Air Masses and ITCZ

Topic 4: Air Masses and ITCZ Global wind circulation and ocean currents are important in determining climate patterns. These are the origins of the air masses which affect our weather. In some areas of the world air accumulates and adopts uniform characteristics of temperature and humidity. Their characteristics remain similar as the air moves away from the source.

Air Masses As you have learned the transfer of solar energy is achieved through circulation systems – primarily the Hadley Cells.

Air Masses Warm moist air is warmed at the equator, it rises, is cooled by convection, clouds form and thunderstorms develop. This releases heat into the upper atmosphere. This heat is carried away to the higher latitudes, sinks and returns as the trade winds. These winds have very different properties – both are different air masses. Over Africa the north east trades bring the Tropical Continental air mass and the south east Trades bring the Tropical Maritime.

Air Masses A tropical maritime air mass originates in tropical latitudes over the Atlantic Ocean. It brings warm, moist, unstable air. It is moist, as it has travelled across the ocean, picking up moisture as it moves. Since it originates in tropical latitudes it brings hot/very hot conditions. A tropical continental air mass originates over the land, e.g. the Sahara Desert, in tropical latitudes. It brings warm, dry, stable conditions as it travels over land as opposed to water. In both summer and winter it brings hot/warm and dry conditions.

Air Masses and ITCZ Where these two air masses meet they form the Inter- Tropical Convergence Zone (ITCZ).

ITCZ Around Africa cT air from the Sahara prevents the mT air mass from rising – thus it cannot rise, cool and condense forming rain. Rainfall only occurs where the compressed edge (also known as the squall line) reaches around 200m. This usually occurs 400km south of the actual position of the ITCZ. The weather associated with the ITCZ is towering cumulonimbus clouds, heavy precipitation and high intensity rainfall ( mm) per hour.

ITCZ Movement of the ITCZ We know that the zone of maximum insolation (thermal equator) is not fixed. It moves north in June and south in January because of the tilt of the earth. It follows then that the ITCZ also moves north in summer and south in winter.

ITCZ

ITCZ, Climate and West Africa The climate is highly seasonal in West Africa due to the movement of the ITCZ. In the winter the dry season, the dominant air mass is Tropical Continental with the dry Harmahatten wind blowing across nearly all of West Africa. Only the coastal south remains under the influence of the mT air. In June/July the thermal equator moves north to the Tropic of Cancer, dragging with it the ITCZ. This brings with it the convectional rainfall of the ITCZ. Following on behind is the mT air mass which is carrying large amounts of potential precipitation.

ITCZ, Climate and West Africa This pattern results in a marked decrease in rainfall away from the equator, with the rainfall occurring at different times of year. Lagos Jos Timbuktu

Rainfall in West Africa Rainfall varies dramatically over the region and can be described using – incidence, intensity and irregularity. The three ‘i’s. 1. Incidence is the variability within the year. – Seasonality is the key word here. – Areas have a distinct wet and dry season. – The wet season falls later in the year the further north you go. – Areas in the south have a twin maxima (double peak) caused by the ITCZ passing over twice. – The southern areas receive a far higher total than those in the north. – This is because even when the ITCZ has moved north, the mT air mass is still overhead – bringing mild wet weather to the southern areas.

Rainfall in West Africa 2. Intensity means how heavy is the rainfall. – One of the problems of high intensity is that much of the water will be lost to overland flow and run-off (refer to hydrosphere notes). 3. Irregularity - the extent that rainfall varies from year to year compared to the long term average. – In much of the West Africa, variability is similar to the UK (16- 20% from the mean), but it can vary by up to 60%. – The graph below shows how variable rainfall can be. The Sahel

Rainfall in West Africa This has had a profound effect on the environment. During the 1950s the mT air mass was powerful keeping the ITCZ in higher latitudes. – This increased rainfall encouraged settlement north to the margins of the Sahara (Sahel region). However since then there has been a southward shift of the ITCZ some km. – This means that land which was previously able to support agriculture can no longer do so. In 1974 one million people migrated southwards in West Africa, increasing population density there. The strength of the cT air mass has resulted in an extended dry season, and this has had major effects on agriculture, with people being forced to adapt to new conditions and grow crops with which they may not be familiar.

Summary The coastal and southern area of Africa lie south of the ITCZ for most of the year. This area receives moist mT air for most of the year. mT air originates over the Atlantic Ocean and brings hot, wet weather. This area can have twin rainfall peaks as the ITCZ moves north and south. The northern areas of Africa lie north of the ITCZ in winter. This area receives dry cT air. In summer the ITCZ moves north and brings moist (mT) air – giving rain. Rainfall becomes more and more limited the farther north you go.