Weather plays a critical role in aviation safety and flight planning. To understand weather phenomena, pilots must first understand the atmosphere, its composition, circulation patterns, and the effects of atmospheric pressure. These fundamental concepts explain how weather develops and how changing atmospheric conditions affect aircraft performance.
The atmosphere is a blanket of air made up of a mixture of gases that surrounds the Earth and extends nearly 350 miles above its surface. This mixture is in constant motion. If the atmosphere were visible, it might look like an ocean with swirls and eddies, rising and falling air, and waves that travel for great distances.
Life on Earth is supported by the atmosphere, solar energy, and the planet’s magnetic fields. The atmosphere absorbs energy from the sun, recycles water and other chemicals, and works with the electrical and magnetic forces to provide a moderate climate. The atmosphere also protects life on Earth from high energy radiation and the frigid vacuum of space.
Composition of the Atmosphere
In any given volume of air, nitrogen accounts for 78 percent of the atmosphere, while oxygen makes up 21 percent. Argon, carbon dioxide, and traces of other gases make up the remaining one percent. This volume of air also contains some water vapor, varying from zero to about five percent by volume. This small amount of water vapor is responsible for major changes in the weather. [Figure 1]
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| Figure 1. Composition of the atmosphere |
The envelope of gases surrounding the Earth changes from the ground up. Four distinct layers or spheres of the atmosphere have been identified using thermal characteristics (temperature changes), chemical composition, movement, and density. [Figure 2]
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| Figure 2. Layers of the atmosphere |
The first layer, known as the troposphere, extends from 6 to 20 kilometers (km) (4 to 12 miles) over the northern and southern poles and up to 48,000 feet (14.5 km) over the equatorial regions. The vast majority of weather phenomena, including clouds, storms, and temperature variations, occur within this first layer of the atmosphere. Within the troposphere, the average temperature decreases at a rate of approximately 2 °C (3.5 °F) per 1,000 feet of altitude gain, and the pressure decreases at a rate of about one inch per 1,000 feet of altitude gain.
At the top of the troposphere is a boundary known as the tropopause, which traps moisture and the associated weather in the troposphere. The altitude of the tropopause varies with latitude and with the season of the year; therefore, it takes on an elliptical shape rather than a uniform spherical shape. Location of the tropopause is important because it is commonly associated with the location of the jet stream and possible clear air turbulence.
Above the tropopause are three more atmospheric levels. The first is the stratosphere, which extends from the tropopause to a height of about 160,000 feet (50 km). Little weather exists in this layer, and the air remains relatively stable, although certain cloud formations may occasionally extend into it. Above the stratosphere are the mesosphere and thermosphere, which have little influence over weather.
Atmospheric Circulation
As noted earlier, the atmosphere is in constant motion. Certain factors combine to set the atmosphere in motion, but a major factor is the uneven heating of the Earth’s surface. This heating upsets the equilibrium of the atmosphere, creating changes in air movement and atmospheric pressure. The movement of air around the surface of the Earth is called atmospheric circulation.
Heating of the Earth’s surface is accomplished by several processes, but in the simple convection-only model used for this discussion, the Earth is warmed by energy radiating from the sun. The process causes a circular motion that results when warm air rises and is replaced by cooler air.
Warm air rises because heat causes air molecules to spread apart. As the air expands, it becomes less dense and lighter than the surrounding air. As air cools, the molecules pack together more closely, becoming denser and heavier than warm air. As a result, cool, heavy air tends to sink and replace warmer, rising air.
Because the Earth has a curved surface that rotates on a tilted axis while orbiting the sun, the equatorial regions of the Earth receive a greater amount of heat from the sun than the polar regions. The amount of solar energy that heats the Earth depends on the time of year and the latitude of the specific region. All of these factors affect the length of time and the angle at which sunlight strikes the surface.
Solar heating causes higher temperatures in equatorial regions, making the air less dense and causing it to rise. As the warm air flows toward the poles, it cools, becomes denser, and sinks back toward the surface. [Figure 3]
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| Figure 3. Circulation pattern in a static environment |
Atmospheric Pressure
The unequal heating of the Earth’s surface not only modifies air density and creates circulation patterns; it also causes changes in air pressure or the force exerted by the weight of air molecules. Although air molecules are invisible, they still have weight and take up space.
Imagine a sealed column of air that has a footprint of one square inch and is 350 miles high. It would take approximately 14.7 pounds of force to support or lift that column. This represents the air’s weight; if the column is shortened, the pressure exerted at the bottom (and its weight) would be less.
The weight of the shortened column of air at 18,000 feet is approximately 7.4 pounds, or about 50 percent of that at sea level. For instance, if a bathroom scale (calibrated for sea level) were raised to 18,000 feet, the column of air weighing 14.7 pounds at sea level would be 18,000 feet shorter and would weigh approximately 7.3 pounds (50 percent) less than at sea level. [Figure 4]
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| Figure 4. Atmosphere weights |
The actual pressure at a given place and time differs with altitude, temperature, and density of the air. These conditions also affect aircraft performance, particularly during takeoff, climb, and landing.
Quick Review: Atmosphere & Weather Basics
Which atmospheric layer contains the majority of weather, and what are its standard lapse rates?
Why is the location of the tropopause operationally significant to pilots?
What is the primary driving mechanism behind global atmospheric circulation?
How dramatically does atmospheric weight and pressure drop with altitude?
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