Composition - P-3030018

CHAPTER ONE

AVIATION WEATHER

The stratosphere is the layer 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. The general lack of weather in this layer

makes for outstanding flying.

106.

COMPOSITION

Air is a mixture of gases having weight, elasticity, and compressibility. Pure, dry air contains

78% nitrogen, 21% oxygen, a 1% mixture of ten other gases. The atmosphere also contains water

vapor ranging from 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, which are referred to as condensation nuclei. When these particles are relatively

numerous, they appear as haze and reduce visibility.

107.

LAPSE RATES TEMPERATURE AND PRESSURE

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

(ELR)). The average or standard lapse rate is 2C (3.5F) 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 3.0C/1000 feet). Any lapse rate greater than 3 Celsius/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.

Example of using standard lapse rates: If the temperature is 2C at 8000 feet and the existing

lapse rate is standard, at what altitude is the temperature +4C?

Algebraic difference of 2C and +4C =

= 3000 feet

2C/1000 feet

2/1000 feet

8000 feet - 3000 feet = 5000 feet

1-4

General Structure of the Atmosphere, and Atmospheric Temperature and Pressure