The Atmosphere Science of Flight – Chapter 1
The Atmosphere Know basic facts and general principles of the atmosphere. 1. Define a list of terms related to the atmosphere. 2. Describe the roles of water and particulate matter. 3. Identify the primary causes of atmospheric motion.
Describing the Atmosphere atmosphere troposphere thermosphere mesosphere stratosphere atmospheric pressure
Atmospheric Elements • Mass of air surrounding the Earth. • Made up of a mixture of gases: • 78% Nitrogen (N2) • 21% Oxygen (O2) • 1% Mixture of other gases
Atmospheric Elements • The Atmosphere: • Absorbs energy from the Sun • Recycles water and other chemicals • Works with the electrical and magnetic forces to provide a moderate temperature • Protects us from high-energy radiation and the frigid vacuum of space. • Four distinct layers have been identified + one “indistinct” layer
Troposphere & Tropopause • Starts at Earth’s surface and extends 10 to 14 kilometers (6 – 8 miles). Temp to -62°F • Lowest, densest part of Earth’s atmosphere in which most weather changes occur. • Tropopause separates the troposphere and the next layer. • Troposphere and tropopause are known as the lower atmosphere. -2° C/1,000’ Lapse rate
Troposphere & Tropopause • Greek “tropein” means ‘turn’ or ‘change’ • You could say: ‘well-mixed’ • What causes weather is the natural tendency for temperature to equalize • Thunderstorms don’t generally climb above troposphere • Almost all weather occurs in this layer..
Stratosphere & Stratopause • Starts just above the tropopause and extends to 50 kilometers (31 miles). • Very dry and less dense compared to troposphere. • The ozone layer is located within the stratosphere. • Separated from the next layer by the stratopause. Temp increases to -3°C by UV rays
Mesosphere & Mesopause • Starts above the stratosphere and extends to 90 kilometers above the Earth’s surface (53 miles). • Temperatures as low as -90o C. (Chemicals in this region are in an excited state) • Mesopause separates the mesosphere from the next layer. Temp falls to -93°C • Mesosphere and stratosphere form the middle atmosphere.
Thermosphere & Thermopause • Starts just above the mesopause and extends to 600 kilometers high (373 miles). • Temperatures reach up to 1,727o C. • Chemical reactions occur much faster. • Known as the upper atmosphere. Temp rises with altitude due to sun • Exosphere extends beyond.
Atmospheric Pressure • The downward pressure exerted by the weight of the overlying atmosphere. • Greatest at sea level and decreases outward from the Earth. • Plays a significant role in the type of weather that occurs.
Atmospheric Pressure • By watching atmospheric pressure changes at a location, forecasters can obtain clues to the expected changes in other weather elements.
Atmospheric Pressure • Low pressure regions usually experience more stormy weather with more cloudiness, higher humidity, and unstable conditions. • High pressure regions are more likely to be associated with fewer clouds, lower humidity, and more stable conditions. • Winds blow because of the differences in air pressure on and above the Earth’s surface.
Water in the Atmosphere • The water content of the atmosphere is almost entirely restricted to the troposphere. • Occasionally, a thunderstorm will produce enough energy to thrust part of its top into the stratosphere. • Water may also be injected into the stratosphere by the engines of high-flying aircraft.
Water in the Atmosphere • In the troposphere, water goes through a cycle from vapor to condensation to precipitation. As it goes through this cycle, it takes on several forms. • Liquid • Solid • Condensation See visualization of hail forming in cloud:
Evaporation • A simple example of evaporation due to temperature is seen with boiling water. • The high temperature causes the water molecules to escape from the liquid surface and rise into the air. • The molecules immediately condense as steam because they saturate the air into which they are escaping. • The steam disappears as it evaporates into the surrounding air because its dew point temperature is raised so that it can contain more water vapor.
Evaporation • Evaporation of water on a global scale takes place in a manner similar to the example, but in a much more subtle manner. • Most of the water vapor in the atmosphere comes from the oceans and other large bodies of water. • The water is heated by a process called solar radiation. • The higher the temperature, the greater the rate of evaporation.
Humidity • Absolute Humidity • The actual amount of water vapor in the air at a given time. • The amount of water vapor the air is able to hold depends on the temperature.
Humidity • Relative Humidity • The amount of water vapor that can still enter the air mass before it becomes saturated. • Expressed by a percentage figure which is the ratio of the amount of water vapor in the air to the maximum amount that the same volume of air could contain at a given temperature and pressure.
Condensation & Precipitation • When part of the water vapor in the air returns to a liquid, it is seen as condensation. • When it returns to solid form, it becomes precipitation.
Condensation and Precipitation • If a cloud’s droplets grow until the buoyancy of the air and any existing updrafts will not support them, they fall as precipitation.
Condensation and Precipitation • If the water vapors that fall are not visible, it is condensation.
Dew Point Temperature • The temperature at or below which water vapor will condense is the key factor in condensation and precipitation. • Dew point temperature does not indicate that there will be clouds, rain, snow, fog, etc. as a result of the condensation; only that some type of condensation will take place if the atmospheric temperature drops to a certain level.
Particulate Matter • Dust and other small particles play an important role in the water cycle. • Without suspended particles in the atmosphere, certain forms of condensation and precipitation would not exist.
Particulate Matter • Water molecules attach themselves to these condensation nuclei when the temperature is right. • Continual accumulation leads to liquid and solid formation of condensation and precipitation. • These particles serve as condensation nuclei for water vapor.
Radiation • The heat energy of the Sun reaches the Earth as radiation or solar energy. • Radiation transfers heat by means of heat waves. • Radiation that reaches the Earth is absorbed by land and water surfaces. • Surface features influence the amount of radiation absorbed by the Earth.
Conduction • The passage of energy through something, particularly heat and electricity. • Heated molecules move more rapidly than cold molecules. • Heat is transferred from fast moving molecules to slow moving molecules until all are moving at the same speed. • A good example of conduction is a stove heating a pan.
Convection • The most efficient method of heating the atmosphere. • Air is first heated by radiation and conduction. • The air absorbs the heat energy. Warm air is forced upward as cold air flows in displacing the warm air.
Convection • Convection currents cause a constant exchange of cold air for warm air until heat is distributed evenly. • Convection also determines the movement of large air masses above the Earth, the action of the winds, rainfall, ocean currents, and the transfer of heat from the interiors of the Sun to its surface.
Advection • The horizontal transfer of a property such as heat, caused by air movement. • When the wind blows, it is simply movement by or within the local air mass. Advection is an important factor in the global circulation of air.
Insolation • The rate at which the Earth’s surface is heated by solar radiation. • The amount received at any point on the Earth’s surface is dependent on the angle that the Sun’s rays make with the horizon, the distance of the Earth from the Sun, and the amount of radiation absorbed by the atmosphere. • Greater in the equatorial zone than anywhere else on the Earth’s surface due to the angle of incidence.
Heat Balance • If there was no balance of heat among the Earth, its atmosphere, and space, the Earth would become increasingly warmer. • Of all the solar radiation arriving at the top of the atmosphere, • 42% is reflected into space by clouds and atmospheric dust; • 15% is absorbed directly into the atmosphere; • 43% reaches the Earth directly.
Heat Balance • Of the 15 percent absorbed directly into the atmosphere, • 4% eventually reaches the Earth as diffused sky radiation. • Thus, a total of about 47% of the incident solar radiation finally reaches the Earth and heats it. • The heating process that tends to maintain the Earth's heat balance is primarily responsible for worldwide weather.
Wind • When air is heated, it rises. This occurs because the heat applied to it has decreased its density to the point where it is lighter in weight than the surrounding air. The surrounding cooler air pushes the lighter, heated air upward.
Wind • When the heated air rises, cooler, higher pressure air flows laterally to fill the lower pressure area created. • This lateral movement is referred to as wind. • Other factors that affect the circulation of the air are: • Gravity • Friction • Centrifugal force
Coriolis Effect • The Earth rotates on its axis in such a way that an observer in space over the North Pole would see the Earth turning in a counterclockwise direction. (Clockwise in the Southern Hemisphere.)
The Pressure Gradient • The atmosphere is a constantly changing landscape of invisible mountains and valleys. • Some of the influences that cause this are: • Irregular distribution of oceans and continents. • Heat-transferring qualities of different Earth surfaces. • Daily temperature variations.
The Pressure Gradient • The high-pressure areas of the atmosphere are the mountains, and the low-pressure areas are the valleys. The wind flows from these high-pressure mountains into the low-pressure valleys.
The Pressure Gradient • The slope of the high-pressure mountain is called the pressure gradient. • On weather maps lines called isobars show the degree of steepness. • Isobars are drawn through points of equal sea-level atmospheric pressure. • Isobars identify five different types of pressure patterns.
Local and Surface Air Movement • The general circulation of air is complicated by the irregular distribution of land and water areas. • Different types of surfaces differ in the rate at which they absorb heat from the Sun and transfer heat to the atmosphere. • In some regions local low-pressure areas form over hot land surfaces and over warmer water surfaces in the winter.
Local and Surface Air Movement • Convection currents are formed along shorelines. • These currents cause the wind to flow from the water over the land during the day. • During the night, they cause the wind to blow from the land toward the water. • Local air circulation of limited scope is caused by variations in the Earth’s surface.
Local and Surface Air Movement • Some surfaces give off or reflect a great amount of heat. • Sand • Rocks • Plowed areas • Barren land • Other surfaces tend to retain heat. • Meadows • Planted fields • Water
Local and Surface Air Movement • Rising air currents are encountered by aircraft flying over sand, rock, and other surfaces that give off considerable heat. • Descending air currents are encountered over surfaces that retain heat.
Local and Surface Air Movement • Moving air flowing around obstructions tends to break into eddies. • On the leeward side of the mountain there are descending air currents. Such conditions cause turbulent air. The stronger the wind, the greater the descending air currents and turbulence. • Aviators flying into the wind toward mountainous terrain should place enough distance between their aircraft and the mountain tops to avoid dangerous descending air currents.