Temperature and moisture

Flying would be much easier if moisture were not such an influential component found in the atmosphere. Moisture in the air creates more hazards during flight than any other weather phenomenon. Water in the atmosphere is measured by relative humidity and dew point accompanied by a temperature-dew point spread. Knowing the conditions during which water changes state also helps pilots to avoid moisture-related problems during flight.

Relative humidity relates the actual amount of moisture in the air (in the form of a percent) to what total amount of moisture could be held in the air (That means it is also a ratio!). Relative humidity expresses the degree of saturation. As a rule cold air holds fewer water molecules than warmer air holds. If air is completely saturated with water molecules the humidity is 100%.

Jet Stream in USA


In relationship to the humidity is dew point. Dew point is the temperature (in degrees) to which air must be cooled in order to be saturated with water vapor already in the air.

Weather reports for pilots usually include the dew point as well as the temperature. When the two are compared, the difference reveals to the pilot how close the air is to being 100% saturated. This difference is called the temperature-dew point spread.

Warm Air rising on land during day and from water at night

Advection fog off CA coast

Advection fog on gulf coast

On a clear night when the dew point colder than 32° F and the temperature-dew point spread is 50° F or less and decreasing, then frost will form. Fog is most likely when the temperature-dew point spread is 50° F or less and decreasing. The fog would be lifting when the temperature-dew point spread begins increasing.

Fog usually forms when the dew point and the temperature are within a few degrees of each other. The air temperature being lowered to the dew point, or the dew point being increased to the air temperature causes fog formation. Air temperature can be lowered as the air crosses over a colder surface like cold lake waters or a snow-covered area. Increasing the atmospheric moisture occurs when air flows from a water source (large lake, ocean) then moves over land.

Pilots need to be mindful of the conditions which cause radiation fog and advection fog. Of the two types of fog, radiation fog does not hang around as long, is less hazardous and more localized. This means that when flying at low altitudes, a pilot will encounter patches of it and be able to fly through it quickly.

Radiation fog (also known as ground fog) occurs most often during clear, cool autumn nights while the Earth's surface is rapidly cooling. It may hang in the air through the morning, but dissipates a few hours after sunrise.

Advection fog however, forms when air laden with moisture from a maritime area moves from the water area over higher terrain while gradually cooling. As the air temperature is reduced to the dew point advection fog forms. This happens most often during the winter months over the eastern half of the United States as moist air flows northward from the Gulf of Mexico across the land increasing in elevation and cooling as it moves.

This same phenomenon occurs along the coastal region of California during the summer as warm winds blow across the chilled California Current resulting in advection fog that can stretch from San Francisco to San Diego.




Valley Wind

Precipitation refers to all different kinds of moisture that falls from the sky: drizzle, rain, snow, ice pellets, hail and ice crystals (whether they contact the ground or not).

As air becomes saturated from cooling temperatures or increasing dew point, water vapor starts to condense on the microscopic particles that are suspended in the air. Once the water droplet or ice crystal forms it will continue to grow by continued condensation or sublimation. When enough of these water or ice particles merge they form a visible collection called a cloud. Depending upon air currents and temperature, clouds can take on a sheet-like shape or become towering masses.

Precipitation that forms and remains liquid will fall as drizzle or rain. If water vapor condenses at temperatures below freezing, snowflakes can form. Precipitation can change its state (vapor, liquid or solid) depending upon the temperature of its immediate environment. As ice pellets fall through a warmer layer of air they will turn to raindrops.

In order to generate significant precipitation, clouds are commonly 4,000 feet or more in thickness. The heavier the precipitation, the thicker the cloud cover is likely to be.

When the destination airport is reporting precipitation that is light to moderate, expect the cloud cover to be greater than 4,000 feet thick.




One of the major weather hazards to aviation is icing. Icing is the formation of ice on parts of a vehicle. Pilots and controllers need to be aware of the icing process, under what conditions ice will form on an aircraft, the different forms it takes on an aircraft and its effects on the aircraft's flight characteristics. Icing occurs when an aircraft flies through visible water and the temperature at the point where the moisture strikes the aircraft is 32° F (0° C) or colder. Even though the air temperature around the airplane may be a few degrees warmer than freezing, aerodynamic cooling can occur (due to the rapid movement of the airplane through the air creating a wind chill effect) and lower the temperature of the airplane's surface thus inducing icing. Supercooled water increases the rate of icing. As a supercooled water droplet hits the airplane's surface a part of it freezes instantaneously. The manner in which the remaining portion of the water droplet freezes determines whether the ice formation is clear ice, rime ice or mixed ice.

sideview of <a href=wing with Clear Ice" width="280" height="235" />


clear ice
After the initial impact of supercooled droplets from large raindrops strike the surface, the remaining liquefied portion flows out over the surface and freezes gradually. This freezes as a smooth sheet of sold ice. It is hard and heavy and is difficult to remove.

Sideview of <a href=wing with rime ice" width="280" height="231" />


rime ice

Formed from small supercooled droplets when the remaining liquefied portion after initial impact freezes rapidly before the drop has time to spread over the surface. This traps air between the droplets, and gives the ice a white appearance. It is lighter in weight than clear ice. Its formation is irregular and its surface is rough. It is brittle and more easily removed than clear ice.

Side view of <a href=wing with mixed ice" width="280" height="233" />


mixed ice
ormed when supercooled water droplets are of various sizes or are intermingled with snow or ice particles. After initial impact, the remaining portion freezes rapidly and forms a mushroom shape on the leading edges of a wing. Ice particles are imbedded in clear ice and form a hard and rough-edged mass.

Icing is considered a cumulative hazard as it takes time for the ice to build up on the aircraft and increasingly changes the aircraft's flight characteristics.

Normal Conditions

Normal Conditions   With Icing

Click the above links to see the effects of icing on airplane performance.

freezing rain with a cold front will cause icing

Basically all clouds with temperatures that are sub-freezing have the potential for icing conditions. See the cloud sub-section for specific cloud types. Other conditions that are conducive to icing include mountainous regions and certain fronts. Icing is most hazardous and most probable over mountainous regions than any other type of terrain. Rapid movements of upward air coupled with cooling support large water droplets. Each mountain region has its own areas that are prone to icing conditions. Pilots need to be familiar with these zones. The most dangerous icing takes place over the windward side of ridges and above the crests. This icing zone can extend 5,000 feet or higher above the top of the mountain.

The effects of icing can be devastating to a flight. A pilot can avoid conditions in which icing occurs or upon encountering icing, react quickly by ascending or descending to a different altitude where the temperature is warm enough to melt the ice. Avoiding quick maneuvers when icing occurs is recommended, as the aircraft is not operating at peak efficiency. If choosing to ascend to a warmer flight level, increasing airspeed to a faster than normal speed when climbing will avoid a stall. Overall, recognizing the conditions in which icing occurs, knowing what to do when icing takes place and recognizing the type of icing that has afflicted the flight will all be required reporting in the pilot's flight log.


Iced jet engine In an effort to improve aircraft safety by reducing the number of in-flight icing events, the NASA Glenn Icing Branch conducts tests using the Icing Research Tunnel (IRT), the largest refrigerated icing wind tunnel in the world. In it, researchers have developed icing technologies that are used on many of today's aircraft. The IRT is able to duplicate the icing conditions that aircraft typically encounter in nature; it has played a critical role in developing, testing and certifying methods to prevent ice build-up on aircraft. The IRT is a closed-loop refrigerated wind tunnel with a test section 6 feet high and 9 feet wide. The airspeed in the test section can be varied from about 25 to over 400 mph at essentially a sea-level pressure. The total air temperature can be independently varied from about +30 to -45° F. Spray nozzles produce the icing spray cloud of very small, unfrozen, subcooled droplets.

NASA Glenn researchers also conduct icing research flights to measure the characteristics of severe icing. Flights of the deHavilland DHC-6 Twin Otter icing research aircraft have been used to sample cloud characteristics, document the resultant ice accretions, and measure their effect on aircraft performance.