3: Meteorology

Cause of Weather

Standard Day Conditions

Altitudes

Air Density

Layers of the Atmosphere

Temperature Inversions

Icing

Rain

Weather Information

Cause of Weather

The primary cause of all changes in the Earth's weather is the variation in solar energy received in different regions. This variation causes differences in temperature, pressure and moisture content that bring about all the weather daily phenomena.

Standard Day Conditions

In the United States, both English and Metric units are used to express temperature and pressure. The formula used to convert from Celsius (C) to Fahrenheit (F) is:

F = 9/5 C +32

The International Civil Aviation Organization (ICAO) has established standard conditions for tem-perature and pressure. The standard temperature at sea level is +15°C (+59°F). The temperature decreases with altitude at a rate of 2°C (3.5°F) per 1,000 feet of altitude increase. This means that each altitude has its own standard temperature. For example, the standard temperature at 5,000 feet is +5°C (+41.5°F). This standard temperature lapse rate continues to an altitude of 38,000 feet on a standard day. See FAA Figure 6.

FAA Figure 6 shows the standard day conditions for altitudes from sea level to 60,000 feet. The standard day temperature for each altitude is sometimes referred to with the term "ISA" (International Standard Atmosphere). Deviations from standard day conditions are expressed with reference to ISA. For example, ISA at 8,000 feet is -0.8°C (30.5°F); ISA +34°C at 8,000 feet is 33.2°C (91.76°F).

The standard metric measure for sea level pressure is 1,013.2 millibars. The English equivalent is 29.92" Hg (Hg is the symbol for mercury). The standard pressure also decreases with altitude, but not at a linear rate as with temperature.

The term hectopascals is replacing "millibars." Millibars is the term used throughout the current flight engineer test.

Altitudes

Pressure Altitude—the height above the standard datum plane. To obtain the pressure altitude in flight, adjust the altimeter to 29.92" Hg and read it directly from the altimeter.

Indicated Altitude—the height the altimeter displays when set to the current local altimeter setting. Indicated altitude is greater than pressure altitude when the altimeter setting is greater than 29.92" Hg. It is less than pressure altitude when the altimeter setting is less than 29.92 inches Hg. Indicated altitude is a close approximation of the actual height above sea level.

True Altitude—the actual height above sea level. Non-standard temperatures affect the pressure lapse rate. When the temperature is warmer than standard, the true altitude is higher than the indicated altitude. In cold temperatures, the true altitude is below the indicated altitude.

Density Altitude—pressure altitude corrected for non-standard temperatures. Density altitude equals pressure altitude at standard temperature, is higher in warm temperatures and lower than pressure altitude in cold temperatures.

Air Density

Air density is the air's thickness determined by pressure, temperature and humidity. Air density increases when the temperature or humidity is lowered or the pressure is increased. Also, air density decreases when the temperature or humidity is raised or the pressure is lowered.

Engine output is directly related to air density. Greater density means more oxygen available for combustion and an increase in engine performance.

Layers of the Atmosphere

The two layers of the atmosphere in which turbojet aircraft operate are the troposphere and the stratosphere. The boundary between these two layers is the tropopause. Notice from Figure 3-1 that the troposphere is thicker over the equator than it is over the poles. Its thickness also varies with the seasons, with the tropopause being higher in the summer than in the winter.

A primary attribute of the troposphere is the decrease in temperature with increasing altitude (lapse rate) from sea level to the tropopause. At the tropopause there is an abrupt change in the lapse rate to near zero. The stratosphere has relatively small changes in temperature with increasing altitude.

Figure 3-1. Layers of the atmosphere

Temperature Inversions

The normal temperature lapse rate is reversed in temperature inversions. Instead of the temperature decreasing with increases in altitude, it becomes warmer. Inversions can occur at any level in the atmosphere and have a number of different causes.

One common type of ground based or surface inversion is caused by ground radiation on clear, cool nights when the wind is calm. The ground cools after sunset, and in turn, cools the air near the surface. The temperature of the air at higher altitudes remains almost the same, and an inversion forms. If the temperature/dewpoint spread was small before the start of cooling, fog or low clouds can develop in this type of inversion.

Icing

Ice can form on an aircraft in flight when the temperature is below freezing and visible moisture is present. The moisture must be in the form of supercooled water droplets. The temperature of these droplets is below freezing, but they have not yet undergone the change of state from liquid water to ice. When these droplets strike an airplane, they freeze almost instantly forming a layer of ice on the wings, fuselage and engine intakes. The moisture can be in the form of clouds, fog or rain, but the highest rate of ice accumulation is associated with freezing rain.

The optimum temperature for inflight icing is between 0°C to about -15°C (32°F to 5°F). Water cannot exist in a liquid state below about -40°C (also -40°F) and so there is almost no icing hazard at or below that temperature.

Frost is a type of icing that forms through the process of sublimation. In this situation, water goes directly from its vapor state to ice without ever being liquid. This occurs when a collecting surface (such as an airplane) has a temperature at or below the dew point of the surrounding air and the dew point is below freezing.

Ice and frost are a hazard to flight because they disrupt the airflow over the aircraft's wing resulting in a loss of lift. This can cause an early airflow separation, which means the aircraft will stall at a lower angle of attack and higher airspeed.

Rain

Rain is not normally the hazard to flight that icing is, but when it is very heavy it can cause a deterioration in performance. The rain forms a film of water over the wing. This film is then roughened by the impact of raindrops and that causes a loss of lift.

Weather Information

Many airports have ATIS (Automatic Terminal Information Service) broadcasts to inform flight crews of the latest weather conditions. These broadcasts are updated upon the receipt of any official weather information, regardless of whether or not it changes. This normally takes place every hour, but can happen much more frequently in rapidly changing conditions.

ATIS broadcasts normally include sky conditions, visibility, temperature, wind and altimeter settings. When the sky condition and visibility are not transmitted it means the ceiling is above 5,000 feet and the visibility is more than 5 miles.