What is the atmosphere? • The atmosphere is an envelope of gases that surrounds the Earth. • Although we think of air as just one gas, it is actually a mixture of several different gases. • It can be up to 10000 km thick, although most of the gas particles are found in the 30 km closest to the Earth's surface.
What does the atmosphere do? • Protects the Earth from excess ultraviolet (UV) rays. • Protects the Earth from meteoroids. • Reduces temperature differences; maintains a fairly regular average temperature. • Without the atmosphere, the daytime temperature would be 80°, dropping to -140° at night.
The atmosphere is divided into five layers: • Troposphere • Stratosphere • Mesosphere • Thermosphere • Exosphere
Troposphere • Most important layer for living things, since it contains 80% of all breathable air. • Contains almost all water vapour in the atmosphere, so this is where all of our weather occurs. • Average temperature drops 6° with every kilometer of altitude
Stratosphere • The ozone layer is found here, which blocks most ultraviolet light coming from the Sun. • Airplanes usually fly through the stratosphere, since the air is thinner so there is less friction.
Mesosphere • Very little air at this altitude • Protects the Earth from meteoroids, since most of them burn up in the mesosphere.
Thermosphere • Makes up about half the atmosphere. • Contains the ionosphere, which is made up of electrically charged particles. • The ionosphere allows us to bounce radio waves from one end of the Earth to another • Meteoroids that burn up in the thermosphere are called shooting stars. • Also the home of the polar auroras.
Shooting Stars • Note: these aren’t stars… they’re meteoroids!
Exosphere • This layer is practically empty; very few air particles can be found here. • Satellites sent into space travel are found in this layer.
Atmospheric Pressure • Atmospheric pressure is the pressure of the air in the atmosphere. • Air exerts pressure as air particles collide with each other: the more collisions, the higher the pressure. • Pressure is measured in pascals (Pa). • The average pressure at sea level is 101.3 kPa. (or 101 300 Pa)
Pressure Factors • When the number of particles increase, they collide more often and the pressure increases. • When the number of particles decrease, they collide less often and the pressure decreases.
Pressure Factors • When the air warms, particles move more rapidly, and therefore collide more often. The pressure increases. • When the air cools, particles slow down and therefore collide less often. The pressure decreases.
Pressure + Temperature • Warm air has a lower density than cool air. • Hence, warm air is lighter than cold air and tends to rise.
Particles try to move away from high-pressure areas (where there are lots of particles) to low-pressure areas (where there are few particles). • This movement gives us winds.
Atmospheric Circulation • Air is warmed at the Equator, heads towards the poles and cools down and descends. • At the same time, the cool polar air moves back towards the Equator. • This creates our weather!
Recall… • Hot air rises because it is less dense than cold air. • Pressure increases as temperature increases. • Pressure increases as the number of particles increases. • Particles are always trying to move from areas of high pressure to areas of low pressure wind
Air Mass • An air mass is a large expanse of the atmosphere with relatively similar temperature and humidity. When two air masses meet, they do not mix together: the lighter warm air rises above the denser cold air.
The line where these two air masses meet is called a front. • At the front, temperature, wind strength and direction and humidity change rapidly: this causes much of our weather. • Fronts can be either warm or cold.
Warm Front • Form when a mass of warm air moves towards a mass of cold air. • Warm fronts often bring cloudy weather and rain showers. Warm fronts are indicated by a line, usually red, with a row of semi-circles.
Cold Front • Form when a mass of cold air meets a mass of warm air. • The warm air rises rapidly, and then cools. This rapid condensation often produces strong winds and heavy rain. Cold fronts are indicated by a line, usually blue, with a row of triangles.
Fronts are generated by the horizontal movement of air. • However, air is also moving vertically. • The vertical movement causes anticyclones and depressions.
Anticyclone • When air cools, particles do not collide as often and the pressure drops. • This causes particles to move closer together, increasing the density of the air. • This air mass becomes heavier and sinks towards the ground, creating an area of high pressure. • This area is called an anticyclone, and is designated by the letter H.
Depression • On the other hand, when air warms up, its density decreases. • This causes the air to rise, leaving an empty space beneath it. • This space becomes an area of low pressure called a depression, designated by the letter L.
Anticyclones and depressions are directly linked with weather: • Anticyclones lead to clear skies and stable weather. • Depressions encourages cloud formation, which leads to precipitation. • Strong depressions create huge cloud systems, strong winds, and lots of precipitation: we know these as cyclones, hurricanes, or typhoons.