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Weather Theory (Part 1) (Private Pilot)


Weather plays main roles in earth’s atmosphere and for aviation and aircraft performance. The term “weather” is the state of the atmosphere at a current time and location to variables in temperature, moisture, wind, visibility, and pressures. It can be also be applied to adverse or destructive atmospheric conditions.

Earth’s atmosphere is made up of mixture of different gases that are allows in motion. The atmosphere is what supports and protects life on earth. It absorbs the suns energy, recycles water along with working with the electro and magnetic fields.

Composition of the Atmosphere:

The Composition of Earth’s atmosphere is made up

of the following molecules:​​

  • Nitrogen — 78 percent

  • Oxygen — 21 percent

  • Other Gases — 1 percent


The first layer of the atmosphere is known as the Troposphere, is between 4 - 12 miles over the North and South poles and up the 48,000 feet over the Equator regions. (The atmosphere is thinner at the poles, and thicker at the equator.)

​​​The Troposphere is the layer that supports life and contains the most weather, clouds, storms and temperatures.  

For every 1,000 feet in altitude the temperature decreases 2 degrees Celsius, and pressure decreases one inch per 1,000 feet in altitude.

There is a boundary at the top of the Troposphere called the Tropopause. The altitude of the Tropopause varies with latitudes and seasons, which takes on an elliptical shape. The Tropopause is associated with jet streams and clear air turbulence.

Above the Troposphere lies the stratosphere which, extends from the Troposphere up to 160,000 feet. In this layer, there is little weather with stable air, and certain types of clouds.


Earth's atmosphere is a constant circulation; the major part is due to the unequal heating of the Earth's surface.  The heating changes air movement and air pressure, this movement of air around the surface is called, atmospheric circulation. 

The heating of the surface is caused by radiation from our Sun. This causes a circular motion, which causes warm air to rise thus replaced by cooler air. 

  • Warm air rises; heat causes the air molecules to spread apart. As warm air expands, it becomes less dense.

  • When air-cools, the molecules become closer and tighter together becoming more dense and heavier than warm air. As cool air becomes heavy, it tends to sink and begins to replace warmer air. 

Since the equator is closer to the Sun, it gets more heat, higher temperatures and less dense air. Unlike the Polar Regions, which are farther away from the Sun resulting in cooler and denser air, which begins to sink toward the surface.

Air has weight:

The unequal heating of the Earth’s surface also causes atmospheric pressure. Air molecules are invisible to the naked eye but these air molecules have weight to them

  • At sea level air weighs 14.7 pounds per 1 square inch.

  • At 18,000 feet, air weighs 7.4 pounds per 1 square inch.


The actual pressure at a given time and place will be different do to altitude, temperature, and density of the air. These atmospheric pressure conditions affect aircraft performance, especially takeoff, rate of climb, and landing performance.


Coriolis force

​The Coriolis force is the force created by the rotation of the earth. We as humans cannot feel this force, but large masses of air or bodies of water an effected. 

In the Northern Hemisphere, the air is deflected to the

right at a curved path, rather than in a straight line.

As does the same goes for the Southern Hemisphere,

with the exception that the air is defected to the left.


The Coriolis force causes the air to separate into three different cells in both hemispheres. In the Northern hemisphere the warm tropical air rises upward from the surface and begins to travel north, then is deflected to the right, due to the Coriolis force, which then the air is now moving eastward. 


This circulation patterns of air become even more complicated caused by changes in seasons, continents, ocean's and frictional forces caused by the topography of the Earth's surface such as; mountains, canyons, valleys etc. Just 2,000 feet above the surface, the wind is beginning to slow down due to friction as well as change directions. The direction of the wind is different from that of the wind within a couple thousand feet above the surface. 


Measurement of Atmosphere Pressure

The standard atmospheric conditions are used for most aircraft instruments and performance charts and data.

  • Standard atmosphere pressure at sea level is 29.92” Hg (1,013.2 millibars (mb).

  • Standard temperature at sea level is 15⁰C (59⁰F).

Weather stations convert barometric pressure to sea level pressure. This is accomplished by adding 1 inch Hg for every 1,000 feet in altitude.

For an example, if a weather station is located at 5,000 feet above sea level and is reading of 24.92”

As altitude increases atmospheric pressure decreases.


Altitude and Atmosphere Pressure

Atmospheric pressure decreases 1 inch with every 1,000 feet increase in altitude. The air becomes less dense at higher altitudes.

As mentioned previously in the beginning of this lesson, temperature decreases 2⁰C with every 1,000 feet gained in altitude.


Winds and Currents

Air always flows from a high pressure to a low pressure. Atmospheric pressures cause convective currents and wind, such as; Coriolis force, friction, and changes in temperature in the surface.

Convective currents are the upward and downward motion of air and wind is the horizontal (side-to-side) motion.


Wind patterns:

Since we are located in the Northern Hemisphere, airflows form a high pressure to a low pressure and is deflected to the right, which cause a clockwise rotation around

high pressures. 

High-pressure systems are of

dry descending air, which is typically good weather.

  • Clockwise motion and anticyclonic circulation.

Low-pressure systems are of ascending or rising air,

which is typically bad weather. These systems bring

cloudiness and precipitation along with bad weather,

which are not uncommon.

  • Counter clockwise motion and cyclonic circulation.


Convective Currents:

Convective currents are the bumpy turbulent air felt when flying at lower altitudes during warm days. This is caused by rocks, sand, barren land, which gives off more heat. Whereas water, tress and vegetation absorbs heat and gives off very little.

Convective currents are more noticeable when flying around landmasses near large bodies of water such as the ocean, large lakes.

The land heats faster than water, causing the air over land to become less dense, this air rises and is replaced with the cooler air from the water.

Pilots should be aware of convective currents near the ground on final approach, which can cause the airplane to balloon and be hard to control.


Effective of Obstructions on Wind:

Obstruction on the ground effect the flow of wind and create problems for pilots. Types of obstructions and man-made structures, are:

  • Hangars

  • Large buildings

  • Mountains

  • Bluffs

  • Canyons

This ground obstructions, depending on the size and the speed of the wind can cause turbulence, which can effect takeoff and landing aircraft performance and be seriously hazardous.

Pilots should be aware during the landing phase. The aircraft may “drop in” due to turbulent air on approach to land, causing the airplane to be too low to the ground.

Greater awareness should be taken in to account when flying in mountainous areas. As the wind flows down the leeward side of mountains.  The stronger the wind greater the downward pressure and turbulence.


Low Level Wind Shear:

Low Level Wind Shear (LLWS) is undetected and dangerous to aircraft; it is a sudden change in wind speed and or wind direction at any altitude.  Wind shear can affect aircraft with strong updrafts or down drafts, and horizontally across the aircraft.

Another type of low-level wind shear and most severe is a microburst. 

  • Microbursts can last anywhere from 5 – 15 minutes

  • Downdrafts up to 6,000 feet per minute, along with headwind loss of 30-90 knots.

  • 1 – 2 miles in diameter

Remember wind shear is undetected and a silent danger to aircraft at any altitude and time. Be alert when flying around thunderstorms and frontal systems.


Wind & Pressure on Surface Weather Maps:

Pilots can gain a great deal of information from surface weather maps, which provide information about weather fronts, high and low pressure areas, surface winds and pressures.

On these surface weather charts, winds that are reported by showing an arrow attached to a circle, which indicates a station. The station circle is the head of the arrow, the arrow points in the direction with wind is from.

The speed of the wind, which are represented by barbs or pennants on the arrow. Each barb or pennant represents a curtain speed in knots.


Pressures are recorded on surface weather charts for each station in millibars (mb).  Isobars are lines of pressure that are drawn on charts.

  • If the Isobar lines are closely spaced, that

indicates a steep pressure and strong winds.

  • If the lines are spaced far apart, the winds are light.          


Isobars also show high and low pressure systems, along with ridges, and troughs on charts.

  • A ridge is an elongated, upward curve of High pressure.

  • A trough is an elongated, downward curve of Low pressure.          



Isobars give information about winds in the first few thousand feet above the surface. The wind direction is different from winds above and the speed is decreased, due to friction along the surface.  At 2,000 to 3,000 feet in altitude, the winds speed up and the direction becomes parallel to the indicated isobars.

The wind at 2,000 feet AGL is from 20⁰ to 40⁰ to the right of surface winds. Wind direction is greater over rough surfaces, such as mountains, valleys, and canyons than over flat surface i.e. plains, fields, etc.



Private Pilot ACS PA.I.C.K3a – PA.I.C.K3I

PHAK Ch.12

AC 00-6

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