Understanding the Atmosphere
The earth is a sphere surrounded by a fluid 'blanket' of gases and
pollutants. This blanket is held to the earth by gravity and has mass, and
therefore weight and has constant composition up to a height of approximately
85km. Above that height satellite drag has detected the atmosphere out past
2000km but as can be expected the atmosphere is very tenuous at those great
heights. The entire column of the atmosphere weighs approximately 1.04 kg for
a 1 square cm column at the surface (on average). The density of the
atmosphere on average is 800 times less than that of water. The mass of the
air in a bedroom would be about 75kg.
The atmosphere is made up of
molecules and has the properties of being mobile, occupying space and as
mentioned above, having weight. Air does not occupy a fixed space due to the
great freedom of motion of it's molecules. Air can be compressed or expanded
almost limitlessly. Air that has been compressed weighs more than expanded air
due to it's greater density. Due to air's compressibility a full 1/2 of the
atmosphere's weight is contained below 5.5km above the earth's surface. As you
climb through the atmosphere the air becomes less dense and lighter than the
air below.
The atmosphere is a mixture of dry air and water vapour.
This dry air is a mixture of gases and the amount by volume of each gas is
relatively constant up to 85km. The gases most in abundance in the atmosphere
are Nitrogen (78%), Oxygen (21%), Argon (1%) and Carbon Dioxide (.3%).
Everything else is in trace amounts.In addition to the gas molecules, the
atmosphere contains a considerable number of impurities and various
microscopic particles held in suspension.
Water Vapour
Water vapour in the
atmosphere is the most important constituent weather-wise. This is because
water has the ability to change state within the limits of the temperature,
pressure and density found in the atmosphere. Water vapour can become opaque
(clouds and fog) and can precipitate out of the atmosphere. The amount of
water vapour in the atmosphere is not constant, but varies between 0 and 4%
with an average being 1% of the atmosphere's composition.
Water
vapour is concentrated in the lower levels of the atmosphere and is
distributed by turbulent mixing. The efficiency of this mixing decreases
rapidly at high levels and water vapour is seldom encountered above 12km. The
maximum amount of water vapour that can be held by the atmosphere is dictated
by it's temperature. Since air decreases in temperature a high levels the
amount that the atmosphere can hold also decreases.
Impurities
The solid particulate
impurities in the atmosphere; dust, smoke, etc. play a very large role in
weather. Without these condensation nuclei water vapour would have a much
harder time accumulating into droplets the size of which are needed to make
cloud or precipitation. This is most evident in very cold climates where ice
fog builds up around towns where there is a lot of particulate pollution from
vehicle exhausts and behind aircraft at high altitudes (contrails).
The water vapour in the atmosphere moderates the temperatures in the lower
layers. Radiation incoming from the sun comes in the form of short wavelength
which heats the ground. At night, the ground re-radiates long wavelength
radiation which is reflected back by clouds and warms the lower levels of the
atmosphere. This is why clear nights are cold and cloudy nights milder.
Understanding Meteorological
Scales
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