Climate is the average state of the weather. And weather is concerned with daily changes in temperature, wind, cloud and rain in general the state of the atmosphere. When we examine the climate of Jamaica we are chiefly concerned with:
- Temperature (degree of heat and cold)
- Winds (their movement and direction)
- Rainfall (its causes and its seasons).
TEMPERATURE
How hot or cold a place is depends mainly on how far it is north or south of the equator. The higher the latitude the colder the climate. What is the latitude of Jamaica? If you look at the globe you will see that Jamaica is in the tropical zone south of the Tropic of Cancer. However, its distance north of the equator has a moderating effect on its temperature, and hence Jamaica is said to have a semi-tropical climate.
Apart from latitude, the greatest factor in determining the temperatureof a country is altitude. Most of the effective heat we enjoy is radiatedfrom the earth which has been warmed by the sun. In reaching the surfaceof the earth the sun's rays have to pass through layers of atmosphere,a process which causes it to lose some of its heat. If we imagine theselayers of atmosphere as blankets retaining the heat radiated by the earth'ssurface we will see that by climbing above these layers it will becomecolder. Temperature decreases by 1.7 degrees Celsius (or 1 degree Fahrenheit)for every 100 metres (300 ft) of ascent.
Because Jamaica is a very mountainous country, temperatures vary widelyin different parts of the island. For example, the temperature might dropto about 10 degrees C or 50 degrees F in Mandeville, 626 m (2,061 ft),while in Kingston the mean temperature is 26 degrees C or 78 degrees F.On the whole, Jamaica's climate has no extremes, especially since the surroundingsea has a moderating effect on the weather, and the variety of climateis considered healthy and beneficial.
WINDS
Local winds, from the sea by day and from the land at night,are very noticeable in Jamaica because it is an island. The prevailingwinds in the West Indies are from the north-east. Remember that warm airis lighter than cold air, and that winds blow from an area of high pressureto an area of low pressure. At the equator, the hot air is continuallyrising, creating an area of low pressure. The air on both sides of theequator, being cooler and heavier, moves in to take the place of the risingair, hence there is a constant movement of air towards the equator, fromthe north and south. However, since the earth turns from west to east,the winds do not blow due south or north, but are slightly deflected sothat they come from a north-east direction north of the equator, and froma south-east direction south of the equator. These movements of air arecalled the Trade Winds. Because they are blowing from a cooler to a warmerpart of the earth, they are able to hold more moisture.
RAINFALL
The heat of the sun, like the fire under a kettle, turnswater from the various water bodies of the world into water vapour. Thisprocess is called evaporation. If the temperature of air is lowered, thewater vapour it contains in the form of clouds will fall as rain. Thisprocess is called condensation. Jamaica receives two kinds of rainfall:convectional and relief. It does not receive monsoon rains because it isnot a large land mass.
Moisture-laden air may be cooled and thus forced to give up its water vapourin the form of rain by 1) rising to colder regions of the atmosphere or2) being forced upwards by land masses in its path. In both cases condensationtakes place and rain falls.
In the first case, when evaporation takes place, the moist air rises, coolsand condenses and rain falls. Rains originating in this way are known asconvectional rains. When the moisture-bearing trade winds come upona high mountain range, they are forced to rise; condensation takes place,and the rain which results is known as relief rainfallbecauseit is caused by a change in altitude. The wind deposits its moisture onthe windward side of the mountains, and then descends as a dry wind onthe leeward side of the mountains. This dry area is known as the rain shadow.
Relief rainfall causes Port Antonio on the windward side of the Blue Mountainsto receive an average of 430 mm (17 inches) of rain in November while Kingston,in the rain shadow, receives only 175 mm (7 inches). Antigua and Barbados,which are two flat islands, have a lower rainfall than the more mountainousCaribbean islands.
As a rule, rainfall is much heavier on the North Coast of Jamaica, whichreceives the relief rainfall provided by the mountains running from westto east, than on the south coast, which receives chiefly convectional rain.
Jamaica has two rainy seasons, the first in May and the other in Octoberand November. As a rule, rain follows the sun. It appears to pass overall places between the Tropics of Cancer and Capricorn twice a year. InJamaica the sun is directly overhead about the second week in May and inearly August. Notice that the periods of heavy rainfall reach their maximumshortly after the sun has been directly overhead.
HURRICANES
A hurricane is a storm revolving around a centre of low pressurewhich contains almost no wind. As a hurricane develops, the winds fromthe area of high pressure rush towards the low-pressure centre, and astheir centrifugal force intensifies, powerful gales of up to 200 kilometres(120 miles) per hour are built up. The calm vortex in the centre, the 'eye'of the hurricane, varies in diameter from thirty to a few hundred miles,and usually moves westwards. Hurricanes are invariably accompanied by drivingrains.
Nowadays weather stations discover hurricanes as soon as they develop andgive warning to those places towards which a hurricane may be heading.On the approach of a hurricane the barometer falls and the thermometerusually rises.
The hurricane season is between July and October, though a hurricane mayoccasionally arise in June or November. On September 12, 1988, Jamaicasuffered the worst hurricane in living memory when Hurricane Gilbert devastatedthe island.
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Now, let's break down the concepts mentioned in the article about Jamaica's climate:
Temperature:
- Jamaica's temperature is influenced by its latitude and altitude.
- Being in the tropical zone south of the Tropic of Cancer, Jamaica has a semi-tropical climate.
- Altitude plays a significant role, and Jamaica's mountainous terrain leads to varying temperatures across the island.
- Temperatures can drop to about 10 degrees Celsius in high-altitude areas like Mandeville, while coastal areas like Kingston have a mean temperature of 26 degrees Celsius.
Winds:
- Local winds in Jamaica are noticeable due to its island nature.
- Prevailing winds in the West Indies, including Jamaica, come from the north-east, known as the Trade Winds.
- Trade winds are deflected due to the Earth's rotation, resulting in a north-east direction north of the equator and a south-east direction south of the equator.
Rainfall:
- Jamaica experiences two types of rainfall: convectional and relief.
- Convectional rains result from moist air rising, cooling, and condensing.
- Relief rainfall occurs when moisture-bearing winds are forced to rise over mountain ranges, causing condensation and rain. The leeward side of mountains experiences a rain shadow.
- Rainfall is heavier on the North Coast of Jamaica due to relief rainfall, while the south coast receives chiefly convectional rain.
- Jamaica has two rainy seasons in May and October/November, following the sun's movement between the Tropics of Cancer and Capricorn.
Hurricanes:
- A hurricane is a storm with a low-pressure center and powerful winds.
- Jamaica is susceptible to hurricanes between July and October, with occasional occurrences in June or November.
- Weather stations provide warnings, and signs include falling barometric pressure and a rising thermometer.
- The devastating Hurricane Gilbert hit Jamaica on September 12, 1988.
This breakdown provides a comprehensive overview of Jamaica's climate, covering temperature variations, wind patterns, rainfall mechanisms, and the impact of hurricanes.
