Equatorial – 0-5 degrees latitude
Weather mainly
influenced by the 'overhead sun' all year and dominance of the ITCZ
zone.
High insolation all the year
– no distinctive season
– average
temperature 30 degrees Celsius
– low pressure
– high humidity
–
heavy convectional rainfall all year averaging 2000 mms.
– some tendency
to higher rainfall at summer solstice in March and September when the
sun is directly overhead.
Savanna Type – 5-20 degrees latitude either side of the
Equatorial belt
As insolation is high, high temperatures prevail all
the year
– 35to 25 degrees centigrade
– this climate type is
distinguished by a hot wet and a cooler dry season
– humidity highest in
the wet season
– evaporation rates remain high during the cooler dry
season.
Rainfall occurrence is associated with the movement of the ITCZ zone towards the tropic in association with the apparent movement of the overhead sun’s position
– as this occurs low pressure prevails with moist in blowing winds and rising air currents leading to convection rainfall during the hot season
– amounts are most reliable towards the equatorial latitudes averaging 800 mms.
– becoming less reliable towards the hot desert margins averaging 300 to 400 mms. (In some years persistence of high pressure over the outer margins of the Savanna e.g. the Sahel zone prevents the ITCZ and in blowing moist winds from extending into these marginal areas leading to drought conditions. The variation from the normal seasonal weather patterns is partly linked to deviations in the path of Jet Streams in the higher atmosphere. Global warming is a further contributory factor.
The cooler dry season in the savanna belt occurs as high pressure and dry out blowing winds prevail when the overhead sun and ITCZ move away to extend beyond the Equator towards the other tropics.
Monsoon Type – occurs mainly on the eastern side of the
continental land masses in the Tropics extending approximately across 5
– 20 degrees of latitude.
The climate is marked by a distinct hot
wet and a cooler dry season which is determined by the annual
movement of the ITCZ between the Tropics and associated movement of
pressure belts and the seasonal reversal of winds consequent upon this.
The Monsoon climate regime is most clearly seen over India but exists in
other regions north and south of the Equator on the eastern edge of
continents e.g. East Africa.
The wet monsoon season occurs with the movement of the ITCZ into
the region bringing an area of low pressure and drawing in hot moist
winds from off the ocean.
- average rainfall is around 2000 mms decreasing with distance inland.
Cyclones and hurricanes are frequent towards the end of the rainy season.
The cooler dry season coincides with the extension of continental high pressure as the ITCZ moves back towards the Equator and across into the tropics beyond. With high pressure dominating there is air subsidence and out blowing winds are dry.
Temperatures remain relatively high at 25 plus degrees centigrade in lowland areas and evaporation rates are also high. But the weather is much more severe in mountain areas.
Hot Desert Climate Regime – occurs in continental
areas/western side beyond the Savanna belt approximately between 20 - 30
degrees latitude.
It is characterised by high insolation rates,
subsiding air and high pressure conditions with out blowing winds.
These
conditions result in generally clear skies, very high insolation and
evaporation rates with temperatures in the day time averaging over 40
degrees centigrade and exceptionally dry conditions.
On the western land
margins cold ocean currents deprive in blowing marine winds of their
moisture before they reach land thus contributing to the exceptionally
dry conditions and leading to fog conditions offshore as e.g. off
northern Chile and Namibia. Occasional incursions of the ITCZ into
desert latitudes may allow the penetration of moist winds bringing very
heavy but short lived convectional downpours of rain.
Average
precipitation is 100-200 mms. but effective rainfall is less due to the
very high evaporation rates.
Diurnal contrasts are more significant than
seasonal variations as rapid radiation at night due to clear skies can
result in below freezing temperatures and heavy morning dew in contrast
to temperatures of 40 degrees plus and very low humidity conditions
during the day time.
The main influence on climates in the TROPICS are –
1. The overhead or near overhead position of the sun giving high insolation throughout the year.
2. The position and seasonal movement of the ITCZ together with the topical pressure belts wind systems.
3. The path of the upper jet streams affecting the path of low pressure systems.
4. Differential heating of land masses and oceans in the tropics affecting air pressure patterns and seasonal wind directions. (Monsoon)
5. The effects of offshore cold currents on western land margins (e.g. Canary Current) and warm currents on eastern margins.
6. The position of mountain ranges and their effects on incoming moist winds off the ocean. (Relief Rainfall)
