CHAPTER 11

CLIMATE

Photograph courtesy Richard Edwards

Photograph courtesy Satee Boodoo

Photograph courtesy Richard Edwards

11 CLIMATE 11.1 Climate Trinidad and Tobago the two most southerly islands of the Eastern Caribbean have

two distinct and seasonal climatic types:

1. Tropical Maritime: - warm days and cool nights with rainfall mostly in the form of showers due to daytime convection. This typifies the dry

season months of January to May.

2. Modified Moist Equatorial: - low wind speeds with hot humid days and nights and a marked increase in rainfall, not always convective.

During this period, the islands come repeatedly under the influence of

equatorial weather systems.

The two climatic types result in two distinct seasons a dry season from January to

May and a wet season from June to December. Tobago, the more northerly of the

two islands, experiences drier seasons in comparison to Trinidad.

11.1.2 Climatic Determinants The main climatic determinants of Trinidad and Tobago are:

1. The latitudinal position and strength of the sub-tropical high pressure

(Bermuda-Azores High) and semi-permanent northern hemispheric

feature.

2. The Inter-Tropical Convergence Zone (ITCZ) a major rainfall

producing system, which is responsible for the rainy season.

3. The Mid-Atlantic Upper-level Trough an upper tropospheric feature

which assume prominence during the months of October to

December.

4. Orography, Land size and sea breeze effect

5. Tropical waves and cloud clusters

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11.2 Rainfall

The annual rainfall pattern in Trinidad is bimodal in distribution (see Figure 11.1). In

an average over the period 1971-2000, the first major peak occurred during June or

July and was associated with the northward movement of the ITCZ. The second

peak occurred in November. Tobago also has a bimodal rainfall distribution,

however, the major peak occurred in November and the secondary peak occurred in

June (see Figure 11.2)

FIGURE 11.1: COMPARISON OF THE LONG-TERM MONTHLY AVERAGES OF RAINFALL FOR THE STATIONS AT HOLLIS, NAVET AND PIARCO IN TRINIDAD, 1971- 2000

0

50

100

150

200

250

300

350

400

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Rai

nfal

l (M

illim

etre

s)

PiarcoHollisNavet

Source: Meteorological Services Division

287

FIGURE 11.2: COMPARISON OF THE LONG-TERM MONTHLY AVERAGES OF RAINFALL FOR THE STATIONS AT CROWN POINT AND HILLSBOROUGH DAM IN TOBAGO 1971 - 2000

0

50

100

150

200

250

300

350

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Rai

nfal

l (M

illim

etre

s)

Crown PointHillsborough

Source: Meteorological Services Division 11.2.1 Mean Annual Rainfall (Trinidad 1961 to 1990)

Map 11.1: Mean Annual Rainfall for Trinidad, 1961-1990 Climatologically, Trinidad had a

mean annual rainfall (1961 to

1990) maximum along the eastern

Northern Range, with the centre

on the eastern part of the range

between Sangre Grande and

Arima. The maximum also extends

southward on the eastern side of

the island through Rio Claro,

towards Moruga. The western side

of Trinidad is relatively dry in

comparison to the eastern side

(see Map 11.1), especially in the

southwestern peninsula. Source: Water Resources Agency/MET Office

288

The mean annual rainfall ranges from in excess of 3500mm centred in the Northern

Range, to values below 1500mm along the western coast of the island and along the

south coast to the south east of Siparia.

11. 2. 2 Trinidad (1999) Map 11.2: Isohyetal Map for Trinidad, 1999 The rainfall pattern for 1999

differs slightly from the mean

annual rainfall pattern. The

major difference is a minimum,

which was located to the south

east of San Fernando and on

the eastern side of the island

to the north west of Point

Radix. (see Map 11.2). Further

an area of maximum rainfall

was located to the north east

of Point Fortin.

Source: Water Resources Agency/MET Office

Map 11.3: Isohyetal Map for Trinidad, 2000

11. 2. 3 Trinidad (2000)

Trinidad, during the year 2000,

experienced similar rainfall patterns to

that of 1999, albeit with higher values.

The location of the maximum shifted

to the Central Range, where the

precipitation exceeded 3500mm (see

Map 11.3).

Source: Water Resources

Agency/MET Office

289

Map 11.4: Isohyetal Map for Trinidad, 2001

11. 2. 4 Trinidad (2001)

During the year 2001, the rainfall maximum

shifted to the eastern Northern Range as per the

mean annual pattern.However, due to the

extremely arid dry season, precipitation totals for

the year were lower than the mean especially on

the western side of the island (see Map 11.4).

Source: Water Resources Agency/MET Office

Map 11.5: Isohyetal Map for Trinidad, 2002 11. 2. 5 Trinidad (2002) Rainfall during 2002 was quite similar

to the mean in its major features. The

maximum was located along the

eastern Northern Ranges and its axis

was along the range. The western

side of the island continued to have

lower rainfall values in comparison to

eastern areas (see Map 11.5).

Source: Water Resources Agency/MET Office

290

Map 11.6: Mean Annual Rainfall for Tobago, 1971- 2000 11.2.6 Mean Annual Rainfall

(Tobago 1971 to 2000) Tobago had much lower rainfall than Trinidad;

the mean annual maximum rainfall for the

island exceeds 2500mm compared to 3500mm

maximum in Trinidad.

The maximum was along the Main Ridge to the

north of Roxborough. Along the southwest it

was relatively dry in comparison to the

northeast (see Map11.6)

Source: Water Resources Agency/MET Office

Map 11.7: Annual Isohyetal Map for Tobago, 1999

11. 2.7 Tobago (1999) There was more precipitation during 1999 when

compared to the mean annual rainfall for the

island. During 1999, Tobago measured in

excess of 3250mm in areas along the Main

Ridge. The precipitation tapered in the

southwest but the values were higher, with

measurements in excess of 1300mm (see Map

11.7) Source: Water Resources Agency/MET Office

Map 11.8: Annual Isohyetal Map for Tobago, 2000 11.2.8 Tobago (2000)

The maximum during 2000 was lower than the

maximum of 1999 and the mean, which was

indicative of a drier regime in Tobago during that

year. The maximum precipitation recorded was in

excess of 2250mm, which was much lower than

the value above 3250mm recorded in the previous

year and the mean in excess of 2500mm (see Map

11.8) Source: Water Resources Agency/MET Office

291

Map 11.9: Annual Isohyetal Map for Tobago, 2001 11. 2. 9 Tobago (2001)

During 2001, the location and amount of

maximum rainfall was almost the same as the

mean. However, in the south west of Tobago

precipitation was much less than the mean (see

Map11.9)

Source: Water Resources Agency/MET Office

Map 11.10: Annual Isohyetal Map for Tobago, 2002

11. 2. 10 Tobago (2002) There was an abundance of precipitation

during 2002, which enlarged the area of

higher than normal accumulation. This

maximum was spread over a larger area

when compared to the mean (1971 to 2000)

or in any of the previous three (3) years.

In the southwest of Tobago, the amount of

precipitation was much less than along the

Main Ridge, however, the areal extent was

smaller than the mean (see Map11.10).

Source: Water Resources Agency/MET Office

292

11.3 Temperature 11.3.1 (Trinidad, 1999 to 2004) Climatologically, the long-term mean maximum temperature measured at Piarco, for

the period 1971 to 2000 is 31.3C, whereas the long-term mean minimum

temperature for the same period is 22.7C and the mean daily temperature for the

same period is 26.5C.

During the period 1999 to 2004, the mean annual maximum, minimum and average

daily temperatures were consistently above the long-term (1971 to 2000) average.

The average yearly maximum for the period 1999 to 2004 was 0.3C to 1.0C above

the long-term mean maximum. The yearly mean minimum temperature ranged

between 0.6C to 1.0C higher than the long-term mean minimum, the same was true

for the daily average temperature, but the range was smaller from 0.2C to 0.6C

higher than the long-term daily average temperature (see Figure 11.3)

FIGURE 11.3: BAR CHART OF THE MEAN MAXIMUM, MINIMUM AND MEAN TEMPERATURES FOR PIARCO, TRINIDAD, 1999-2004

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

1999 2000 2001 2002 2003 2004

Tem

pera

ture

Mean MinMean Temp.Mean Max.

Source: Meteorological Office

293

11.3.2 Temperature (Tobago, 1999 to 2002) The long-term mean maximum temperature measured at Crown Point, Tobago, for

the period 1971 to 2000 was 30.5C, whereas the long-term mean minimum

temperature for the same period was 23.7C and the mean daily temperature for the

same period was 26.8C.

The mean maximum temperature for the period 1999 to 2002 exceeded the long-

term mean maximum (1971 to 2000) in every year, except in 2000, when the mean

was 0.1C lower. The mean minimum temperature for the period was higher than the

long-term mean minimum temperature (Figure 11.4), as was the mean temperature

except in 2000, when the mean yearly temperature equalled the long-term average.

FIGURE 11.4: BAR CHART OF THE MEAN MAXIMUM, MINIMUM AND MEAN TEMPERATURES FOR CROWN POINT, TOBAGO, 1999-2002

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

Year

Tem

pera

ture

Mean Min.Mean TempMean Max.

Mean Min. 24.5 24.2 24.7 24.1

Mean Temp 27.2 26.8 27.2 27.2

Mean Max. 30.7 30.4 30.9 30.7

1999 2000 2001 2002

Source: Meteorological Office

294

11.4 Relative Humidity and Sunshine Hours 11.4.1 Trinidad (1999 to 2004) Trinidad has an average (1971 to 2000) relative humidity of 82.2% and the average

number of sunshine hours per day is 7.2. The period 2001 to 2003 had more

sunshine hours per day than the average (see Figure 11.5), whereas the years of

2000, 2001, 2003 and 2004 were relatively drier than average.

Generally, relative humidity and sunshine hours have an inverse relationship; higher

numbers of sunshine hours leads to lower relative humidity values as shown in

Figure 11.5. The year with the most sunshine hours (2001) had the lowest relative

humidity. Concomitantly, the dry season of 2001 was extremely arid, with rainfall

stations reporting little or no rainfall in the period January to April.

FIGURE 11.5: AVERAGE SUNSHINE HOURS PER DAY AND AVERAGE RELATIVE HUMIDITY PER DAY FOR PIARCO, TRINIDAD, 1999-2004

7.0

7.2

7.4

7.6

7.8

8.0

Hou

rs

80.0

80.5

81.0

81.5

82.0

82.5

83.0P

rece

nt

Sunshine Hrs.Rel. Humidity

Sunshine Hrs. 7.2 7.1 7.9 7.5 7.8 7.2

Rel. Humidity 82.4 81.8 80.7 82.4 80.8 82

1999 2000 2001 2002 2003 2004

Source: Meteorological Office

295

11.4.2 Tobago (1999 to 2002)

Sunset in North Trinidad

Tobago had an average (1971 to 2000) relative humidity of 79.5% while the average

number of sunshine hours was 7.7%. Compared to the period 1971 to 2000, the

period 1999 to 2001 had more sunshine hours per day than the average at Crown

Point, as well as a higher humidity than average (see Figure 11.6).

FIGURE 11.6: A COMPARISON OF AVERAGE SUNSHINE HOURS PER DAY TO AVERAGE RELATIVE HUMIDITY PER DAY FOR CROWN POINT, TOBAGO, 1999-2002

7.0

7.2

7.4

7.6

7.8

8.0

8.2

Year

Hou

rs

80.0

80.2

80.4

80.6

80.8

81.0

81.2

Perc

ent

Sunshine 7.8 7.8 8.1 7.4

Rel. Humidity 81.1 81.1 80.6 80.7

1999 2000 2001 2002

Source: Meteorological Office

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11.5 Wind Direction and Wind Speed Generally, the average wind speed

over Trinidad and Tobago shows a

pattern consistent with the

establishment of the sub-tropical high

pressure closer to tropical latitudes

during the northern hemisphere

winter/spring months and a northward

migration of the high-pressure system

as the northern hemisphere moves

into summer and autumn.

Figure 11.7 shows that during the

months of January to May, Piarco and

Crown Point experienced a steady

increase in the mean wind speed, with

the peak in May. Thereafter, there was

a steady decrease in the wind speed

until December when the mean wind

started to increase.

FIGURE 11.7: MEAN WIND SPEED FOR PIARCO (TRINIDAD) 1961-1990 AND CROWN POINT (TOBAGO) 1971-1990

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

Month

Win

d Sp

eed

(kt)

Piarco 5.5 6.2 6.7 6.9 7.1 6.3 5.0 4.1 4.0 4.2 4.2 4.7

Crown Point 5.8 7.3 7.6 8.1 8.8 8.2 6.7 5.8 5.8 5.9 5.6 5.5

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Source: Meteorological Office

297

Trinidad and Tobago experiences a northward migration of the Equatorial Trough

during the month of August to November, which is characterized by weak easterly

wind speeds, high temperatures and localized torrential rainfall. During this period, a

sea breeze develops blowing from the Gulf of Paria inland. At times, the sea breeze

penetrates to the east of Arima. (see Appendix 11.1 for Wind Roses; Piarco -Trinidad

and Crown Point -Tobago)

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