temperature ► Temperature is a measure of how hot or cold something is. It is measured using a thermometer, which consists of a glass tube containing a liquid. The liquid expands when the temperature rises and contracts when the temperature falls. Sometimes the liquid used is alcohol containing a red dye, but many scientific thermometers use mercury. The glass tube is calibrated so that the temperature is easy to read off. Scientific temperature measurements are made using the Celsius scale, where 0°C is the temperature at which water freezes and 100°C is the temperature at which water boils.
For recording climatic temperature data, it is usual to use maximum and maximum thermometer ► minimum thermometers. The maximum thermometer records the highest temperature reached during the period of measurement and the minimum minimum thermometer ► thermometer records the lowest temperature. Inside each of the glass tubes of the
thermometers, there is a small piece of glass called an index.
index glass tube The maximum thermometer contains mercury and when the
temperature rises, the mercury pushes the index upwards. When
,,
o o ,o p b o ° the temperature falls, the index is left behind and the maximum
temperature reached can be read by looking at the lower end of the index and reading the figure from the scale.
mercury
Note: The maximum temperature was 35°C The minimum thermometer contains alcohol. The alcohol expands
as the temperature rises and it rises up the tube, flowing past the
index glass tube index. When the temperature falls, the alcohol contracts and the
index is dragged down the tube. To read the minimum temperature,
x
0 x w . . ro the position of the end of the index closest to the edge of the alcohol
is used.
When readings have been taken, the thermometers need to be
alcohol
Note: The minimum temperature has been 20°C shaken to restore the index to the level of the fluid. Figure 9.2 A maximum thermometer (top) and a The Stevenson screen
minimum thermometer (bottom). If temperature comparisons between different areas are to be made, then thermometers have to be kept in standard conditions. For this purpose, the thermometers are kept in a Stevenson screen.
A Stevenson screen has the following features: • it is painted white to reflect the
sunlight
• it has louvred sides allowing air to flow freely around the instruments • it should have a double roof;
the air space created is a poor conductor of heat and the effect of the heat from the sun will be less • it is located on a grassy surface
well away from trees and buildings • it should stand 112 cm above
ground level to decrease the effect of heat conduction from the ground.
Stevenson screens often contain other recording instruments, such as wet wet and dry bulb hygrometers ► and dry bulb hygrometers to measure
humidity.
Figure 9.3 A Stevenson screen for housing weather-recording instruments. 128
Practical activity:
Use the internet to find your local weather forecast and the forecast for the Caribbean region. How does the forecast weather differ across the region?
,tors. meteorological organisations ►
9.3 Interpreting weather records
The measurements made by meteorologists are used to forecast weather, either on a short-term basis or over longer periods. Short-term forecasts are usually made for periods of 5 to 7 days, but weather patterns can change quickly and farmers should check the forecast on a daily basis, particularly if considering harvesting or planting.
Observations are collected by the Caribbean meteorological organisations and farmers obtain weather reports using the internet, radio and television. Certain symbols, recognised internationally, are used to indicate the weather on maps. Some of these are illustrated in Figure 9.4.
Interpreting symbols on weather maps is relatively straightforward. The symbols for rainfall and wind speed indicate the quantity and nature of the rainfall expected and the severity of the winds.
(a) Total cloud cover
0
C-.-
Cill
(111 Ell 4111 0 • 0
No cloud 1/8 o less 1/4 cloud 3/8cloud 1/2 cloud 5/8cloud 3/4 cloud 7/8 cloud No blue Sky
cover cover cover cover cover cover cover sky obscured
(b) Rainfall
•
Light and intermittentV
Showers• •
Light and continuousA
Hail••
Moderate and intermittent , Drizzlet
o Moderate and continuous
R.
Thunderstorm•••
Heavt and intermittent . .R.
Thunderstorm with hail••• ••
Heavy and continuous — Fog=
Mist — Haze(c) Wind strength Symbol Speed (knots) Speed (km/hr) Symbol Speed (knots) Speed (km/hr)
0 Calm Calm III 0 35 65 Less than 5 Less than 9 11 11 0 40 74
O
5 9 1111r 0 45 83 0 1 0 10 19 1 Q 50 93 15 28 ∎, 0 55 102 Q II Q 20 37 ∎\ Q 60 111 II, 25 46 k\' Q 65 120 Q III 30 56 1\\ 0 70 130 Q (d) Fronts Symbol System Cold front p Warm front Quasi-stationary frontIntertropical Convergence Zone (narrow zone, one area of activity) Il
Ca
Intertropical Convergence Zone (wide zone, two areas of activity)Note: Each half feather — - 5 knots A whole feather I--- -10 knots A shaded triangle L— - 50 knots
Figure 9.4 Some international weather symbols representing cloud cover rainfall, wind strength and fronts.
129
0 Q Dominica Barbados 0 Piarco Trinidad Guyana Section B: Crop Production
Fronts
A front forms where different air masses meet. The symbols for fronts indicate the boundaries between air masses that have different properties. The Caribbean is affected by two air masses:
polar air mass ► • the polar air mass which originates in Canada and the USA in winter and moves southward to the Caribbean
tropical maritime air mass ► • the tropical maritime air mass consisting of warm, moist air in the Gulf of Mexico and the Caribbean Sea.
cold front ► Where the cold air mass meets the warm air mass, there is a cold front at the edge warm front ► of the polar air and a warm front at the edge of the tropical air. When cold air
meets the warm, moist Caribbean air, it causes the warm air to rise. The warm air is cooled, condensation takes place and rain clouds form. As a result, there is heavy rainfall, thunderstorms, the temperature decreases and the wind changes speed
Name THREE sources of information about the and direction. This type of weather is called the Northers because it originates
weather. from North America.
As the cold air mass continues to move southwards, it becomes warmer until eventually there is little difference between the temperatures of the two air masses. Under these conditions, the cold air mass does not move much, the weather quasi-stationary front ► conditions are more stable and a quasi-stationary front is established.
r The ITCZ
Describe what happens when the polar air mass
1
When two major air streams (the North East Trades and the South East Trades)
meets the tropical maritime air mass. meet, rain and thunder showers are produced. This happens because the air of the South East Trades is cooler than that of the North East Trades and causes the warmer air to rise and bring about the wet conditions.
Intertropical Convergence Zone Where these two air streams meet is called the Intertropical Convergence Zone (ITCZ) ► (ITCZ). This zone moves northwards during the northern summer, bringing heavy rain to Trinidad and Tobago from June to August. During winter in the northern hemisphere, the wind systems shift south of the Equator and from January to May areas north of the Equator have their dry season.
Gulf of Mexico V Miami
o^ 0
'a
N 0 500 Km Tropic of Cancer - -- -- --- --- --- --- 23½'N O o 2 0 0 ATLANTIC OCEANCuba o NE Trade Winds
o Jamaica Caribbean Sea Key t Warm air E'er Cold air o a
Areas of convergence of air 1 NE Trades and SE Trades 2 Cold air from N. America
and warm air from the Tropics
Figure 9.5 Convergence zones in the Caribbean.
Name TWO major effects of heavy rainfall on farming.
clearing ►
9.4 Weather records and farming decisions
Effects of heavy rain
leaching ► During the wet season, nutrients are lost from the soil by leaching. In order to maintain the fertility of the soil, farmers have to add fertilisers and manure, together with using cultivation methods such as rotation of crops. Application of fertilisers has to be timed to avoid heavy rainfall and fit in with the growth of the crop. A farmer would be wasting money if he added fertiliser when heavy rainfall was due — much of it would be leached.
Heavy seasonal rainfall floods low-lying areas and this can disrupt farming activities. For example, if heavy rains come at the end of the sugar cane harvesting season, then it becomes difficult to use heavy machinery. The fields are muddy, the soil is churned up and the crop may be spoilt. To overcome this problem, extensive drainage schemes have been set up in areas that are likely to be flooded. Often, drainage channels can serve two purposes: removal of excess water in the rainy season and irrigation in the dry season.