Composition • The atmosphere is composed or made of certain substances that are gases at “normal” temperatures. • Nitrogen: 78% • Oxygen: 21% • Carbon Dioxide: <1% • Argon: <1% • Water vapor: <1%
Atmosphere layers • Thickness: since the Earth is spinning, it bulges slightly around the equator. • The atmosphere is thus thicker at the equator where it bulges and thinner at the poles.
Troposphere • The troposphere stretches from the surface to up to 20 kms in the air. (About the distance from here to Orland Park) • Almost all air movement occurs in the troposphere. • All weather occurs in the troposphere. • The troposphere is the warmest and densest layer, but cools near the top.
Stratosphere • The stratosphere extends from 20 to 50 km in the air. • The stratosphere contains a layer of ozone (O3) that absorbs ultraviolet light before it can reach Earth’s surface. • The stratosphere warms near its top because of the energy the ozone layer absorbs. • As altitude increases, air pressure continues to decrease.
Mesosphere • Temperatures continue to drop as you rise through the mesosphere. • Falling meteors experience enough friction in the mesosphere that they heat up to become glowing hot: falling stars. • Most meteorites burn up completely as they fall. • Ice clouds are sometimes found in this layer.
Thermosphere • In this layer, temperatures begin to rise again. As the atmosphere gets thinner (density and pressure decrease) the remaining particles absorb so much energy from the sun that they are very hot, if rarely found. • This is where the International Space Station and most satellites are found.
Exosphere • The exosphere is what we would consider “outer space.” • Here, gases are so thin and spread out, that the temperature, pressure and density are basically zero. • In the exosphere, there is so little matter to absorb electromagnetic energy that electromagnetic waves can go on forever.
Atmosphere Conditions • Temperature increases with direct sunlight and longer days. • Temperature: average temperature increases closer to the equator, and decreases closer to the poles.
Atmosphere Conditions • Humidity: humidity measures how much water vapor (water in gas phase) is in the air. High humidity means there is lots of water vapor. • Humidity can increase as temperature increases.
Atmospheric Conditions • Pressure: Pressure is the force on an area. In the atmosphere, that force is caused by gravity pulling a column of air towards the planet. • The weight of that air causes pressure. • Pressure decreases with altitude, because there is less air pressing down from above.
The water cycle • Evaporation – As liquid or solid water is heated, the fastest moving molecules escape as gas: water vapor. • Evaporation rates increase as temperature increases. • Evaporated water moves throughout the atmosphere. • Evaporated water is relatively fresh or pure water.
The water cycle • Condensation: the process where water vapor cools and forms liquid or solid drops. • As evaporated water (water vapor) moves into the upper troposphere, the temperature decreases as altitude increases. • Water vapor condenses around ice crystals and dust to form droplets.
The water cycle • Precipitation: this is when water droplets or crystals become large enough that the air can no longer hold them aloft, and they fall to the ground as rain, snow, hail or sleet. • Precipitation is fresh water.
The water cycle • Ground water: Liquid precipitation that meets the ground and soaks in gets filtered as it passes through layers of soil and rock to become groundwater. • Runoff: water that cannot soak into the ground moves along the surface as runoff. This water joins together into creeks, stream, rivers, lakes, seas and finally oceans.
Fresh Water • Fresh water: only 1% of the water on earth is fresh. The rest is all salt water. • Fresh water is found trapped in ice, snow, glaciers, streams, rivers and lakes. • As groundwater filters through soil and rock, salts and minerals dissolve into it, forming salt water.
Brackish water • As fresh water in rivers flows into the oceans and seas, it mixes to form partially salty water called brackish water. • Sedimentation, evaporation and organic processes all work to keep ocean water at the same levels of saltiness.
The air cycle • As visible and ultraviolet radiation is absorbed by land or water, it heats up, and reflects infrared radiation. • The land or water then heats nearby air through conduction. • As the air near the ground warms, its density decreases, and the air rises as a convection current.
Greenhouse Effect • As infrared radiation reflects from the ground, it can go back into the exosphere. • Certain substances can absorb infrared radiation, converting the energy into molecular motion, also known as temperature increase. • If present in the atmosphere, these are called greenhouse gases (Methane, carbon dioxide, water vapor), although glass can work similarly.
Wind • The movement of air as a part of convection currents causes wind. There are three major wind belts: • The trade winds blow out from the equator from the east between the equator and the tropics. (Hadley Cells) • The Prevailing Westerlies blow in towards the equator from the west between the tropics and the arctic circles. (Mid-latitude/Ferrel Cells) • The Polar Easterlies blow away from the equator from the east between the arctic circles and the poles. (Polar Cells)
The Coriolis Effect • Winds blow directly away from high temperature areas at the equator. • Because the Earth is rotating towards the east (counterclockwise when viewed from above the North Pole,) winds appear to move in relation to the ground. • They move West as they move away from the equator, and East as they move away from the equator. • This causes winds in the northern hemisphere to move counterclockwise, and those in the southern hemisphere to move clockwise.
Winds • Winds always blow from high pressure (cold, dense areas of air) to low pressure (hot, low density areas of air.) • Differences in temperature cause differences in pressure, which cause winds.
Land and Ocean Winds • Land heats and cools quickly, and water heats and cools slowly. • During the day, land heats up, and heats the air above it. That air floats upwards, and colder air from the ocean blows in as an ocean breeze to replace it. • During the night, land cools off, but the ocean stays warmer, heating the air above it. That air floats upwards, and colder air from the land blows in as a land breeze to replace it.
Jet streams • Air near the top of the troposphere moves from West to East at high speed in currents called jet streams. • Jet streams can carry cold polar air towards the equator, creating storm fronts.
Air masses • Temperature differences can form differences in pressure and density. • Cold air has high pressure, and is denser. • Warm air has low pressure, and is less dense. • Bodies of moving air with differences in density do not easily mix.
Air Masses • Tropical masses: warm, with low pressure. • Polar masses: cold, with high pressure. • Maritime masses: forms over oceans, with high humidity. • Continental masses: form over continents, and are dry. • Air masses usually move in the same direction as the wind belt they are in.
Air Fronts • Weather changes as conditions in the atmosphere change with moving air masses.
Cold fronts • A moving mass of colder air forces its way under the less dense warm air as it moves. • This forces the warm air higher in altitude, cooling it. The water vapor in the warm air can then condense to form storm clouds, possibly causing precipitation. • This warm air moves in the opposite direction as the cold front. • Cold fronts are sometimes called storm fronts, and can move quickly to cause dramatic changes in weather.
Warm Fronts • A moving mass of warm air can collide with a cold mass, sliding up on top of it. • The warm air usually is moving in the same direction as the cold air. • As the warm air increases in altitude, it cools. Condensation forms clouds and sometimes precipitation.
Stationary Fronts • If neither of two air masses is moving with enough force to slide over or under the other, a stationary front forms. • At the boundary, warm air cools and condensation occurs, forming clouds and precipitation. Ongoing storms can result.
Occluded fronts • An occluded front forms when two cold air masses move together with a warm air mass between them. • The warm mass is forced higher in altitude, and cools to form clouds and storms.
Tornadoes • When cold air masses slide up atop warm masses, a vortex can form as they switch positions, resulting in a small rotating tube of air that. • High wind speeds, short durations
Hurricanes • As air passes over warm ocean water, it picks up water vapor and heat. • This humid, warm air rises and moves away from the equator in a curve because of the Coriolis Effect. • As it rises, it cools. Condensation and then precipitation forms. This rainfall warms the air, causing more evaporation.
Hurricanes • The constant evaporation of water and heating of air creates an ongoing current of air in a giant circle, curving to the east as it moves away from the equator and to the west as it cools and returns to the equator. • Precipitation releases more energy into the system as rain warms air as it falls, accelerating the system further until hurricane strength winds result.
Weather maps • Weather maps represent cold fronts with blue lines with semicircles. • Warm fronts are represented with red lines with triangles.