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Atmospheric Circulation and Weather. Composition of the Atmosphere. The lower atmosphere is a nearly homogeneous mixture of gases. The most abundant gas in our atmosphere is nitrogen. Air is never completely dry; water vapor, the gaseous form of water, can occupy as much as 4% of its volume.
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Composition of the Atmosphere • The lower atmosphere is a nearly homogeneous mixture of gases. • The most abundant gas in our atmosphere is nitrogen. • Air is never completely dry; water vapor, the gaseous form of water, can occupy as much as 4% of its volume.
Composition of the Atmosphere • Sometimes liquid droplets of water are visible as clouds or fog, but more often the water is simply there, invisible. • The water in our atmosphere has entered from the ground, plants and the sea surface. • The residence time of water vapor in the lower atmosphere is 10 days.
Atmospheric Properties • A 1-square cm column of air extending from sea level to the top of the atmosphere weighs about 1.04 kilogram (2.3 pounds). • A square foot column of air at the same height weighs more than a ton. • The temperature and water vapor content of air is greatly influenced by density.
Atmospheric Properties • Because of molecular movement associated with heat warm air occupies more space than an equal mass of cold air. • In other words, warm air is lighter than cold air. • Humid air is less dense than dry air, because water vapor molecules weigh less than the nitrogen and oxygen molecules they displace.
Atmospheric Properties • Air lifted from near sea level to a higher altitude is subject to less pressure and will expand. • As air expands it cools. • The opposite is also true. Air descending from higher altitudes towards sea level warms as it is compressed
Atmospheric Properties • Warm air can hold more water vapor than cold air. • Water vapor in rising expand, cooling air will often condense into clouds (aggregates of tiny droplets) because the cool air can no longer hold as much water vapor.
Atmospheric Properties • If rising and cooling continue the atmosphere will loose water as precipitation, raindrops or snowflakes. • These rising-expanding-cooling and falling-compressing-heat relationships are important in understanding atmospheric circulation, weather and climate.
Weather and Climate • Weather is the state of the atmosphere at a specific place and time. • Climate is the long-term average of weather in an area. • Both are influenced by the amount of solar radiation an area receives, by local terrain and nearby bodies of water, changing geological and biological conditions as well as other factors.
Weather and Climate • We know that climate has changed in the relatively recent past, the last ice age peaked only 18,000 years ago. • We can not yet predict climate change, the data and models on which to base these predictions are still incomplete.
Weather and Climate • Accurate climate predictions will soon be as critical as accurate weather predictions. • Human induced changes in the quantities of atmospheric carbon dioxide and ozone will lead to climate changes, but we’re not sure what these will be.
Atmospheric Circulation • Air flow over the Earth is in large patterns shaped by variations in solar heating with latitude, seasonal changes and the rotation of the Earth. • The mass movement of air is known as wind. • About half of the radiation directed at the Earth from the sun is absorbed.
Atmospheric Circulation • The amount of radiation reaching the Earth’s surface per minute, called the insolation rate, varies with transparency of the atmosphere, the angle of the sun above the horizon, and the local reflectivity of the surface. • The most important factors that affect solar angle are latitude and season.
Atmospheric Circulation • More sun falls in equatorial regions than strikes the poles. • Light striking to poles comes in at a higher angle of incidence than the close to 90-degrees at the equator. The light striking the poles is also spread over a greater area.
Atmospheric Circulation • At mid-latitudes the Northern Hemisphere receives about three times as much solar energy per day in June as it does in December. • This is due to the 23.5-degree tilt of the Earth’s axis to the plane of the ecliptic.
Atmospheric Circulation • The plane of the ecliptic is the plane in which all the planets orbit around the sun. • It is the inclination of the Earth’s axis that causes the change of season. • As the Earth revolves around the sun, the constant tilt of its rotational axis causes the Northern Hemisphere to lean toward the sun in June but away from it in December.
Atmospheric Circulation • The sun appears higher in the sky in summer and lower in winter. • Most of the sun’s warmth is concentrated between 23.5-degrees north to 23.5-degrees south. • This zone is known as the tropics.
Uneven Solar Heating • The concentration of solar energy near the equator has important effects on the atmosphere. We know that warm air rises and cool air sinks. • The circulation of air in a room with a source of heat on one side and a window on the other side exhibits what we call a convection cell or convection circulation.
Uneven Solar Heating • A similar process occurs over the surface of the earth. • Surface temperatures are higher at the equator than at the poles so the air at the equator gains heat and rises.
Uneven Solar Heating • In an ideal model of air circulation over the surface of the earth we would find air rising at the equator. • This warm air would expand and become less dense. It would rise to high altitudes and turn toward the poles and pile up. • This air would then cool, give off its heat to space, sink down at the poles and return to the equator.
Uneven Solar Heating • But in reality it does not work like that. • The eastward rotation of the Earth deflects the air, water (or any freely moving object) to the right of its path in the Northern Hemisphere and to the left of its path in the Southern Hemisphere.
The Coriolis Effect • This apparent deflection is called the Coriolis effect named for a French Mathematician Gaspard Gustave de Coriolis. • To an Earth bound observer the deflection is caused by the observers frame of reference on the spinning Earth.
The Coriolis Effect • The Coriolis effect increases as one moves to the higher latitudes because the earth is spinning faster at the higher latitudes than at the equator. • For all practical purposes there is no Coriolis effect between 0 and 5 degrees of latitude.
The Coriolis and Atmospheric Circulation • As air rises at the equator, cools, expands and heads toward the north pole. But instead of continuing all the way it is gradually deflected to the right of its direction of motion. • As air rises at the equator, it loses moisture (rainfall) by expansion and cooling.
The Coriolis and Atmospheric Circulation • At about 30-degrees the air firm the equator falls back to the surface still being deflected to the right and produces the North East Trade Wind Belt. • The atmospheric circulation cell form the equator to 30-degrees latitude known as the Hadley Cell.
The Coriolis and Atmospheric Circulation • Between 60-degrees latitude and 90-degrees a similar circulation pattern occurs and this cell is called the Polar Cell. • The Polar Cell produces winds out of the east which are called the Polar Easterlies. • Between the Hadley Cell and the Polar Cell is another cell called the Ferrel Cell which produce the Westerlies between 30-degrees and 60-degrees.
