This document looks at the factors that affect weather and climate. The weather systems that also affect the Caribbean is also examined. These include ITCZ, Hurricanes, Cold Fronts, Easterly waves, Anticyclones

1. CSEC GEOGRAPHY
Weather and Climate
O.Johnson
The layer of gases which surrounds the earth is called the atmosphere. The terms weather and
climate both have to do with the atmosphere, but they do not mean exactly the same, and they
cannot be used in place of each other. The term climate refers to the average as well as the
highest and lowest rates of temperature, rainfall, wind, cloud cover and air pressure over a period
at least 30 years. NASA defines climate as the description of the long-term pattern of weather in
a particular area. By contrast, the term weather refers to the atmosphere at a particular instant. In
general we talk about a few days or up to a week. The term micro climate refers to the distinct
climate associated with small areas such as a city, a woodland, a coastal area or even a school.
The main difference between weather and climate is time.
Factors affecting weather and climate
The factors that influences weather and climate are: latitude, altitude, distance from the sea and
relief/ topography.
Latitude

2. Temperatures decreases as you move away from the equator.
The further you move away from the equator the sun's rays are dispersed over a larger area of lan
d. In addition areas far away from the equator for e.g. the poles are cooler because the sun's rays
have a further distance to travel before reaching the earth's surface and some of the heat energy
would have lost during the journey.
At the equator the sun is often overhead. Therefore the rays of the sun fall directly or vertically at
the equator. It hits on a very small area so the heat gets concentrated in this area which gets very
hot. This causes the temperatures at the equator to be higher. In addition at the equator the sun’s r
ays have a shorter distance to travel through before reaching the earth's surface. As a result less h
eat is lost during the journey.
Relief/topography/ Mountain
Relief can influence the amount of precipitation an area receives. The winds which blow over the
Caribbean (over the Atlantic) are known as the north-east trade winds (NETW)
1) Rain-bearing wind – AS they (NETW) pass over the sea, they are able to take/pick up
moisture from the sea by the process of evaporation.
2) Air forced to rise – On meeting a mountain range (Blue Mountain) the rain bearing wind
is forced to rise over it. Upon rising the air cools and becomes less capable of holding
water vapor.

3. 3) Condensation takes place- Because the air can no longer hold all the water vapor in it.
Some water vapor changes back into tiny drops of water. Millions of tiny droplets of
water accumulate together to form clouds over the top of the mountain range.
4) Precipitation- Heavy rainfall occurs on the top of the mountain range and on the
windward facing slope.
5) Air now descends – The air can now sink down the leeward side of the mountain and so
becomes warmer. The leeward side of the mountain range gets less rainfall than the
windward side because a) the air is now much drier, having deposited much of its
moisture on its journey over the mountain range b) the air is descending and getting
warmer, and so it can now hold much more water vapor than it would when rising. much
of its
Parishes such as Portland and St Thomas in Jamaica receives between 1,200 and 1,800 mm
(47-70 inches) per year. Kingston receives about 800m (31.5 inches) of rain per year. This is
because it is sheltered from the trade winds.

4. Altitude (Height Above sea level)
The atmosphere is not warmed directly by the sun. When the sun's rays heat the earth the heat fro
m the earth warms the air above it.

5. Water vapour and dust particles in the air prevent the heat from escaping into outer space, becaus
e they have the effect of trapping the heat forming a blanket/canopy over the earth's surface.
At higher altitude (the top of a mountain) the air contains very little vapor or dust. So little heat is
trapper at higher altitudes. In addition the air gets thinner because the molecules in the air which
receive and retain the heat became fewer and are widely space (less compact). As a result they ar
e not able to trap much heat to the earth's surface, the heat rapidly escape resulting in cooler temp
eratures.
In contrast at lower altitudes (bottom of a mountain) the air contains more vapor and dust particle
s. Therefore more heat is trapped. In addition, the air is also dense meaning the molecules in the
air which retain heat are more/greater and they are more compact. As a result the air is able to tra
p more heat to the earth's surface which results in warmer temperatures.
Temperature decreases by 6.5 degrees celsius for every 1000m in height. This is called the Envir
onmental lapse rate. The environmental lapse rate is the decrease in temperature usually expected
with an increase in height through the troposphere.
Continentality/ Distance from the sea
This is of particular importance in those places which are far away from the equator. These
places have well developed summer and winter seasons. The sea has a moderating influence on
places close to it and they usually have moderate temperatures.
IN THE SUMMER
The sun's heat is absorbed and released more slowly by water than by land. The land absorbs
heat faster resulting in higher temperatures over land surfaces. The sea is cooler in the summer as
it takes up a longer time to heat up. Cool winds are brought onshore and this reduces the
temperature of coastal areas. As a result places near the coast tend to have cooler summers than
places further inland.

6. IN THE WINTER
In the winter the sea loses heat more slowly than the land. The sea is therefore warmer in the
winter. The land releases heat faster resulting in colder/lower temperatures over land surfaces.
Warm winds are brought onshore and this increases the temperature of coastal areas. Those
places further inland have really cold winters while those close to the coast have milder winters.
The places nearer to the sea have cooler summers and warmer/mild winters (maritime/oceanic
climate). They have a moderate climate due to the influence of the sea. Those further away and

7. further inland generally have extremes of climate that is really hot summers and very cold
winters (continental climate).

8. Caribbean Weather Systems
The weather systems that affect the Caribbean are: cold fronts, easterly waves, hurricanes,
intertropical convergence zone and Anticyclone
Cold Front
When air remains relatively stagnant over an area for some time, it takes on the temperature and
moisture characteristics of the area. Therefore, air which remains stagnant over a cold and dry
area will become cold and dry. Air which remains stagnant over the sea or ocean in the tropics
will become warm and moist. A body of air which has similar temperature and moisture
characteristics throughout is known as an air mass.
Over time, air masses may move out of the areas over which they develop. As they do so, they
encounter other air masses with different characteristics. When two air masses with different
characteristics meet, they do not readily mix. There is usually a sharp or well defined boundary
between the two air masses. This boundary is known as a front. There are different types of
fronts such as warm fronts and cold fronts. Warm fronts occur when a warm air mass moves
into a region of colder air. Cold fronts occur when a cold air mass moves into a region of warm
air.
Cold fronts affect the Caribbean region (especially the northern part of the region) during the
northern hemisphere winter. During this period, cold air from North America (Northers) moves
into the Caribbean. There is a well-defined boundary where this cold, dry air meets the warm,
moist air of the Caribbean. The cold air is denser and heavier and so it pushes its way beneath the
warm air. The warm air, which is lighter and less dense than the cold air, is forced to rise. The
rising of air will result in cooling. Cooling will result in condensation and then condensation will
result in cloud formation (cumulonimbus clouds). Eventually rain will fall (heavy rain and
thunderstorms). This is called FRONTAL RAINFALL.
How cold front affects the weather
- As the front passes the temperature may drop by several degrees (sometimes 50
C)
- As the font passes the wind speed increases
- Heavy rainfall along the line of the front and either side of it

9. - There is an increase in the amount of cloud with the development of particularly cumulonimbus
clouds
Cold Fronts affect the countries in the Northern Caribbean e.g. Jamaica, Bahamas Cuba and
Haiti
Cross section through a cold front
Symbol for cold fronts on weather maps

10. Easterly waves
Among the most important rain producing system in the Caribbean are easterly waves. They are
also called tropical easterly waves or African easterly waves. These are areas of low pressure that
originate off the coast of West Africa. These are areas of low pressure that originate off the coast
of West Africa.
For much of the year, the trade winds which blow across the Caribbean bring dry, settled
conditions, but from time to time weather systems develop in the trade wind belt which bring
unsettled weather and rain. These weather systems develop more frequently in the rainy season,
from May to November. Tropical waves are one type of weather system. It is a wave-like
disturbance in the north-east trades and it may takes several days to pass over the region.

13. Hurricanes
A hurricane is one of the most dangerous natural hazards to people and the environment. Every
year immense damage is done by tropical storms and hurricanes. Hurricanes are also known as
tropical cyclones in Asia. These storms are essential features of the Earth’s atmosphere, as they
transfer heat and energy between the Equator and the cooler regions towards the poles.
Some tropical waves travelling over the Atlantic become unstable and develop into hurricanes. A
hurricane is a tropical revolving storm. It is a very large circular area of low pressure, driven by
evaporation from warm seas. In areas away from the Equator, air is drawn into the hurricane to
replace the rising air. Owing to the Earth’s rotation, winds in a hurricane spiral inwards in an
anticlockwise direction. Wind speeds can reach 300 km/hour. The hurricane is driven by the
latent heat released as water vapour condensed into droplets.
Hurricanes form between latitudes 50
and 300
and initially move westward and slightly towards
the poles. Hurricanes don’t usually form close to the equator as the rotation of the earth (Coriolis
Effect) is not strong enough to cause the system to spin or spiral. An average hurricane can move
between 192km and 768km per day, and travel over 4000km before it dies out. Hurricanes are
measured using the Saffir-Simpson Scale. Hurricanes in the Atlantic occur between June and
November.
Several different factors are required to transform storms, which occur frequently, into rarer
hurricanes. These trigger mechanisms depend on several conditions being ‘right at the time’. The
most influential factors are:
 a source of very warm, moist air derived from tropical oceans with surface temperatures
greater than 260
C
 sufficient spin or twist from the rotating earth- this is related to latitude
As the warm sea heats the air above it, very warm most air rises up quickly, creating a center of
low pressure at the surface. Winds rush in towards this area of low pressure and the inward-
spiraling winds whirl upwards releasing heat and moisture. The rotation of the Earth causes the
rising column of air to twist. The rising air cools and produces towering cumulus and
cumulonimbus clouds. Further aloft at around 10km the cloud tops are carried outwards to create
a thick layer of clouds which mark the outward-spiraling winds as they leave the hurricane core.

14. The whole system may move slowly, at speeds of 15-20km/hour on average. Hurricanes start off
as very small areas of low pressure – often less than 10km in diameter. The eye itself can be up
to 40km wide.
Saffir-Simpson Scale
Structure of a Hurricane
A mature hurricane is roughly circular in shape and may be hundreds of miles across. The entire
feature rotates around a relatively calm center which is known as the eye of the hurricane. In the
northern hemisphere, hurricanes rotate in an anti-clockwise direction. Rain bands containing
massive cumulonimbus clouds spiral in toward the eye. These clouds form as a result of the
strong updraughts (rising air currents) within the hurricane. The rain bands are capable of
producing the very heavy rainfall associated with hurricanes. Some of the clouds tower up to a
height of 16km. Where the clouds are thickest, there is very little light, even at midday. At high
level, there is a canopy of cirrus clouds.
Strong winds spiral in towards the eye. Wind speed increases toward the center of the hurricane.
The strongest winds occur in a part of the hurricane which is known as the eyewall. The eyewall
is the part of the hurricane immediately surrounding the eye. Winds spiral into the eyewall and

15. then they spiral upward toward the top of the hurricane. At the top of the hurricane, winds spiral
outward.
At the center of a hurricane is the eye. The eye of a hurricane is the “hole” in the middle of the
hurricane. A typical hurricane has an eye which is about 20 – 40 miles (32 – 64km) across.
Conditions within the eye are relatively calm. Winds are light and there is little or no rain.
Within the eye, air is descending. The sky may be clear. As the eye of a hurricane passes over an
area, people who don’t know better may think the hurricane has passed.
Structure of a Hurricane

16. Description of the weather associated with a hurricane
Before
The approach of a hurricane is usually indicted by the appearance of feathery cirrus clouds at
great height. Then a veil of cloud appears across the sun, and the sky is bright red at dawn and at
sunset. The air is calm. Humidity is high, so the air feels hot and sticky.
During
As the hurricane gets closer there are occasional gusts of wind and showers of rain. During the
hurricane the wind speed is very strong and the rainfall is very heavy. To the west of the eye, the
wind blows from the north. The heavy rains can cause serious flooding and the heavy winds can
damage infrastructure. When the eye of the hurricane passes overhead, there will be a short
period of calm. After the eye has passed, the wind suddenly starts again. To the east of the eye,
it blows from the opposite direction- from the south. Heavy rain may continue for some time.
After

17. With the passage of the hurricane the winds gradually weaken, but steady rain can continue for
several days.
How Hurricanes die
Hurricanes derive their energy from warm tropical seas. They are weakened when they pass over
land. Even crossing an island the size of Jamaica will reduce the strength of a hurricane.
Some tropical storms and hurricanes lose their strength while they are still over warm tropical
seas in the Caribbean or Atlantic. This may happen when upper-level winds disrupt the outward
spiral of air above the hurricane.
Hurricane on weather maps

18. On weather maps hurricanes are represented by circular pattern of isobars with the lowest
pressure in the center (eye). The isobars are packed closely together and this tells us that winds
are likely to be strong.
Anticyclone
An anticyclone is a large area of high atmospheric pressure. Anticyclone may be thousands of
kilometers in diameter. In an anticyclone the air tends to sink or descend. As the air descends or
sinks, it is compressed and warmed (this means it can hold more moisture). These conditions are
not suitable for cloud formation and clearly is not a condition which favors rainfall. Therefore,
anticyclone are associated with long periods of sunshine or fair weather with few or no clouds
and no rainfall. Periods of fine settled weather are generally associated with anticyclones
Figure 8.8 shows that on weather maps anticyclones are represented by a roughly circular pattern
of isobars. The highest pressure is found in the middle of the anticyclone, and is lowest at the
edges. Winds move around the anticyclone in a clockwise direction in the northern hemisphere.
As indicated by the wide spacing of the contours in figure 8.8, differences in pressure are slight.
As a result, winds are very light.

19. The Intertropical Convergence Zone (ITCZ)
The name Intertropical convergence zone (ITCZ) is given to the zone low of pressure which
occurs roughly at the equator. The North East Trade Winds, and the South East Trade Winds
blow from the sub-tropical high pressure belts and converge there. Where these two air masses
come into contact they force each other to rise. On rising the air is cooled, thus creating
conditions favorable for to rainfall.
The conditions which occur at the ITCZ depend to a great extent upon the angle at which the two
trade winds converge. If the angle is small, then only a limited amount of air rising takes place
and the weather is likely to be fine. But if the angle is large, a great deal of air rising occurs. This
results in the formation of a dense layer of cloud, and rainfall is likely to be heavy and
prolonged.
The ITCZ is not stationary, but moves slowly northwards and southwards. It follows with a lag
of 1-2 months the passage of the overhead sun. The sun is at its furthest position south on
December 22. At this time the ITCZ is moving southwards across coastal Guyana. December is
one of the wettest months in the coastal belt of Guyana. In the Caribbean region the ITCZ
reaches its most southerly position in February, when it is located over Southern Guyana (20
C).
It then starts to move back northwards again. The sun is at its furthest point north on June 21st
.
The ITCZ reaches its most northerly position in about August, when it is centered around
latitudes 100
N to 120
N. This helps to explain why Trinidad and Tobago gets a great deal of
rainfall at this time.
ITCZ- FROM NASA