The Structure of the Atmosphere Weather Whether the weather be fine, Or whether the weather be not, Whether the weather be cold, Or whether the weather be hot, We'll whether the weather Whatever the weather Whether we like it or not.

(How many of those can you remember?) What’s it made of? The atmosphere is a mixture of transparent gases held to the Earth by its gravitational force. It consists of mainly nitrogen (78.09%) and oxygen (20.95%) by volume. Other gases include argon, carbon dioxide, water vapour containing hydrogen, neon, helium, krypton, xenon, ozone, methane and radon. (How many of those can you remember?)

The Upper Atmosphere By international convention the top of the upper atmosphere is assumed to be at 1000km, but due to gravity and compression, most of the atmosphere is concentrated near to the earth’s surface. About 50% lies within 5.6km of the surface and 99% within 40km. You’ll be ready for University Challenge after this ...

The Troposphere NB: This is the important bit! Most of our climate and weather processes operate within 16-17 km of the earth’s surface in this lower atmospheric zone. In the troposphere temperatures generally decrease with height (averaging 6.5˚C per km). The top of this layer is marked by a boundary called the tropopause where temperatures remain fairly constant. This occurs at a height of about 8 km at the poles and 17 km in the tropics and it can vary with seasonal changes in climate.

The Tropopause The tropopause acts as a temperature inversion forming an effective ceiling to any convection in the troposphere and so provides an upper limit to the earth’s weather systems. Within the troposphere vertical convection currents disturb the atmosphere and masses of air flow horizontally from one latitude to another. These movements provide a background to the study of climate.

The Stratosphere The stratosphere is the next layer of the atmosphere extending to about 50km above the Earth’s surface and within this layer temperatures actually increase with height. Within this cloud and dust free layer, ozone absorbs and filters out ultraviolet radiation. Warming is greater over the polar regions than in the tropical latitudes and these temperature differences lead to strong horizontal air movements at great heights. Global warming has been linked to recent changes in the stratosphere, brought about by human activity. The upper limit of the stratosphere is marked by the stratopause.

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Mesosphere and Thermosphere The final two layers of atmosphere are shown on this diagram. Within the mesosphere temperatures again decrease with altitude. However, in the atmosphere’s most distant layer, the thermosphere, temperatures start to rise again at a constant rate up to as much as 1500ºC.

The Atmospheric Heat Budget The earth and its atmosphere receive heat energy from the sun. The atmospheric heat budget of the Earth depends on the balance between incoming solar radiation (insolation) and outgoing radiation from the planet . Geological records show that, on average, the energy budget has remained constant over the last few thousand years. This means that the incoming and outgoing energies must be equal. However, there is evidence to suggest that global warming has occurred over recent decades.

The Heat Budget continued ... The Earth receives energy from the sun as incoming radiation. Some of this is lost on passing through the atmosphere but overall the surface has a net gain of energy. The only places where there is a deficit are the polar regions where only about 24% of the incoming solar radiation reaches the surface of the Earth, because of absorption, reflection and scattering. The atmosphere, in contrast, has a net deficit of energy. To compensate for this difference, heat is transferred from the surface of the Earth to the atmosphere by radiation, conduction and by the release of latent heat.

More Heat Budget stuff ... There are variations in energy and heat between different latitudes. Low latitudes have a net surplus of energy, due mainly to their relative proximity to the sun. The high latitudes (polewards 40N and 40S) have a net deficit. Theoretically, this differential heating should result in the equatorial regions being much hotter and the poles much colder than they are. Since the poles are not becoming colder, and the equatorial regions are not becoming hotter, heat must be being transferred between the two. This occurs by means of air movement (winds), and water movement (ocean currents).

Planetary Surface Winds Wind is the horizontal movement of air on the Earth’s surface. Winds result from differences in air pressure and they always blow from high to low pressure. Pressure differences occur spatially because of global and local variations in temperature. When the air temperature of a place increases, the air in that area expands and rises, thus reducing air pressure. Conversely, when the temperature falls, the air becomes denser, it sinks and air pressure increases. The gradual change in air pressure over an area, seen in the pattern of isobars on a weather map, is called the pressure gradient. This gives rise to the movement of air from an area of relatively high pressure to an area of relatively low pressure.

That Wind Chill Factor ... Winds can also have a marked influence on temperatures; the temperature of the wind is influenced by its area of origin and by the characteristics of the surface over which it has travelled. Generally, winds blowing onto the land from the sea tend to relatively warm in winter and help to raise air temperatures. In the summer onshore winds tend to depress air temperatures as the sea over which the wind has travelled is much cooler than the land surface.

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