Other Instruments

A different device for measuring humidity is used in radiosonde equipment (Section 1.6). It involves materials which are hygroscopic (or water-absorbing), such as lithium chloride, deposited in a thin layer on a slip of glass. Its electrical resistance is lowered when it absorbs water in a damp atmosphere, and this alters the transmitted radio signal, indicating the relative humidity of the air around the balloon up to several kilometres aloft. The water-absorption process is reversible, but sluggish above about 6 km, where temperatures are below -20°C. So water-vapour concentrations in the upper

Vapor Hpa Licl

Figure 6.6 A psychrometric chart, to determine humidity variables (plus temperature) from any set of two known variables at a pressure of 1,000 hPa. Temperature is on the horizontal axis, vapour pressure on the vertical. The slanted, curved lines indicate relative humidity, and the diagonal straight lines show wet-bulb temperature.

Figure 6.6 A psychrometric chart, to determine humidity variables (plus temperature) from any set of two known variables at a pressure of 1,000 hPa. Temperature is on the horizontal axis, vapour pressure on the vertical. The slanted, curved lines indicate relative humidity, and the diagonal straight lines show wet-bulb temperature.

troposphere are measured nowadays by means of satellites.

Dewpoint hygrometers are used for accurate automated measurement of humidity. They determine the temperature of a tiny metal mirror, which is cooled electrically until the reflection of a laser beam is scattered by the formation of dew on the mirror.

An alternative to measuring atmospheric moisture is estimation, which can be done approximately by means of Table 6.1. This was derived empirically from measurements of daily extreme temperatures and dewpoints at many places, on the basis that nocturnal cooling (to the minimum) depends chiefly on the previous daily maximum and on the air's moisture (Section 3.4). The notion can then be turned into the proposition that the air's moisture content is roughly indicated by the daily extremes.

Table 6. 1 Approximate estimation of the dewpoint from the daily extreme temperatures, based on monthly mean data from 127 places worldwide; values in italics at the bottom right have been extrapolated

Daily max. Daily minimum temperature (°C)

temperature

Table 6. 1 Approximate estimation of the dewpoint from the daily extreme temperatures, based on monthly mean data from 127 places worldwide; values in italics at the bottom right have been extrapolated temperature

16

-

-

-

-

-

-

-

-

-

-

-

18

-

7

10

-

-

-

-

-

-

-

-

20

5

7

9

12

-

-

-

-

-

-

-

22

-

6

9

11

14

16

-

-

-

-

-

24

-

5*

8

10

13

15

18

-

-

-

-

26

-

-

7

9

12

14

17

20

-

-

-

28

-

-

5

8

11

13

16

19

22

24

-

30

-

-

-

7

9

12

14

18

20

23

26

32

-

-

-

5

8

11

13

16

19

22

25

34

-

-

-

-

6

9

12

15

18

21

24

36

-

-

-

-

5

7

10

13

16

19

22

38

-

-

-

-

-

5

7

10

13

17

20

40

-

-

-

-

-

-

5

8

11

14

17

* That is, the dewpoint is approximately 5°C when the daily maximum is 24°C and the minimum 8°C

* That is, the dewpoint is approximately 5°C when the daily maximum is 24°C and the minimum 8°C

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