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Lessons and Modules

Introduction to Clouds

Atmospheric Pressure

The Earth's atmosphere is divided into four layers that begin at sea level and extend to a height of about 400 km (260 miles). The lowest layer, the troposphere, starts at sea level and reaches a height of 10 km (7 miles). 90% of all the molecules in the Earth's atmosphere are found here. The layer above this, the stratosphere, contains another 9.9% of the atmosphere. The stratopause, the boundary between the mesosphere and stratosphere, has a pressure of 1 mb (1/1000 of standard sea level pressure). This makes the vast distance from 50 km to 400 km, containing the mesophere and thermosphere, almost a vacuum with only .1% of the atmosphere.

Layers of the Atmosphere

Red columns indicate atmospheric pressure.

The troposphere is the layer of the atmosphere around you that effects your daily life. Almost all weather phenomena occurs here. Most clouds are created here, and it is here that storms unleash their fury through rain, snow, and hail. The greatest amount of air pressure is in this layer because most of the air molecules are in this area. Each red column in the image below represents an equal volume of space with a decrease in the amount of air pressure as they get higher up in atmosphere. This pressure is the mass of air being brought down by gravity causing an increase in weight exerted on you as you descend lower into the atmosphere. Sea level is the mean boundary level between the sea and the atmosphere. At this point the sea level pressure (the weight the atmosphere puts on any point on Earth that has the level the sea does) is 1000 mb.


Atmospheric pressure in the troposphere.

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