Seasonal and Latitudinal Variations of Surface Temperature

Variations of the incoming solar radiation with season and latitude cause corresponding changes in surface temperatures which are observed all over the globe. An example is presented Fig. 8.6 for two stations in Asia; an equatorial station in Borneo and Beijing near 40 °N latitude.

It is evident from Fig. 8.6 that the amplitude of the seasonal oscillation is close to 1 °C in Borneo, whereas it is about 15°C at Beijing.

WINTER VERNAL SUMMER AUTUMNAL WINTER

SOLSTICE EQUINOX SOLSTICE EQUINOX SOLSTICE

DEC FEB MARCH MAY JUNE AUG SEPT NOV DEC

WINTER VERNAL SUMMER AUTUMNAL WINTER

SOLSTICE EQUINOX SOLSTICE EQUINOX SOLSTICE

DEC FEB MARCH MAY JUNE AUG SEPT NOV DEC

Seasonal Solar Radiation Surface

LONGITUDE OF THE SUN

Fig. 8.4 Receipt of solar radiation at the earth's surface in Langley (Ly) per day with no loss in the atmosphere (1Ly day"1 = 0.484 Wm"2) (After Milankovich)

LONGITUDE OF THE SUN

Fig. 8.4 Receipt of solar radiation at the earth's surface in Langley (Ly) per day with no loss in the atmosphere (1Ly day"1 = 0.484 Wm"2) (After Milankovich)

8.6.5 Diurnal Variation of Radiation with Clear and Cloudy Skies

We are all familiar with the diurnal cycle of temperature at a place. With the rising of the morning sun, air temperature goes up, reaches a maximum soon after local noon and then drops as the sun goes down in the evening. However, it is observed that more often than not this regular cycle is disturbed when there is excessive moisture in the atmosphere with clouds overhead. An example is given in Fig. 8.7 which shows the diurnal cycle of surface temperature at Washington, D.C. over a period of 5 days (from 1 to 5 July, 2003).

WINTER VERNAL SUMMER AUTUMNAL WINTER

SOLSTICE EQUINOX SOLSTICE EQUÎNOX SOLSTICE

DEC FEB MARCH MAY JUNE AUG SEPT NOV DEC

WINTER VERNAL SUMMER AUTUMNAL WINTER

SOLSTICE EQUINOX SOLSTICE EQUÎNOX SOLSTICE

DEC FEB MARCH MAY JUNE AUG SEPT NOV DEC

Seasonal Variation Examples
LONGITUDE OF THE SUN

Fig. 8.5 Same as in Fig. 8.4, but with an atmospheric transmissivity of 70%.

Fig. 8.5 Same as in Fig. 8.4, but with an atmospheric transmissivity of 70%.

Diurnal Variation Surface Temperature

2 3 JULY 5 2003 6

Fig. 8.7 Diurnal variation of surface temperature (TEMP) and dew-point (D.P) in °C at Washington, D.C., at 4-hourly intervals from 00 EST on 1 July to 24 EST on 5 July, 2003 (Daily maxima and minima are not marked)

2 3 JULY 5 2003 6

Fig. 8.7 Diurnal variation of surface temperature (TEMP) and dew-point (D.P) in °C at Washington, D.C., at 4-hourly intervals from 00 EST on 1 July to 24 EST on 5 July, 2003 (Daily maxima and minima are not marked)

It shows how the diurnal cycle of temperature was disrupted by an almost continuous rise of dewpoint from a value of about 17.7 °C late on 1 July to about 20.6 °C around noon the following day. The increase of the water vapour content of the atmosphere suppressed the amplitude of the diurnal cycle by causing the daytime maximum air temperature to come down and the nighttime minimum temperature to go up, while the dew-point rose rapidly. There was rainfall in the city during the period of moisture build-up between 1 and 2 July and the weather remained largely cloudy and disturbed till 4 July.

8.7 Reflection of Solar Radiation at the Earth's Surface - The Albedo

Not all the solar radiation that reaches the earth's surface is absorbed by it, since a part of the radiation is reflected back to space by what is known as the earth's surface albedo. The albedo is defined as the fraction of the incident radiation that is reflected by the surface. The earth's surface not being uniform, the albedo varies widely from place to place depending upon the nature and composition of the underlying surface. For example, the albedo of a dense forest is very different from that of a freshly-covered snow surface, or, for that matter from a given water surface at different solar elevations, as shown in the Table 8.1, which gives the approximate values of albedoes for a few selected types of surfaces (after Riehl, 1978).

Table 8.1 Typical values of earth's albedo, (After Riehl, 1978)

Surface type

Albedo, a (%)

Forests

3-10

Fields, green

3-15

Fields, dry, plowed

20-25

Grass

15-30

Bare ground

7-20

Sand

15-25

Snow, fresh

S0

Snow (old)/ice

50-70

Water, solar elevation > 40°

2-4

Water, solar elevation 30-5°

6-40

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