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Structural Icing in VMC
Temperature - Dew Point Spread
International Standard Atmosphere (ISA)
Structural Icing in VMC

Structural icing in Visual Meteorological Conditions may sound like an oxymoron to many pilots. If we examine the conditions in which structural ice forms, we shall find the  following:

    1. the aircraft must be flying through visible moisture, and
    2. the temperature at the point where the moisture strikes the aircraft is at or below 0 degrees Celsius.
These conditions constitute the physical phenomenon known as supercooled water.

Supercooled Water  One of the qualities of water is that it can be found in its  three forms at or below 0 degrees Celsius. That is water,
 vapor and ice. The state of water at or below freezing is defined as supercooled water. Supercooled water is in an
 unstable liquid state. Even a small impact on a supercooled droplet will violate the fine balance, causing it to freeze.
 As an aircraft moves through supercooled water the formation of structural ice is imminent. The ice forms on all
 the surfaces that are exposed to the initial impact. That includes the leading edge of the wings, the empennage and
 the propellers, as well as the struts, the windshield, etc.  

Types of Structural Ice

 The types of structural ice are clear, rime and a mixture of  the two. The type of ice depends on the size of the water
 drops. Clear ice forms when the drops are large. Upon impact, the liquid portion of the drop flows over the aircraft
 surfaces, gradually forming a smooth sheet of solid ice. Clear ice will normally form while flying through cummuliform clouds and through freezing rain. Rime ice forms while flying through stratified clouds and freezing drizzle. When the drops are small, the drop freezes prior to spreading. Air is trapped between the small frozen drops, giving the ice a white frostlike appearance.

VMC and Icing

The term visible moisture may lead to a wrong conclusion about the imminence of structural icing. Clouds are the most
common form of visible moisture. Rain, drizzle, and mist are often transparent to the naked eye and should also be
Meteorological Conditions exist, icing may occur when flying through freezing rain or freezing drizzle. 
Freezing rain and drizzle may be found in frontal zones. When rain is formed aloft and drops through air at a temperature below freezing, it becomes supercooled, and source for structural icing. Icing may occur in cold, warm and occluded fronts and over high terrain. Areas of freezing rain may exist in Visual Meteorological Conditions, especially under the frontal line of warm and occluded fronts. Icing may also be present in a cold front inside and under cumuliform clouds. Freezing rain may be associated with critical icing because of the large amount of supercooled water and because of the size of the drops.
Remember, structural icing is dangerous because of the increase in weight and drag, and the decrease in lift and thrust. A flight without ice removal equipment into areas of known or forecast icing should be avoided.

Temperature - Dew Point Spread

"This is Sky Harbor airport information Kilo at 0040 Zulu, wind 180 at 6 knots, visibility 7sm, sky clear, temperature 10
degrees Celsius dew point nine degrees Celsius...."

These are the stereotypical conditions for the formation of radiation fog. Every text book about aviation weather has
undoubtedly reference this phenomenon. The quickness with which fog can form, makes it hazardous for flying especially
for arriving aircraft. Under certain conditions, fog can form in just a few minutes. To understand fog, one should
understand the basic qualities of the air.

The air is a mixture of several gases. When the air is completely dry, it is composed of about 78% nitrogen, 21% oxygen
and one percent of other gases. In nature, the air always contains some amount of water. This varies from near zero to
about 5% in volume. As the quantity of water in the air increases, the amount of other gases is decreased proportionally.

The air temperature is a major factor in the air's ability to hold water vapor. Warm air can hold more water vapor than
cold air. When the air can no longer hold the water vapor, it is said to be saturated. The ratio between the actual amount
of water vapor in the air and the amount that the air can hold under a given condition is defined as the RELATIVE
HUMIDITY. Relative humidity is expressed as a percentage. When the relative humidity is 100%, the air is saturated,
otherwise it is unsaturated.

There are two ways to bring air to saturation, one is by adding water vapor and the other is by cooling it. The point at
which the air becomes saturated as it is being cooled is defined as the DEW POINT. As the actual air temperature drops to the dew point, the air becomes saturated and either a cloud or fog will form.

The difference between the actual air temperature and the dew point is called the Temperature - Dew Point Spread. The
temperature-dew point spread is another indicator of the relative humidity. As the temperature reaches the dew point, the air becomes saturated, or in other words, the relative humidity is 100%. The temperature-dew point spread is important in anticipating fog and the height of the clouds' bases.

There are a few ways in nature in which the temperature-dew point spread is modified. The temperature-dew point
spread is reduced by either cooling or by adding humidity. The most common examples where the air is being cooled are
radiation, advection and convection. radiation cooling occurs mostly at night. After the sunset, when the source of heat is
no longer available. The air progressively looses heat energy by radiating it outside of the earth's atmosphere. At the same, with the absence of a heat source, the surface is cooling down, which causes the cooling of the nearest layer of air.
advection cooling is the process when warm air is being forced over a colder surface. convective cooling happens when
air is being forced vertically. The amount of water in the air determines whether the cooling is sufficient for it to become

Air can also become saturated by added water. In frontal areas, specifically in warm and occlude fronts, warm
precipitation falls through colder air, thus increasing the amount of water. Another typical phenomenon is the release of
water vapor by bodies of warm water which underlie colder air.

In short, these two magic numbers, that of the temperature and that of the dew point and the spread between the two,
give pilots and meteorologists the most valuable information about formation of clouds and fog.


The terms hail, sleet and ice pellets are often confused by pilots. While sleet and ice pellets are always associated with
cold weather, hail is generally associated with warm weather. The formation of sleet and ice pellets is a result of water
droplets passing through cold saturated air. These droplets change directly from liquid to solid state (by sublimation). As
the frozen particles fall through a cloud, they grow by the impact with supercooled water. The height of the cloud and the
freezing level are the major factors which determine the size and texture of the frozen particles.
Hail, however is a product of highly turbulent thunderstorms. There are three stages in a thunderstorm cloud's (Cumulonimbus) life, the Cumulus, the mature and the dissipating stages. The mature stage of a thunderstorm is characterized by extremely rapid updrafts, downdrafts and by heavy rain. At the mature stage, the base of the cloud could start at less than 1,000 feet with tops as high as 66,000 feet. The updraft speed may exceed 6,000 feet per minute while the downdrafts may exceed 2,500 feet per minute.

Supercooled rain drops freeze a result of the violent turbulence and start falling through the cloud. As the frozen droplets reach the melting level they lose weight. If the updrafts are very strong they will carry the frozen drops upward. As the frozen drops move upward they latch onto supercooled moisture and grow. The increase in weight causes the frozen drops to start falling through the cloud again, thus collecting more ice and becoming even heavier. This process continues until the hailstone is heavy enough to overcome the updraft. By this time the hail will fall under the cloud onto the surface.

Formation of hail is anticipated in any thunderstorm. Whether the hail will reach the ground or not depends on the size of the hail and the prevailing temperatures. Hailstones in the size of a baseball are common in highly violent thunderstorms.

Hail may be encountered in clear air when flying in the vicinity of a thunderstorm. That includes flying on top and behind the thunderstorm, and when flying underneath the anvil. The largest amount of hailstones that moves in high velocity is naturally found inside the thunderstorm itself. Flying through hail can significantly cause damage to an aircraft in just a few seconds, especially when the size of the hailstone exceeds one half of an inch.


International Standard Atmosphere (ISA)

The term ISA (pronounced as eyes-zha) is the abbreviation for International Standard Atmosphere. ISA was established
by the International Civil Aviation Organization (ICAO) as a uniform reference for temperature and pressure. The
properties of the Earth's atmosphere are constantly changing. The barometric pressure, temperature and the amount of
humidity in the air are subject to annual, seasonal and diurnal variations. The pressure, temperature and humidity are also
subject to altitude changes over the same geographical location. A uniform reference became a necessity not only for
operational reasons but also essential for aircraft design.

The standard atmosphere was derived from the average conditions for all latitudes, seasons and altitudes. The properties of a standard day are related to sea level at latitude 45 degrees with absolutely dry air. The standard temperature is 15 degrees Celsius (59 degrees Fahrenheit) and a standard temperature lapse of 2 degrees Celsius (3.5 degrees Fahrenheit) per 1000 feet. The standard barometric pressure is 1013.25 hectoPascal (milibars) or 29.92 when expressed in inches of mercury. 

To assure altitude separation, all aircraft altimeters are calibrated under ISA conditions. When the altimeter is set to standard pressure (either 1013.2 hctoPascal or 29.92" of mercury) , it reads pressure altitude. Since the ISA parameters are hypothetical in nature, they constitute an imaginary atmosphere. Pressure altitude is an imaginary altitude that is used for vertical separation.

Since it is not practical for aircraft that fly long distances at high altitude to constantly change their altimeters to a local setting, it is agreed that above a certain altitude all aircraft use pressure altitude as a uniform altimeter setting. Each country publishes its own transition altitude. A transition altitude is the altitude above sea level where pilots must set their altimeters to standard pressure. When flying with the altimeter set to standard pressure, the altitude is expressed in flight level. Flight level is obtained by removing the altitude's three rightmost digits. For example and altitude of 21,000 feet will be expressed as FL210.

Air density is a major factor in aerodynamic performance and engine efficiency. An increase in air temperature, altitude
and humidity causes a decrease in air density. Since lift and drag are proportional to the air density, a loss of lift
is caused by reduced air density. The propeller efficiency is decreased for the same reason. Power decreases because the engine does not get enough air to support an optimum combustion. In jet engines, a lesser mass of gasses is pushed out through the exhaust nozzle. When low air temperature, humidity and altitude conditions exist, the air density increases resulting in improved aerodynamic performance.

The same as with pressure altitude, a uniform atmospheric conditions reference became necessary to evaluate aircraft's
performance. Density altitude is pressure altitude corrected for non-standard temperature. Density altitude is not a height
reference; it is rather an index for aircraft performance under certain meteorological conditions. Under standard
atmospheric conditions, the air has a particular air density for each level of the atmosphere. The density altitude indicates
how conditions in a specific airport compare to a hypothetical elevation under standard atmosphere. For example, if an
airport's elevation is 150 feet MSL but density altitude computations indicate a density altitude of 2000 feet, the airplane's
performance will be the same as at 2000 feet regardless of the airport elevation. High density altitude may present a
serious hazard. Density altitude is computed from the pressure altitude (not indicated altitude) and the outside air

Aircraft Performance Data Charts use both pressure and density altitude to determine aircraft's performances. When
using these charts, the pilot must ensure the use of the appropriate units. Temperature is often expressed in terms of ISA+
or ISA - (degrees Celsius). For example, in standard atmosphere the temperature at 4000 feet is 7 degrees Celsius.
However if the actual temperature at 4000 feet is 12 degrees Celsius, and can be expressed as ISA+5.

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Last update June 13, 2009
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