ATMOSPHERE: Composition and Structure
Definitions:
Atmosphere: 'the gaseous envelope that surrounds the earth'
Air: 'the mechanical mixture of gases which makes up the atmosphere'
Boundary layer: 'the layer of the atmosphere which is affected by the earth's surface - around 600m in British Isles, but varies in height'
COMPOSITION
Air: component gases - act independently of each other.
Tend to think in terms of ‘dry' air - free from variable components -measured below 80km
In terms of volume:
Nitrogen 78.08%, Oxygen 20.95. Argon 0.93% (an inert gas)
Remaining 0.4% made up of minute amounts of inert gases (Neon, Helium, Krypton and Xenon), Hydrogen, Ozone, Carbon Dioxide, Methane, Halogen and CFC derivatives, Nitrous Oxide and aerosols.
Although these trace gases are present only in very small quantities, they clearly have an important role to play e.g Ozone layer, role of Greenhouse gases.
Also important are aerosols: 'minute airborne particles held in suspension’.
Aerosols are too microscopic to be observed by the naked eye but they collectively influence visibility as haze. They also colour distant objects and through the evening haze, dark distant objects appear blue and light clouds appear yellow. They also influence the turbidity or reduced transparency of the atmosphere through the so-called dust-veil effect. Aerosols scatter solar radiation back to space. Certain particulates (especially salt and sulphur compounds) are vital for the condensation process since they act as the condensation/hygroscopic nuclei required to effect the change from vapour to liquid.Effect of these on weather processes ?
Have considered 'dry’ air, but atmosphere contains variable amount of water vapour. Varies with latitude, temperature, position of land and sea masses and rates of evapotranspiration.
General global average for water vapour is less than 1% by volume, but hot humid equatorial air can contain as much as 4%.
90% of all water vapour is concentrated within a few km of the earth's surface. Due to increasing distance from sources of water, and lower temperatures with altitude can't maintain water in its gaseous state.
So simple questions to consider in this regard. How far up does the atmosphere go ? Does it have a definite upper limit ? A figure of 560km is sometimes mentioned as being a possible upper limit, but like many of these 'facts', different textbooks will tend to disagree on a figure.
STRUCTURE
Air has a definite weight: I cubic metre of air weighs 113g. Atmosphere therefore exerts pressure on earth's surface of 1.2kg/cubic metre.
Pressure decreases with height as there is less weight of air above, and because air is compressible. Air 'gets thinner' as you move up through the atmosphere. At a height of around 5500m, the density of air is half what it was at sea level. Some people live permanently at high altitude, but their bodies tend to have physiological differences to allow them to cope such as larger lung capacity.
Atmosphere has mass - it presses down on everything.
There are 4 major atmospheric zones:
TROPOSPHERE: extends up to 16km at equator and 9km at poles.
STRATOSPHERE: in the upper part of this layer are powerful winds: the 'Jet Streams' that can reach 400km/hr and steer depressions - sometimes this means they appear over the UK, sometimes they are steered to the north of the UK
MESOSPHERE
THERMOSPHERE - 'sphere' means a deep layer
Separated by isothermal layers:
TROPOPAUSE
STRATOPAUSE
MESOPAUSE - 'pause' means a boundary, or barrier - the coming to an end of one section (think of the menopause for a context..)
Borders are not at fixed intervals, and are not sharply defined.
Atmosphere slowly merges into space above 100km, but 'technically' extends up to approximately 32,000km
99% of atmosphere is within 32km of earth's surface.
90% of water vapour, and all weather systems occur in the lowest layer: the Troposphere.
Troposphere extends up to 18km at equator, but only 8km at poles.
Think of the atmosphere as a pan of water: over the heat of the equator, you would expect things to start bubbling over!
Temperature and pressure drop with height through the troposphere. The usual rate of decrease in temperature is known as the ELR (Environmental Lapse Rate) and averages 0.65 degrees Celsius per 100m
Mt. Everest is 9000m high: a breath taken at the summit would take in less than a third of the air taken in at sea level
At the top of the troposphere is the Tropopause which is an isothermal layer i.e temperature is constant. Above this layer, into the Stratosphere temperatures increase - due to increased ozone concentration - formation and destruction of ozone is a continuous process
Jet airliners would fly in this layer.
Pressure drops rapidly in this layer. Near the stratopause, UV rays from the sun are intercepted, and the thin mixture of gases is easily heated.