JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
Composition
Air is a mixture of gases having weight, elasticity, and compressibility. Pure, dry air contains
78% nitrogen, 21% oxygen, a 1% mixture of 10 other gases. The atmosphere also contains water
vapor amounting to 0% to 5% by volume. Water vapor (for ordinary considerations) acts as an
independent gas mixed with air.
The atmosphere appears clear, but it contains many nongaseous substances such as dust and salt
particles, pollen, etc, which are referred to as condensation nuclei. When these particles are
relatively numerous, they appear as haze and reduce visibility.
Lapse Rates
The decrease in atmospheric temperature with increasing altitude is called the temperature lapse
rate. In order to determine how temperature changes with increasing altitude, meteorologists
send up a weather balloon to take the temperature (among other readings) at different altitudes.
The resulting temperature profile is known as the environmental lapse rate (a.k.a. the existing
lapse rate, or ELR). The average or standard lapse rate is 2° Celsius (3.5° Fahrenheit) per 1000
feet. Even though this is the average lapse rate of the troposphere, close to the surface of the
earth the ELR may indicate an increase, decrease, or a constant temperature when measured at
increasing altitudes. These different ELRs give meteorologists a clue to the type of weather that
exists, and there are names for these various types of ELRs, as well. The standard lapse rate is
actually a shallow lapse rate (between 1.5 and 3.0° C/1000 ft). Any lapse rate greater than 3 °C/
1,000 feet is called a steep lapse rate. An isothermal lapse rate indicates the temperature is the
same at different altitudes, and an inversion is a lapse rate where the temperature increases with
increasing altitude, such as occurs in the stratosphere. Inversions can be anywhere from a few
hundred to a few thousand feet thick, and stable conditions are generally found within them.
These three major types of lapse rates–the standard, isothermal, and inverted–are shown in
Figure 1-2 as a graph of temperature vs. altitude overlaid on a profile of the atmosphere.
There is also a pressure lapse rate, which indicates the decrease in atmospheric pressure with
increasing altitude, to be discussed next. Notice that the values used for lapse rates assume that a
decrease is normal, thus positive lapse rates actually indicate a decrease in the value measured,
and a negative lapse rate (only temperature has this characteristic) would indicate an increase.
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, unlike temperature, always decreases with altitude.
In the lower layers of the atmosphere pressure decreases much more rapidly than it does at
higher altitudes because density decreases as altitude increases.
Units of Measurement
In the U.S., two units are used to measure and report atmospheric pressure: inches of mercury
(in-Hg) and millibars (mb). Inches of mercury is a measure of the height of a column of mercury
that can be supported by atmospheric pressure. The millibar is a direct representation of pressure,
1-3 Version 3.2/Dec 08