Air MassesAir Masses

Science 10Science 10

Air MassAir Mass A large mass of air that has nearly A large mass of air that has nearly

uniform properties such as uniform properties such as temperature, humidity and pressuretemperature, humidity and pressure

There are five air masses of There are five air masses of importance that you should know.importance that you should know.

Arctic Air MassArctic Air Mass Extremely cold temperatures and Extremely cold temperatures and

very little moisture. Originating over very little moisture. Originating over the Arctic ocean in winter, when high the Arctic ocean in winter, when high pressure dominates and differs only pressure dominates and differs only slightly from the continental polar slightly from the continental polar (cP) air masses that develop over (cP) air masses that develop over Siberia and northern Canada.Siberia and northern Canada.

Continental polar (cP)Continental polar (cP) Cold and dry, originating from high Cold and dry, originating from high

latitudes, typically as air flowing out of latitudes, typically as air flowing out of the polar highs. the polar highs.

This air mass often brings the rattling This air mass often brings the rattling cold, dry and clear weather on a perfect cold, dry and clear weather on a perfect winter day and also dry and warm winter day and also dry and warm weather on a pleasant day in summer.weather on a pleasant day in summer.

Maritime polar (mP)Maritime polar (mP): : Cool and moist, often originating as Cool and moist, often originating as

continental polar air over the North American continental polar air over the North American and Asian land masses and is modified as it and Asian land masses and is modified as it moves out over the Atlantic and Pacific moves out over the Atlantic and Pacific Oceans. Oceans.

Heated by the relatively warm water bodies Heated by the relatively warm water bodies this air mass becomes rather unstable this air mass becomes rather unstable resulting in blustery showers over the sea and resulting in blustery showers over the sea and windward coasts.windward coasts.

Continental tropical (cT)Continental tropical (cT): : Hot and very dry, orginating from the Hot and very dry, orginating from the

arid and desert regions during summer. arid and desert regions during summer. The least common air mass to affect the The least common air mass to affect the British weather. However, approaching British weather. However, approaching from the south and south-east in from the south and south-east in summer it can bring record heat to summer it can bring record heat to southeast Britain, particulary in late-southeast Britain, particulary in late-summer.summer.

Maritime tropical (mT)Maritime tropical (mT) Mild and damp in winter, very warm and Mild and damp in winter, very warm and

muggy during summer. muggy during summer. Originating from the Azores this air mass Originating from the Azores this air mass

approaches the British Isles from the west, approaches the British Isles from the west, leading to overcast skies with prolonged leading to overcast skies with prolonged rain for the western half of the country and rain for the western half of the country and to many clouds, sometimes broken with to many clouds, sometimes broken with drizzle for the eastern half. drizzle for the eastern half.

In summer tropical maritime air often In summer tropical maritime air often produces warm weather with abundant produces warm weather with abundant sunshine in the southeast. sunshine in the southeast.

High Pressure SystemsHigh Pressure Systems When an air mass cools over an ocean When an air mass cools over an ocean

or a cold region of land, a high or a cold region of land, a high pressure system forms.pressure system forms.

The cooling, descending air exerts The cooling, descending air exerts more pressure on the region below. more pressure on the region below. This is why it is called a high pressure This is why it is called a high pressure system.system.

High pressure systems bring clear High pressure systems bring clear bright skies.bright skies.

Low Pressure SystemsLow Pressure Systems Air masses traveling over warm land Air masses traveling over warm land

or oceans may become low pressure or oceans may become low pressure systems.systems.

The warming air rises and leaves less The warming air rises and leaves less dense air below.dense air below.

Water vapour in the air condenses as Water vapour in the air condenses as it reaches the higher altitudes, it reaches the higher altitudes, producing clouds and precipitation.producing clouds and precipitation.

How wind forms:How wind forms: Wind is caused by the movement of Wind is caused by the movement of

air from an area of higher pressure to air from an area of higher pressure to an area of lower pressure.an area of lower pressure.

The rotation of the earth cause wind The rotation of the earth cause wind to move in a clockwise direction to move in a clockwise direction around the high pressure centre.around the high pressure centre.

The Coriolis EffectThe Coriolis Effect The Coriolis effect is the change in The Coriolis effect is the change in

the direction of moving air, water or the direction of moving air, water or any objects on the Earth’s surface any objects on the Earth’s surface due to the rotation of the Earth.due to the rotation of the Earth.

It deflects winds to the right in the It deflects winds to the right in the Northern Hemisphere and to the left Northern Hemisphere and to the left in the Southern Hemisphere.in the Southern Hemisphere.

Polar EasterliesPolar Easterlies also also Polar Hadley cellsPolar Hadley cells dry, cold prevailing winds that blow dry, cold prevailing winds that blow

from the high-pressure areas of the from the high-pressure areas of the polar highs at the north and south polar highs at the north and south poles towards low-pressure areas poles towards low-pressure areas within the Westerlies at high latitudes within the Westerlies at high latitudes

often weak and irregular often weak and irregular

Prevailing WesterliesPrevailing Westerlies prevailing winds in the middle latitudes prevailing winds in the middle latitudes

between 30 and 60 degrees latitude, blowing between 30 and 60 degrees latitude, blowing from the high pressure area in the horse from the high pressure area in the horse latitudes towards the poles. latitudes towards the poles.

blow from the west to the east, and steer blow from the west to the east, and steer extratropical cyclones in this general manner extratropical cyclones in this general manner

strongest in the winter hemisphere and times strongest in the winter hemisphere and times when the pressure is lower over the poles when the pressure is lower over the poles

Trade WindsTrade Winds the prevailing pattern of easterly surface the prevailing pattern of easterly surface

winds found in the tropics winds found in the tropics The trade winds blow predominantly from The trade winds blow predominantly from

the northeast in the Northern Hemisphere the northeast in the Northern Hemisphere and from the southeast in the Southern and from the southeast in the Southern Hemisphere Hemisphere

the trade winds have been used by the trade winds have been used by captains of sailing ships to cross the captains of sailing ships to cross the world's oceans for centuries world's oceans for centuries

Horse LatitudesHorse Latitudes Horse LatitudesHorse Latitudes or or Subtropical HighSubtropical High

are subtropical latitudes between 30 and are subtropical latitudes between 30 and 35 degrees both north and south 35 degrees both north and south

This region, under a ridge of high pressure This region, under a ridge of high pressure called the subtropical high, is an area called the subtropical high, is an area which receives little precipitation and has which receives little precipitation and has variable winds mixed with calm.variable winds mixed with calm.

The consistently warm, dry conditions of The consistently warm, dry conditions of the horse latitudes also contribute to the the horse latitudes also contribute to the existence of temperate deserts existence of temperate deserts

DoldrumsDoldrums a low-pressure area around the equator a low-pressure area around the equator

where the prevailing winds are calm where the prevailing winds are calm The low pressure is caused by the heat at The low pressure is caused by the heat at

the equator the equator the air rise and travel north and south high the air rise and travel north and south high

in the atmosphere in the atmosphere The doldrums are also noted for calm The doldrums are also noted for calm

periods when the winds disappear periods when the winds disappear altogether, trapping sail-powered boats for altogether, trapping sail-powered boats for periods of days or weeks periods of days or weeks