What are the names for the various levels of the atmosphere and of what significance are they?
The atmosphere is divided up into layers with names which describe the dynamic or thermal structure of that particular layer. The two layers which are of most interest to us are the troposphere and the stratosphere.
Troposphere: (overturning or changing sphere) - The lowest layer of the atmosphere. Positive lapse of temperature (positive lapse rate: temperature overall decrease with height). It is the most important for operational meteorology, as this layer contains almost all the water vapour, and by far the greatest part of the mass of the atmosphere. Because of its mean thermal structure, it is the region of greatest vertical motion (up and down) in the atmosphere, even without the help of vigorous thunderstorm complexes, which in themselves may occupy the entire depth of the troposphere. At some level, there is usually an abrupt change in the lapse rate from positive (decrease with height), to isothermal (no change), or a slight rise. This level is the tropopause. Typical heights of the tropopause, and therefore thickness of the troposphere, are:
- High arctic/antarctic latitudes: 6 to 8 km (20000-25000 ft)
- On/near the equator: 16 to 18 km (50000-60000 ft)
In mid-latitudes, the temperate zone, which is of most interest to us in NW Europe, the tropopause is highly variable, from cold to warm season, and from cold to warm air mass. For example, it is lower in winter, and in cold/polar air masses (typically 8 to 10 km/25000 to 30000 ft), than in high summer, and in warm/sub tropical air masses (typically 12 to 14 km/35000 to 45000 ft)
Stratosphere: (the 'layered' sphere) - the next layer ascending through the atmosphere. Isothermal or negative lapse rate of temperature (i.e the temperatures rises with increasing height). Because of this temperature structure, little natural, or un-forced overturning of air takes place, either within the stratosphere, or in exchange with the troposphere. Once gases, particulates etc. penetrate to this layer, they remain there for very long periods, hence the concern regarding such substances due to both the actions of mankind (e.g. CFCs) and those of natural processes (e.g. volcanic ash). However, near the boundary with the troposphere (q.v.), marked vertical motion can occur under certain circumstances (forced by jet-stream actions), which are important in driving developments in the troposphere.
As with the troposphere, the stratosphere varies in thickness, but as an average figure the top of this layer, the stratopause, occurs around 45-48 km (148000-158000 ft).
The importance of the stratosphere (and the primary reason for its temperature structure) is that much of the atmospheric ozone is found within its lower layers - circa 18 to 30 km amsl. The selective absorption by ozone (and oxygen) of solar ultra-violet radiation leads to warming in the stratosphere - this (and other) factors give rise to its markedly stable nature. Very little water vapour is found here, nor dust (except for dust injected by major volcanic eruptions), but when the stratosphere is anomalously cold, then Polar Stratospheric Clouds (PSC) are sometimes visible.