuddering, buffeting or shaking of the yoke but not the airframe. (This is how you tell the difference between a tail stall and a regular wing stall.)

2) “Softening” of the control feel – “mushiness.”

3) Difficulty in trimming the airplane.

4) Pilot-induced oscillations. (“Porpoising” up and down.)

5) A sudden, forceful, nose-down pitch change. Stick forces may be extreme – 100 pounds or more.

C. What is the recovery procedure for a tail stall?

1) Immediately retract the flaps to their previous setting. (In other words, undo whatever you just did.)

2) Apply NOSE-UP elevator input. (This is the exact opposite from a wing stall recovery! Be careful, because your reflexive tendency to lower the nose will exacerbate the tail stall, making it deeper and even more violent.)

3) Increase airspeed and power gently, smoothly and cautiously. (Just like with a spin, or a deep wing stall, the sudden addition of a large amount of power in a deep tail stall could make it worse!)


Standard Pressure, Temperature, and Lapse RateSea level standard pressure = 29.92" hg
Standard lapse rate = -1" hg. for each 1000' increase in altitude
Sea level standard temperature = 15°C / 59°F
Standard Lapse Rate = -2°C / -3.5°F for each 1000’ increase in altitude

Most structural icing occurs between 0°C to –10°C
Deviate 10-20 miles upwind around thunderstorms; Don’t fly under anvil
Hail may be found 10 miles or more underneath the anvil
Dew point above 10C /53F indicates enough moisture for thunderstorms to form.
Altimeter Error

Assuming a standard lapse rate, an airplane's altimeter will over- or underread by 4 ft per 1°C deviation from ISA per 1,000 ft above the station reporting the altimeter setting.

For example, if you are flying at 11,000 ft indicated altitude using an altimeter setting from a station at 1,000 ft elevation and the outside air temperature is -20°C, your altimeter will be off by -520 ft (4 * -13 * 10), and your true altitude will be slightly below 10,500 ft.


Airplane performance depends on density altitude. To estimate density altitude (at least at lower altitudes), start with pressure altitude and add 120 ft for every degree Celsius above ISA temperature, or subtract 120 ft for every degree Celsius below ISA.

For example, at 3,000 ft pressure altitude, the ISA temperature is 9°C. If the actual temperature is 20°C, add 1,320 ft (11 * 120) to get an approximate density altitude of 4,320 ft.

Humidity also affects density altitude, but not enough to worry about in a rule of thumb.


The International Standard Atmosphere is the reference point for most aircraft performance data. When the real atmosphere varies from ISA, it is necessary to adjust the altimeter for pressure altitude and to modify cruise speeds, power settings, takeoff and landing distances for density altitude.

The ISA has an air pressure of 29.92 inHg at sea level, decreasing by 1.00 inHg for every thousand feet (at lower altitudes), a temperature of 15°C, decreasing by 2°C for every 1,000 ft (ditto), and no humidity.

For example, ISA predicts that the air pressure should be 19.92 inHg at 10,000 ft and that the outside air temperature should be -5°C. If the temperature is different than that, or if there is a non-standard pressure lapse rate, there will be a (possibly serious) altimeter error.


To calculate pressure altitude, set your altimeter to 29.92 inHg and read the value from your altimeter (write down the current altimeter setting first, if you're in the air), or alternatively, subtract 1,000 ft from indicated for every inHg above 29.92, or add 1,000 ft to indicated altitude for every inHg below 29.92 (remember, pressure decreases as you go up, so lower pressure seems like higher altitude).

For example, if the indicated altitude is 3,000 ft and the altimeter setting is 28.50, add 1,420 ft ((29.92 - 28.50) * 1,000) to get the pressure altitude of 4,420.

Estimating Cloud Bases:

Temp. minus dew point divided by 4 and multiply by 1000'
Ex. 72-52=20 20/4=5 5x1,000=5,000' bases = 5,000' agl during instability and warmer part of day

If you're flying towards a stationary thunderstorm, pass it on the right - you'll get a tailwind.

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