Weather

Layers of the Earth's atmosphere

 

Though we live on the surface of the Earth, we actually live at the bottom of an ocean of air. Dynamic layers of air interact with the Earth's surface and the Sun's energy to produce the phenomenon of weather. The atmosphere is classified into layers based upon the characteristics each layer exhibits. Most weather occurs in the troposphere and most flying occurs in the troposphere and stratosphere. The atmosphere is comprised of air. Air is a mixture of gases. It is about 80 percent nitrogen and 20 percent oxygen. Water vapor and many other gases constitute the remainder of the gas mixture.

Air is made up of matter and has weight. Since air is gaseous, it is compressible. This means that the air pressure nearer the surface of the Earth is greater than the air pressure in the stratosphere. Air exerts pressure on everyone and everything. At the Earth's surface the pressure is 14.7 pounds per square inch. That does not sound like much, but it means the air pressure per square foot is 2116.8 pounds. Increase the air pressure and the air's density is increased. Because of air's compressibility flight conditions will vary depending upon the altitude. This is due to the air density. More molecules for a given volume of air will generate greater lift with less thrust. Fewer molecules for a given volume of air will require greater thrust to generate the same lift. Air density decreases with an increase in altitude and this affects people physiologically. Decrease the air pressure and the oxygen pressure is also decreased.

The rate at which our lungs absorb oxygen depends on the partial pressure exerted by the amount of oxygen in the air. Since our atmosphere is about 1/5 oxygen, the oxygen pressure at any given altitude will be about 1/5. Under normal conditions our lungs function under 3 pounds per square inch of oxygen pressure. As an airplane climbs higher into the troposphere, it will encounter less oxygen for a given volume of air. Without supplemental oxygen the people on board such a flight will suffer from hypoxia, a deficiency in oxygen. The symptoms of hypoxia are a feeling of acute exhaustion with an immediate impairment in vision and judgment resulting in unconsciousness and death if the proper amount of oxygen is not soon administered. Prolonged flights at or above 10,000 feet and even short flights above 12,000 should use auxiliary oxygen.

The ocean of air we live in can be calm, delightfully warm and pleasant or it can be turbulent and rainy like a thunderstorm, hurricane or tornado. The air temperature varies from below -100 Celsius to above 1500 Celsius (-150 Fahrenheit to 2700 Fahrenheit). These variations are caused by the uneven heating of the Earth's surface by the sun's energy as well as how the Earth reacts to this energy. The characteristics of a substance (for example water or land) will affect the amount of heat absorbed or released by that substance. Let's say we have a land surface and water surface of equal temperature and we apply an equal amount of heat to each. The land surface will become hotter at a much faster rate than the water surface. The opposite is true when both substances release the same amount of heat. Under equal heat loss the land will become colder at a faster rate than the water.

Click on the sun button below for a demo of land vs. water and heat absorption.

ATMOSPHERE

The differences in the amount of solar energy received by the various regions of the Earth throughout a day or year cause temperature variations that power our dynamic atmosphere. There are 5 temperature variations that need to be considered by pilots whenever planning a flight:

Solar radiation effect on earth temperature day versus night.

 

Diurnal variation is the change in air temperature that occurs from day to night brought about by the Earth's rotation. A unique exchange takes place on the Earth's surface in regards to solar and terrestrial (from or on Earth) radiation. The sun gives off energy to the Earth in the form of solar radiation. Fifty-five percent of the solar radiation received by the Earth and its atmosphere is reflected while the remaining 45% is absorbed and converted into heat. The Earth itself gives off radiation that is referred to as terrestrial radiation. The exchange is worldwide and maintains a delicate balance in the Earth's atmospheric temperature.

Reflection and absorption of solar radiation.  45% absorbed by earth, 55% reflected away from earth

 

That is, the average amount of heat gained through solar radiation is roughly equal to the amount of heat lost through terrestrial radiation. This unique exchange keeps the Earth's atmosphere from becoming progressively hotter or colder. During the day, terrestrial radiation is exceeded by the solar radiation and the Earth's surface becomes warmer. At night, the part of the Earth facing away from the sun receives no solar radiation. Despite the continuance of terrestrial radiation, the Earth's surface cools. The cooling of the Earth's surface continues until about 1 hour after sunrise. Shortly after sunrise, the solar radiation of the next day exceeds that of the terrestrial radiation, and the temperature increases. The continued cooling after sunrise can cause the formation of fog shortly after the sun is above the horizon. Pilots need to keep this in mind when planning flights with an early morning departure. Such morning ground fog can delay the departure time due to limited visibility.

Seasonal variations as the earth moves around the sun

 

Seasonal variation occurs due to the Earth's approximate 23.50 degree tilt and its position relative to the sun during its revolution. The hemisphere that receives more direct rays of sunlight will have warmer temperatures than the opposite hemisphere at that same time of year. Knowing the average daily temperature of a region from which or to which you are flying will assist in your flight planning calculations.

Latitudinal variation between the sun and Earth

 

Along with the seasonal variations are the latitudinal variations. The shape of the Earth directly affects the amount of solar radiation received during certain segments of its revolution around the sun. Equatorial regions receive more direct rays of the sun, thus more solar radiation. Moving farther south or north of the equator will change the angle at which the rays strike the Earth, thus decreasing the amount of solar radiation received at that latitude.

Remembering that water absorbs and radiates energy with temperature changes that are less than land, a pilot must look closely at the terrain over which the flight will progress, especially if flying at lower altitudes. The topographical variations are influenced also by diurnal and seasonal variations and can cause a change in wind direction or wind strength. In general, the partial list below lists some of the general topographical variations:


Warm air versus cold air

Warm Air - Not as many molecules

Cold Air - More molecules

 

A change in air temperature can cause a change in air density. Air density changes will affect the performance of an aircraft. Without changing the thrust force, decreasing air density will lead to fewer molecules in the air. This will decrease the lift force as fewer molecules are available to generate lift. Increasing the amount of molecules in the air will increase lift at the same given rate of speed. A change in air temperature will also create local winds, which if not accounted for in the flight calculations will push a flight off course.

Altitudinal variations need to also be monitored by the pilot before and during flight. Normally in the troposphere temperature decreases as the altitude increases. This is known as the lapse rate. The average temperature decrease (or average lapse rate) in the troposphere is usually given as 2.0 Celsius per 1,000 feet. Remember this is just an average! There are times when the temperature actually increases with height. This increase in temperature with an increase in altitude is known as an inversion. That is, the lapse rate is inverted or moves opposite to what is expected. Inversions can occur at any altitude if conditions are conducive to their development. See the two examples below for a description of such conditions.

Heat rises from the ground at night

 

Ground-based inversion: occurs near the ground on cold, clear nights. Since the ground radiates and cools much faster than the air above it, air in contact to the ground becomes cold while just a few hundred feet above it that temperature has changed very little. This can trap fog or smoke close to the ground and decrease visibility.

Inversion aloft

Inversion aloft: Occurs a little higher in the atmosphere than ground-based inversions. A current of warm air overruns a large patch of cold air trapping it next to the Earth's surface. If rain clouds in the warm air current drop rain, it will pass through the colder air and freeze. This can develop into icing on an aircraft's wings drastically reducing the aircraft's lift force.

Diagram of Earths Atmosphere Layers

Temperature affects an aircraft's performance and is perhaps most crucial when making altimetry readings. The altimeter is basically an aneroid barometer that is graduated to use increments of height. The standard used for graduating the altimeter is that of standard atmosphere. Standard atmosphere was developed by engineers and meteorologists who needed a fixed standard with which they could reference for aircraft performance and weather, respectively. They arrived at standard atmosphere by averaging conditions throughout the atmosphere for all latitudes, seasons and altitudes. This gave them specific sea-level temperature, pressure and rates of change of temperature and pressure with height. In this hypothetical atmosphere, pressure falls at a fixed rate upward through the atmosphere unlike the actual atmosphere. Think of the standard atmosphere as a measuring stick to which pilots compare their altitude based upon other factors (density, pressure and temperature).

So, why is a standard atmosphere necessary? The altimeter measures altitude (that is vertical distance above a level plane, such as sea level), so at any time a pilot should know the aircraft's distance from sea level, right? The airplane's altimeter measures the atmospheric pressure at flight level. In other words it is measuring the weight of the air pressing down on it from above. What the altimeter cannot measure is the air density and the air temperature, both of which affect the flight characteristics of the aircraft. An airplane performs differently in different temperature and air density situations. By establishing a standard atmosphere, pilots can compare their airplanes performance under certain conditions to this standard. Pilots can then make adjustments accordingly to their instruments and flight plan.

Look at the bar graph below for an example of how temperature affects the reading of true altitude. According to standard atmosphere an airplane flies at 10,000 feet given a standard sea level pressure of 29.92 inches of mercury or 1013.2 millibars. Let's take 3 aircraft, each flying atop a measurable column of air. The air pressure at the top and bottom of each column of air is equal. Now let's change the temperature of the columns making one warmer than the standard and the other column colder than the standard. When the air is warmer than the standard, the altimeter reads lower than the airplane's true altitude. When the air is colder than the standard, the altimeter reads higher than the airplane's true altitude.

How temperature affects altitude.

According to standard atmosphere, an airplane flies at 3,000 feet given a standard sea level pressure of 29.92 inches of mercury or 1013.2 millibars. Click the links above to see the relationship.


To account for the effects of pressure, density and temperature on an aircraft's altitude, pilots look at more than one kind of altitude reading. They actually keep track of 5 different altitudes.

Pilots need to bear in mind 5 different types of altitude:

Prior to takeoff, each pilot sets the aircraft's altimeter with the correct altimeter setting of the airport from which the pilot will depart. This setting is given by a controller in the local control tower who also gives the indicated altitude. Along the route the pilot continues to adjust the barometric altimeter setting according to radio reports or controllers contacted en route. From this setting and other data the 5 different altitude readings are derived.


So how do temperature, pressure and density affect an aircraft and its performance?

Temperature in relation to altitude

The effects of temperature changes on an aircraft's altitude:

  • When air is warmer than average the airplane will be higher than the altimeter indicates
  • When air is colder than average the airplane will be lower than the altimeter indicates
  • When temperature lowers en route, the airplane is lower than the altimeter indicates
  • When temperature rises en route, the airplane is higher than the altimeter indicates

Pilots can determine the correct pressure altitude from the on-board altimeter by setting the altimeter at the standard altitude reading of 29.92 inches (of mercury at sea level). The altimeter will then indicate the pressure altitude at which the aircraft is flying.

Lower pressure means lower altitude

The effects of pressure changes on an aircraft's altitude.

  • Flying from a high pressure area to a low pressure area without adjusting the altimeter while maintaining a constant indicated altitude would result in a loss of true altitude
  • Flying from a low pressure area to a high pressure area without adjusting the altimeter while maintaining a constant indicated altitude would result in a gain of true altitude

Density altitude is perhaps the most critical to an airplane's performance during takeoff and landing. Results can be disastrous if the density altitude is incorrectly computed. Density altitude is a comparison between the air density at your aircraft's current altitude to the standard atmosphere where the air density is the same. Temperature, pressure and humidity determine air density. Pilots differentiate between high-density altitude and low-density altitude in terms of the performance of an airplane. Let's say that at an airplane's present flight location the day is hot. That means that the air has become thinner (fewer molecules in the air). When that hot location is compared to the standard atmosphere its density is the same as if the aircraft were located at a much higher altitude. That means that the airplane at its current location will act as if it is flying through air that is at a higher altitude. That means the airplane is flying in high-density altitude conditions.

Now let's say that the present aircraft location is in very cold air. The air has now become heavier than before (more molecules in the air). When that cold location is compared to the standard atmosphere its density is the same as if the aircraft were located at a much lower altitude. That means that the airplane at its current flight location will act as if it is flying through air that is at a lower altitude or low-density altitude conditions. It is crucial for a pilot to know the density altitude of the airport at which takeoff and landings are planned. The pilot also needs to know the runway's length and the height of structures, trees and land immediately following the runway, so as to ascertain clearance at the climb rate. Knowing the performance specifications of one's airplane is also important. After computing all the necessary altitudes, the pilot needs to know if the airplane can perform safely under all these conditions.

Here are some of the effects of density changes on an aircraft's performance:

Flight Conditions

Altitude Conditions

Aircraft performance

Characteristics

High elevations, low atmospheric pressure, high temperatures, high humidity,

High density altitude conditions

Reduction in aircraft performance

- Engine taking in less air so power is reduced;
- Propellers and jet engines have less air to move so thrust is reduced;
- Less molecules in the air, less force on the wings results in reduced lift;
- Reduced thrust and lift means more takeoff runway length needed and more clearance area at the runway’s end needed because of a reduced climb rate.

Lower elevations, high atmospheric pressure, low temperatures, and low humidity are more indicative of low density altitude

Low-density altitude conditions

Increase in aircraft performance

- Greater thrust than normal due to a greater number of molecules in the air with which propellers and jet engines can interact;
- Greater lift force as heavier air exerts more force on the wings;
- Faster speed and faster climb rate as thrust and lift are increased.