THE ATMOSPHERE
Learning Objectives:
The atmosphere surrounds Planet Earth providing protection from and distributing the Sun's energy. The atmosphere is dynamic and can be thought of as a fluid layer that blankets our planet.
Composition of the atmosphere:
Nitrogen 78%
Oxygen 21%
all other gases 1%
Carbon Dioxide 0.3%
Structure of the Atmosphere:
The atmosphere is divided into 4 layers
1) Troposphere: the bottom layer of the atmosphere, it extends from the surface to approximately 11km. This is where "weather" occurs. As you go up in altitude the air becomes colder. There is lots of vertical and horizontal air movements in this layer.
2) Stratosphere: Relatively stable layer, the air becomes warmer with altitude. The stratosphere extends from 11km to 50km. Vertical air movement is weak but there are strong horizontal movements. The ozone layer is within the stratosphere.
Ozone layer: Incoming solar radiation strikes the atmosphere and splits the oxygen (O2) molecule into two individual oxygen atoms. These individual atoms then react with the remaining O2 molecules to form ozone (O3) molecules. The ozone molecule absorbs ultraviolet radiation from the Sun, essentially shielding our planet from UV radiation. Ultraviolet radiation destroys DNA and life could not exist if the ozone layer were not present. There is some concern about the ozone layer because a hole in the ozone layer opens over Antarctica every year during the Southern Hemisphere spring. Refer to class discussion on ozone problem.
3) Mesosphere: Extending from 50km to 80km the temperature drops with altitude in the mesosphere. The coldest area of the atmosphere is in the Mesosphere at 80km.
4) Thermosphere: This area extends from 80km to 600km and the edges blend with space. The atmosphere warms with altitude.
Ionosphere: located in the upper mesosphere and lower thermosphere, the ionosphere is an area with a high concentration of free ions. The amount of ions present vary with the time of day and the season. The ions form from solar radiation striking oxygen and nitrogen releasing positivly charged ions and free electrons. The ionosphere distorts radio waves either disrupting them or causing them to travel great distances. The ionosphere is also responsible for the Aurora Borealis (Northern Hemisphere) and the Aurora Australis (Southern Hemisphere).
Auroras: The magnetic field that encircles the Earth dips inward at the poles. The magnetic field acts to deflect the solar wind. (The solar wind is a stream of protons and electrons thrown outward from the Sun.) Where the magnetic field dips at the poles the solar wind comes in contact with the ionosphere and the solar wind excites the particles in the ionosphere to higher energy state. This causes the ions to emit radiation in the visible spectrum
(ie glow) forming the Northern and Southern Auroras.
related sites:
Atmospheric Pressure:
The atmosphere is composed of molecules of gas. The gas is subject to gravity and therefore under pressure. The molecules that make up the atmosphere have mass and the surface of the Earth is at the bottom of a very tall column of air. We live under the weight of the entire atmosphere. As you go up in altitude there is less of the atmosphere above you and more below you and the pressure decreases. As the atmosphere is heated and cooled the atmospheric pressure will change. This change in atmospheric pressure causes the gas molecules to move from place to place setting up movements within the atmosphere. The movement of the atmosphere is very important to the distribution of heat in the atmosphere and weather systems.
Solar Radiation:
Solar radiation strikes the atmosphere in many different wavelengths. Solar radiation striking the Earth is absorbed or reflected. The energy budget describes how incoming solar radiation affects the atmosphere and the surface of the Earth.
If incoming solar radiation is equal to 100%:
30% reflects off the top of clouds and the atmosphere 51% emitted by oceans and land surface
19% is absorbed by the atmosphere 45% absorbed by the atmosphere
51% is absorbed by the oceans and the land surface 6% transmitted through atmosphere to space
64% emitted from the atmosphere to space
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100% incoming radiation = 100% outgoing radiation
As long as the incoming radiation (19% and 51%) = outgoing radiation (6% and 64%) then the temperature of the Earth will remain the same. Both the land surface and the atmosphere absorb and emit radiation. The atmosphere absorbs radiation in one wavelength and reemits it in another wavelength. This is an important fact and will determine how the atmosphere affects the surface.
The Earth's surface heats up in the day due to solar radiation. At night the Earth's surface quickly loses this heat energy thru emittance into the atmosphere. The atmosphere absorbs this energy and re-emits it much more slowly. This acts to keep the Earth's surface warm.
Greenhouse gases: these are gases that absorb energy and re-emit it slowly. Greenhouse gases include water vapor, carbon diaoxide and methane. Water vapor is by far the largest greenhouse gas in the atmosphere. Its role can be demonstrated by deserts. In a desert there is little water vapor. The Sun rises and the surface heats quickly through absorbance of solar radiation. At night this energy is quickly re-emitted to the atmosphere. Since there is little water vapor present the heat energy quickly rises to the upper atmosphere. Therfore deserts are hot in the day and cold at night.
In the Norfolk area there is alot of water vapor in the atmosphere. During the day the Earth's surface heats up and this is re-emitted into the atmosphere at night. The energy is absorbed by the water vapor (a greenhouse gas) and re-emitted slowly to warm the Earth's surface through the night. This is why it is often very warm at night in the summer.
The atmosphere acts to moderate the Earth's temperature, if there were no atmosphere the Earth would be like the Moon--very hot on the sunlit side and very cold on the dark side.
Solar radiation strikes the Earth unevenly. The Earth is curved; because of this the equator recieves more solar radiation than the poles. The Earth is also tilted on its axis (23.5 degrees). The tilt of the Earth causes one hemisphere to be tilted toward the Sun at any given time. This tilt causes the seasons. The hemisphere that is tilted towards the Sun is in its summer and the opposite hemisphere is in its winter (seasons are reversed in the Southern Hemisphere.)
On a globe the Arctic Circle, Antarctic Circle, Tropic of Cancer and Tropic of Capricorn refer to the amount of the Sun's radiation recieved on those areas of the Earth due to the tilt of the Earth on its axis. Refer to the lab discussion and illustrations in the lab book. Link to an animation showing radiation recieved by season.
Relative Humidity:
Water exists in three states: vapor, liquid and solid. Temperature determines the state. In the atmosphere water exists in all three states and readily changes states as temperature changes. Warm air supports more water vapor than cold air. (You can think of it as warm air holds more moisture than cold air.) As the atmosphere warms more water changes to the vapor state and the air holds more moisture. When the atmosphere cools not as much water vapor is supported and liquid water condenses out of the air. (The air holds less moisture.)
Relative humidity is the ratio of actual amount water vapor in the air compared to the capacity of the air to support water vapor.
Think of a parcel of air. The temperature of that parcel determines the total amount of water that can be supported as a vapor. Measure the amount of water vapor actually present in that parcel.
________water vapor actually present_____________
total amount of water vapor that COULD be supported X 100 = relative humidity
If the temperature of a parcel increases with no increase in water vapor the relative humidity actually decreases.
The amount of water vapor present in a parcel of air changes throughout the day depending on the temperature of the air.
As an air parcel warms it will absorb more moisture and as it cools moisture will condense out of the parcel.
Dew point: the temperature to which a parcel of air must be cooled for condensation to occur.
Cloud Formation:
Sites related to clouds:
University of Illinois
Plymouth State University
Air parcels rise and sink. Since the atmosphere cools with altitude in the troposphere, as a parcel rises it slowly cools. Cool air holds less moisture than warm air and the water vapor condenses. The water vapor condenses around dust and salt particles forming clouds.
Air parcels rise:
1) Surface Heating: Air near the surface is warmed to due surface heating by solar radiation. The Earth's surface re-emits energy to the air above. Air is heated, rises, cools and water vapor condenses.
2) Topography: Air moving across the Earth's surface encounters a topographic high and moves up and over it. As it moves upward it is cooled and water vapor condenses.
3) Frontal Systems: Air masses of different temperatures meet, the warm air mass moves over top the cooler air mass. As the air mass rises it cools and water vapor condenses.
Types of clouds:
1) Cumulus: puffy, cottony looking. Characterized by vertical growth these clouds can be fair weather or rain producers. Usually found in the mid atmosphere 15,000 - 30,000 ft. (Thunderheads can be much taller.)
2) Cirrus: wispy clouds high in the atmosphere 30,000 ft and above. They are not rain producers.
3) Stratus: layered clouds that are very low and cover the sky. "Cloudy day" clouds, stratus clouds are low and produce rain. Usually associated frontal systems.
Prefixes used with clouds:
Nimbus means rain producing and alto mean higher altitude. A cumulonimbus cloud is a rain producing cumulus cloud (a thundercloud) and an altostratus is a higher stratus type cloud.
Global Wind Patterns:
The uneven heating of the Earth sets up global wind patterns.
Refer to the diagram in your book or this link.
The following is for the Northern Hemisphere but it holds true for the Southern Hemisphere as well.
Air in a band around the equator is heated, it rises and moves laterally (poleward) in the upper atmosphere. As it rises it cools, condenses and rain is abundant in the Tropics. At the surface air moves equatorward to replace the rising air. The surface air moves from north to south but the Coriolis Effect deflects it to the right creating the Trade Winds which blow from the northeast.
Air sinks at about 30 degrees latitude. Sinking air warms, expands and evaporates moisture from the surface. As a result a majority of the World's deserts can be found in a band around 30 N and S latitudes. Surface air moves outward from 30 degrees latitude and moves both equatorward and poleward. The air moving equatorward forms the trade winds. The air that moves poleward is deflected by the Coriolis Effect and forms the Prevailing Westerlies.
Air at the pole sinks and moves equatorward. It is deflected by the Coriolis Effect forming the Polar Easterlies. Air rises at 60 degrees latitudes.
These global air circulation patterns have varying affects on climate.
Equator: air is rising, little surface wind (the doldrums). Lots of rain from this area northward.
Trade Winds: in tropics (varies with season). Climate in these latitudes very stable. Trade winds are dominant but some variation with season in the amount of rain received.
30 degrees latitude: sinking air forms deserts (exception is Gulf Coast of US). Little surface wind (the horse latitudes).
Prevailing westerlies: in the mid latitudes. Climate dominated by frontal systems, the prevailing westerlies steer these systems.
Polar easterlies: in the higher latitudes the global wind patterns have a minimal effect. Climate dominated by the absence/presence of sunlight with the season.
Frontal Systems:
A front is where air masses of differing temperatures meet.
Warm front: warm air mass moving into an area of colder air. The warm air will slide up and over the cold air. As it rises the air cools and condenses causing clouds and rain.
Cold front: cold air mass moves into an area with warm air. The warm air is pushed up and out of the way to make room for the cold air. As the warm air moves up it cools and condenses causing clouds and rain.
Cyclone or low pressure system: rising air moving in a counterclockwise rotation formed when waves in the upper atmosphere disturb the frontal boundary. Air at the surface moves into a low and then upward. Associated with unstable weather, cyclones are responsible for Northeasters and our large snow events.
Anticyclone or high pressure system: sinking air moving in a clockwise rotation. Air at the surface moves out of a high. Anticyclones are associated with dry, stable weather.
Storm Systems:
Thunderstorms:
Thunderstorms are characterised by up and down drafts that can exceed 50mph. Horizontal winds are unpredictable and variable. Thunderstorms from from the lifting of warm moist air.
During the summer daily heating of the Earth's surface builds clouds that can develop into "pop-up" thunderstorms. These are localized and last 15 to 20 minutes.
A fast moving cold front rapidly pushes warm air upward forming a thunderstorm. These storms define a frontal boundary and move quickly constantly forming and reforming as the cold front moves into areas of warmer air.
A supercell thunderstorm is a powerful system usually formed in the spring when cold air from the north meets warm air from the south. These storms have complex structure and can last much longer than the other types of thunderstorms.
As warm air is lifted it cools, condenses forming cloud mass. The warm air that has cooled then sinks rapidly only to reheat and rise again. This pattern of rising and sinking or up and down drafts acts to build cloud mass vertically into large cumulonibus clouds. In powerful storms the cumulonimbus cloud can "punch up" into the stratosphere. Heat is the driving mechanism for storm formation. Eventually the cloud has built to the point that precipitation will occur. Rain cools the updrafts and the storm dissipates.
By definition a thunderstorm contains thunder and lightning. Tunder and lightning ALWAYS occur together, there can NEVER be one without the other.
Lightning: as air moves rapidly up and down in the updrafts and downdrafts electrons are stripped off of water molecules. The electrons and positive ions separate in the thunderhead forming an electrical potential within the cloud. Lightning is a flow of electrons from an area of high potential to an area of low potential. Lightning can move from cloud to cloud, within a cloud or cloud to ground.
Thunder: as a flow of electrons move through a column of air the air is rapidly heated. The heated air expands rapidly forming a shock wave that is heard as thunder.
Hail: hail is often associated with thunderstorms. Water condensing in an updraft freezes into an ice crystal. The ice is then pushed lower in the atmosphere by a downdraft where more water condenses on the ice crystal. As this ice and water moves back up in the atmosphere on an updraft it freezes into a larger ice crystal. As long as the updrafts are strong enough to keep the ice crystal suspended it will continue to move up and down within the cloud growing larger and larger. When it is too heavy to stay suspended in the cloud the ice falls to the Earth as hail.
Tornados: tornados can form in association with a thunderstorm. Powerful up and downdrafts in the thundercloud act to spin air between them much like a person rolling a log in the water. If this area of rotation becomes tilted (usually due to updraft or downdraft interference) it stretches out and drops to the Earth as a tornado. Tornados are areas of VERY low pressure. Winds in the tornado can exceed 200 mph. They are local and are usually less than 1 mile in diameter. The last for minutes but can cause extensive damage.
