AIR MASSES AND THEIR MOVEMENTS; SCIENCE NOTE SHS
AIR MASSES AND THEIR MOVEMENTS; SCIENCE NOTE SHS
Air Masses
An air mass is defined as a large body of air that has relatively uniform temperature and humidity characteristics.
The regions where air masses form are referred to as air mass source regions.
If air remains over a source region long enough, it will acquire the properties or characteristics of the surface below.
The areas that air masses cover may be hundreds or thousands of square kilometres.
The best source regions for air masses are large flat areas where air can be stagnant long enough to take on the characteristics of the surface below.
When air masses meet, fronts are formed.
Formation of Air Masses
- Air is not the same everywhere around the globe.
- It can vary in temperature and humidity depending on where it comes from (i.e. its origin or source).
- However, when a large volume of air remains over a location for an extended period, it acquires the characteristics of the underlying surface.
- We call this large body of air an air mass.
- The underlying surface could be water (e.g. ocean) or land.
- The air may acquire the temperature of the underlying surface through heat transfer processes of conduction, convection and radiation.
- The moisture content may also be acquired through processes such as evaporation and condensation.
- The area covered by the air mass could be hundreds or thousands of square kilometres.
- An air mass that forms over the Arctic in winter would be cold and dry, while an air mass that forms over a tropical ocean would be warm and moist.
Types of Air Masses
Air masses are classified according to temperature and moisture conditions (humidity) of where they originate.
These give rise to four main types of air masses that can influence the weather.
They are;
- cold (polar),
- warm (tropical),
- humid (maritime)
- and dry (continental).
Tropical Continental Air Mass: This is a type of air mass which is formed over a warm land.
Tropical Maritime Air Mass: This is a type of air mass which is formed over a warm sea.
Polar Continental Air Mass: This is a type of air mass which is formed over cold land.
Polar Maritime or Arctic Maritime Air Mass: This is a type of air mass which is formed over a cold sea.
Movement of Air Masses
The general principles underlying the movement of air masses:
- Movement of air is caused by temperature or pressure differences and is experienced as wind.
- Where there are differences in pressure between two poles, a pressure gradient exists, across which air moves from the high-pressure region to the low-pressure region.
- Air blows anticlockwise around a low-pressure centre (depression) and clockwise around a high-pressure centre (anticyclone) in the Northern Hemisphere.
- This situation reverses in the Southern Hemisphere.
- Wind caused by differences in temperature is known as convection or advection.
- In the atmosphere, convection and advection transfer heat energy from warmer regions to colder regions.
- Small-scale air movement of this nature is observed during the formation of sea and land breezes, due to temperature differences between seawater and land.
- At a much larger scale, temperature differences across the Earth generate the development of the major wind belts. Such wind belts, to some degree, define the climate zones of the world.
Global Wind Patterns
As a result of the direction of movement of air masses globally, we have different types of wind patterns, namely;
- trade winds,
- westerlies
- and easterlies.
The high temperatures along the equator warm up the air, and the air becomes less dense, which then rises up.
- The warm air moves towards the poles, leaving behind a low-pressure area at the equator.
- At about 30° latitude north and south of the equator, this warm air begins to cool and sink.
- Most of the cooling sinking air moves back towards the equator between 30° latitude and the equator.
- The rest of the air flows towards the poles.
- Winds are normally named according to the direction from which they originate. For example, a wind from the west is a westerly wind but blows eastward.
- The rotation of the Earth deflects or changes the direction of winds as they blow, and this curving motion is called the Coriolis effect.
Trade winds
- The trade winds are part of the Earth’s atmospheric circulation.
- The trade winds are a pattern of winds that are found in bands around the Earth’s equatorial region.
- The trade winds are the prevailing winds in the tropics, blowing from the high-pressure area in both latitudes towards the low-pressure area around the equator.
- The trade wind blows from the northeast in the northern hemisphere, and southeast in the southern hemisphere.
- Trade winds are warm, steady winds that blow almost continuously or all the time.
- The trade winds meet at the doldrums, an area of calm, with only light, variable winds.
- Whether the trade winds are moving from the north or south, they curve to the west due to the Coriolis effect.
The Westerlies
- The Westerlies generally blow between 30° and 60° latitude from the west towards the east in both the Northern and Southern hemispheres.
- The westerlies curve to the east due to the Coriolis effect.
- At the core of the westerly winds lies what scientists call a jet stream.
- Jet streams are super high-speed winds.
- Each hemisphere’s westerly has two main jet streams.
- Closer to the poles, we find the polar jet stream, and at a slightly lower latitude, we find the subtropical jet stream.
The Polar Easterlies
- The polar easterlies are the dry, cold winds over the poles.
- The polar easterlies generally blow from the east to the west at 90° in both the northern hemisphere and the southern hemisphere.
- The cold air sinks and flows away from the poles,
- curving to the west due to the Coriolis effect.
- Unlike the westerlies in the middle latitudes, the polar easterlies are often weak and irregular.
EFFECTS OF AIR MASSES
The following are some of the effects of air masses.
- It causes the formation of storms and cyclones that may, in turn, destroy lives and properties.
- It spreads pollution in the air. This carries harmful or radioactive substances from place to place.
- It causes acid rain.
- It causes global warming.
- It destroys the ozone layer.
- It destroys biodiversity.
- It causes siltation.
Review Questions
- What is an air mass?
- State five effects of air masses
- Explain briefly what is meant by global air pattern.
- Explain the pattern of movement of the following air masses.
- Trade winds
- Westerlies
- Polar Easterlies
- Define wind.
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