Weather 3-1

DEFINE the standard atmosphere

29.92 in-Hg (1013.2 mb) and 15° C (59° F). In the lower atmosphere, and thus for most aviation applications, a 1000 foot increase in altitude will result in a pressure decrease of approximately 1 in-Hg (34 mb) and a temperature decrease of 2° C (3.5° F). These values are the standard day pressure and temperature lapse rates.

Good info that was in end of chapter questions

A highpressure area (or high)-where the pressure in the center is higher than the surrounding areas-may be thought of as a mountain on a surface pressure chart. Similarly, a lowpressure area-where the pressure in the center is lower than the surrounding areas-may be thought of as a basin or valley. A ridge is an extension of a high-pressure area, and a trough is an extension of a low-pressure area. There are certain characteristic winds and weather systems associated with these pressure systems. For example, poor weather such as found with fronts and squall lines are generally associated with troughs and lows, while good weather is associated with highs and ridges.

DESCRIBE the effects of temperature deviations from the standard lapse rate on aircraft altimeters

An incorrect altitude indication will result if the temperature deviates from the standard. For every 11 °C that the temperature varies from the standard, the altimeter will be in error by 4%. If the air is colder than the standard atmosphere, the aircraft will be lower than the altimeter indicates. If the air is warmer than standard, the aircraft will be higher than the altimeter indicates. You may notice that the rules presented in the pressure section, above, also apply to temperature deviations.

DESCRIBE the effects of pressure changes on aircraft altimeters

If the altimeter is not adjusted and your flight path takes you into an area of lower MSL pressure the aircraft will be lower than the altimeter indicates. Conversely, if your flight path takes you into an area of higher MSL pressure, the aircraft will be higher than the altimeter indicates. A change in pressure of 0.10 in-Hg will change the altimeter reading 100 feet.

DEFINE atmospheric pressure

Pressure is force per unit area. Atmospheric (barometric) pressure is the pressure exerted on a surface by the atmosphere due to the weight of the column of air directly above that surface. For example, the average weight of air on a square inch of the Earth's surface at sea level under standard conditions is 14.7 pounds. Pressure ALWAYS decreases with altitude

DIFFERENTIATE between sea level pressure and station pressure

Station pressure is the atmospheric pressure measured directly at an airfield or other weather station. Sea level pressure is the pressure that would be measured from the existing weather if the station were at mean sea level (MSL). For pressure to be meaningful, all stations-even those far from the ocean-will report sea level pressure.

DESCRIBE the characteristics of the stratosphere

The stratosphere is characterized by increasing temperature with increasing altitude. This increase in temperature is due to the gas ozone, which plays a major part in heating the air at this altitude. Flying in the stratosphere is generally smooth with excellent visibility. The air is thin and offers very little resistance to the aircraft.

DESCRIBE the flight conditions associated with the tropopause

The strongest winds, those of the jet stream, occur just below the tropopause. Moderate to severe turbulence is sometimes associated with the wind shear caused by the jet stream. Contrails frequently form and persist near the tropopause since it is normally the coldest area within the lower atmosphere.

DESCRIBE the characteristics of the tropopause

The strongest winds, those of the jet stream, occur just below the tropopause. Moderate to severe turbulence is sometimes associated with the wind shear caused by the jet stream. The average height of the tropopause over the US is 36,000 feet MSL, anvil tops of thunderstorms will spread out at the base of the tropopause, and a haze layer with a definite top frequently exists at the tropopause. The temperature in this layer is constant with altitude

DEFINE indicated altitude

1) Altitude is the height above a given reference Indicated altitude is the altitude read directly from the altimeter

DEFINE the types of altitudes

1)Calibrated altitude is indicated altitude corrected for instrument error. 2)Mean Sea Level (MSL) or True altitude is the actual height above mean sea level (MSL). It is found by correcting calibrated altitude for temperature deviations from the standard atmosphere. On a standard day, MSL/true altitude is equal to calibrated altitude. If there is no instrument error, true altitude would also be equal to indicated altitude 3)AGL or absolute altitude is the aircraft's height above the terrain directly beneath the aircraft and is measured in feet above ground level (AGL). 4)Pressure altitude is the height above the standard datum plane. The standard datum plane is the actual elevation above or below the earth's surface at which the barometric pressure is 29.92 in-Hg Really good illustration on page 211 5)Density altitude (DA) is pressure altitude corrected for nonstandard temperature deviations. On a hot day, air molecules are farther apart, decreasing the air density and increasing the density altitude. In this situation, the DA of an airfield would be higher than both the published field elevation and the pressure altitude. Density altitude is not a height reference; rather, it is an index to aircraft performance

STATE the average lapse rate in degrees Celsius

The average or standard lapse rate is 2° Celsius (3.5° Fahrenheit) per 1000 feet.

DEFINE a lapse rate

The decrease in atmospheric temperature with increasing altitude is called the temperature lapse rate.

DESCRIBE the flight conditions associated with the stratosphere

The general lack of weather in this layer makes for outstanding flying.

DESCRIBE the characteristics of the troposphere

The troposphere is the layer adjacent to the Earth's surface. It varies in height from an average 55,000 feet over the equator to 28,000 feet over the poles. Due to heating, the troposphere extends to a greater height in summer than in winter. The atmosphere becomes less dense with altitude, and roughly 50% of it, by weight, lies below 18,000 feet and 90% within 53,000 feet. Within the troposphere, the temperature normally decreases with increasing altitude. Large amounts of moisture and condensation nuclei are found in the troposphere because of its closeness to the Earth's surface, and nearly all weather occurs here. Winds are generally light near the Earth's surface and increase with altitude. Wind speeds over 200 knots may occur near the top of the troposphere. An abrupt change in the rate of temperature decrease with increasing altitude marks the boundary, called the tropopause.

DESCRIBE the flight conditions associated with the troposphere

Winds are generally light near the Earth's surface and increase with altitude. Wind speeds over 200 knots may occur near the top of the troposphere.

STATE the standard units of pressure measurement

inches of mercury and millibars