The Atmosphere, Troposphere and Tropopause
As with everything to do with meteorology, atmospheric composition plays a massive role in the formation of fronts. By percentage, the atmosphere is 78% Nitrogen and 21% Oxygen leaving a measly 1%to account for everything else including Carbon Dioxide, Ozone and todays primary concern, Water Vapour.
Despite this, local concentrations of water vapour can be up to 4%. This is because on average 50% of the mass of the atmosphere is situated in the 5.5km(18,000 ft) closest to the earth and another 25% in the 5.5km after that.
Layers of the Atmosphere, Bredk, 2007. (Available at https://commons.wikimedia.org/wiki/File:Atmospheric_Layers.svg)
By separating the atmosphere into layers around the earth based on how temperature varies with altitude, we can define the troposphere. In the International Standard Atmosphere (ISA), this is the lowest level stretching from the surface to the tropopause at 11km (36,090 ft) with a temperature lapse rate of 1.98°C per 1000ft with a surface temperature of +15°C. As the majority of aircraft operate in this layer, this is what we are going to examine more closely.
The ISA is an average model used to standardise the basis for calculations around the world but if you’ve stood outside recently, I highly doubt it was exactly 15°C. The variation in temperature with season causes the actual level of the tropopause to rise and fall over the course of the year. In the warmer summer months, you can expect it to be higher while its lower in the colder winter months.
The rotation of the earth also influences the height of the tropopause. As the earth spins, the atmosphere tends to get pulled towards the equator and bulge outwards meaning different latitudes will have different tropopause heights. As a general rule the tropopause at the poles and high latitudes will be almost half the height of the equatorial tropopause.
Variation in Tropopause height with latitude
Air Masses
Air masses are large bodies of air that have remained stationary over a source region for long enough to take on the characteristics of the region. Each source region is a large stable area of high pressure. Some of these exist year-round, while others are dependent on seasons. Air masses tend to extend from the surface to the tropopause and contain the majority of the water vapour, and hence weather, in the atmosphere.
"A body of air with horizontally uniform levels of temperature, humidity and pressure"
Air masses follow a 3-letter naming system based on their track, source region and temperature. For instance, the air mass originating from North Africa travels mostly over the continent to reach the UK bringing hot dry air so is known as the Tropical Continental Air Mass and described by cTw.
Even if we didn’t know the source region, from its code we could determine its basic characteristics. The “c” implies it has travelled over land where there is less moisture than over the sea, so it is likely to be quite dry. “w” implies it has been situated in a hot area for an extended period so is likely to be very warm. This is confirmed by the “T” stating it is from the tropical zone much closer to the equator than we are situated.
6 major air masses affect the UK. 5 of these can be seen on the diagram, with the addition of the Returning Polar Maritime Air Mass.
Like the Polar Maritime Air Mass, this originates in the polar regions like Greenland but travels south before returning to the South West of the UK, extending its sea track to loop around a large depression or area of low pressure situated somewhere to the North West of the UK.
Due to its extended sea track, the air mass has more time to warm in the lower layers and take on moisture creating a stable base layer with unstable air above, leading to a large variety of weather. Stratus clouds and hill fog are likely, and may develop into cumuliform cloud, or even CBs in the unstable layer aloft.
Similarly, Polar Continental air may be divided into two categories based on its travel path. If the air mass follows the Short Sea Track, traveling mostly over land, it tends to arrive in the UK near Dover. The Long Sea Track air which has travelled between Denmark and Scotland tends to be slightly more humid and may bring drizzle or snow, especially in the winter months.
BBC,2011. (Available at: http://www.bbc.co.uk/scotland/science/weather.shtml)
Warm Fronts
A warm front forms where a warm air mass replaces a cold airmass. The warm air is less dense than the cold, allowing it to rise over the top producing a gentle 1:150 slope. As the air rises, it cools causing stratiform cloud to form in layers along the boundary, increasing in altitude the further from it. This band of cloud can be up to 500NM ahead of the front and result in precipitation up to 200NM ahead.
The warm front tends to travel at approximately 1/3 of the speed of the 2000ft wind, slower than its cold front counterpart.
As the front passes through, can expect:
Surface Wind - Veers.
Pressure - Decreases.
Temperature - Increases.
Cloud - Clear edge of high cirrus cloud far ahead of front, followed by Altostratus, Nimbostratus and Stratus.
Cloud Base - Gradually lowering as front approaches.
Precipitation - Light up to 200NM ahead, increasing to moderate then continuous when front directly overhead.
Transitional Zone - Up to 90NM causing gradual changes.
Behind - Low cloud cover and drizzle with possible mist, especially in winter.
