Humidity_chapnumber0Humidity

9.1 General Description

Wyoming's annual average relative humidity (RH) is quite low (Figure 9.1) and is particularly low in summer. During the warmer part of the summer days, the humidity drops to about 25 to 30% and on a few occasions it will be as low as five to 10%. Late at night, when the temperature is lowest, the humidity will generally rise to 65 or 75%. This results in an average diurnal variation of about 40 to 45% during the summer, but in the winter the variation is much less. Low RH, high percentage of sunshine, and rather high average winds all contribute to a high rate of evaporation. Similarly, with dew point temperatures generally in the upper 40s during the summer, Wyoming offers its residents and visitors a break from the sweating and discomfort. Lower dew point temperatures assure cool summer nights (Figure 9.2). For average annual dew points, see Figure 9.2a.

Figure 9.1

Figure 9.1. Annual average relative humidity (%) (PRISM, 1961-90)

Since RH is a function of temperature (assuming there is no additional change in the initial amount of water vapor in the air), outside air heated indoors will reduce RH to values well below 10% in winter, especially on the coldest nights. The use of a humidifier is the easiest way for adding moisture and thus increasing comfort to the otherwise drying effects on our skin and respiratory system. While static electricity is a common annoyance with these very low RH values, it can also be a potentially dangerous problem for those working in coalbed methane mines, where static discharge from clothing could set off an explosion. These conditions, while uncommon, do occur over lower elevations across the state in the summer.

Figure 9.2

Figure 9.2. July average dew point temperature (oF) (PRISM, 1961-90)

Figure 9.2a

Figure 9.2a. Annual average dew point temperature (oF) (PRISM, 1961-90)

9.2 Hourly Humidity Data

9.2.1 Mean RH

In Figure 9.3, Figure 9.4, Figure 9.5, and Figure 9.6, mean hourly RH by month for Casper, Cheyenne, Lander, and Sheridan from 1961-90 reveals that Cheyenne and Sheridan have similar periods of minimum and maximum. Minimum occurs in the afternoon during mid-summer and the maximum occurs in the early morning hours of mid-spring. Casper and Lander also show typical RH minimums in the summer but differ on their period of maximum RH.

Figure 9.3

Figure 9.3. Casper mean hourly RH (%) based on observations from 1961-1990. Annual minimum and maximum values are depicted.

Figure 9.4

Figure 9.4. Same as Figure 9.3 except for Cheyenne

Figure 9.5

Figure 9.5. Same as Figure 9.3 except for Lander

Figure 9.6

Figure 9.6. Same as Figure 9.3 except for Sheridan

9.2.2 Relative Humidity Frequency

In Figure 9.7, Figure 9.8, Figure 9.9, and Figure 9.10, RH frequencies by month for Casper, Cheyenne, Lander, and Sheridan are shown based on daily average observations from 1950-1996. Generally, the highest frequencies of occurrence are winter (70% RH) and summer (40% RH) with the exception of Cheyenne where the highest frequency occurs generally between an RH of 45 to 65% throughout the year.

Figure 9.7

Figure 9.7. Casper RH frequency (%) by month based on hourly observations taken from 1950 to 1996

Figure 9.8

Figure 9.8. Same as Figure 9.7 except for Cheyenne

Figure 9.9

Figure 9.9. Same as Figure 9.7 except for Lander

Figure 9.10

Figure 9.10. Same as Figure 9.7 except for Sheridan

9.2.3 Mean Dew Point Temperature

In Figure 9.11, Figure 9.12, Figure 9.13, and Figure 9.14, mean dew point temperatures by month for Casper, Cheyenne, Lander, and Sheridan from hourly observations from 1961-1990 are characterized by generally constant values across the day for a given month and are highest during summer mornings (mid to upper 40s).

They are lowest during winter mornings (single digit F). The importance of dew point temperatures, besides indicating at what temperature dew or frost will form (equating to 100% RH), is that other moisture calculations can be performed.81 Weather instruments cannot ecomonically measure all atmospheric elements. However, with a number that is directly measured, such as dew point, others can be derived accurately.

Figure 9.11

Figure 9.11. Casper average hourly dew point temperature (degree F) by month (1961-1990). Annual minimum and maximum values are denoted.

Figure 9.12

Figure 9.12. Same as Figure 9.11 except for Cheyenne

Figure 9.13

Figure 9.13. Same as Figure 9.11 except for Lander

Figure 9.14

Figure 9.14. Same as Figure 9.11 except for Sheridan

9.2.4 Dew Point Temperature Frequency

In Figure 9.15, Figure 9.16, Figure 9.17, and Figure 9.18, dew point temperature frequencies by month for Casper, Cheyenne, Lander, and Sheridan are shown based on hourly observations from 1961-1990. Generally, the highest frequencies of occurrence are in spring, fall (20-30 F range), and summer (40-50 F range).

Figure 9.15

Figure 9.15.. Casper dew point temperature frequencies by month based on hourly observations taken from 1961-1990

Figure 9.16

Figure 9.16. Same as Figure 9.15 except for Cheyenne

Figure 9.17

Figure 9.17. Same as Figure 9.15 except for Lander

Figure 9.18

Figure 9.18. Same as Figure 9.15 except for Sheridan

9.3 Monthly Humidity Data

Data in Table 9.A. and Table 9.B. are based on monthly averages of morning (5AM) and evening (5PM) local time observations. Average morning relative humidity values are based on early morning readings when the highest values generally occur.

Average afternoon relative humidity values are based on late afternoon readings when the lowest values generally occur. Wet bulb temperature (Table 9.C.) is defined as the lowest temperature that can be obtained by evaporating water into the air. Dew point (Table 9.D.) is the temperature in which air must be cooled (at constant pressure and constant water vapor content) for saturation to occur. When the dew point falls below freezing it is called the frost point.

Table 9.A. Mean monthly and annual percent RH (%) (morning)

 

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

ANN

Casper

68

70

73

75

77

75

69

66

68

67

69

68

70

Cheyenne

57

61

65

68

71

71

70

69

67

61

60

58

65

Lander

68

68

66

65

65

61

54

54

59

63

69

68

63

Sheridan

69

72

73

73

76

77

72

68

71

70

73

70

72


Table 9.B. Mean monthly and annual percent RH (%) (afternoon)

 

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

ANN

Casper

60

57

48

41

39

32

26

24

30

40

56

62

43

Cheyenne

49

48

47

42

44

41

38

38

38

41

50

52

44

Lander

60

54

45

39

36

31

27

26

32

40

56

62

42

Sheridan

64

60

50

43

46

44

33

30

38

45

61

65

48


Table 9.C. Mean monthly and annual wet bulb temperatures (F)

 

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

ANN

Casper

21

23

30

36

45

52

56

54

47

35

28

22

37

Cheyenne

23

24

29

35

45

52

52

52

44

35

26

21

36

Lander

17

21

29

36

44

51

55

53

46

37

26

16

36

Sheridan

21

23

31

38

47

55

55

63

46

36

26

19

37


Table 9.D. Mean Monthly and Annual Dew Point Temperatures (F)

 

JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

DEC

ANN

Casper

14

16

22

26

35

42

45

42

36

25

20

14

28

Cheyenne

12

15

20

26

36

44

44

44

36

26

18

12

28

Lander

10

13

19

25

33

39

42

40

34

26

17

9

26

Sheridan

14

15

22

28

37

44

46

43

37

29

20

13

29

9.4 Higher Frequency Data

Although ASOS provides more frequent observations than once per hour, these data are often difficult to obtain. However, WYDOT's observations are generally available several times per hour and are more readily available.82 In Figure 9.19, the annual frequency of RH for Gillette is provided to illustrate the signatures of drought and moist years. As noted, bars for the drought years of 2000-2001 show a higher frequency of lower humidity values than the bars for the wetter years of 1996-1997 that show a higher frequency of higher humidity values.

Figure 9.19

Figure 9.19. Relative Humidity from WYDOT's Gillette reporting station based on two-yearly, 20-minute observations from 1994 to 2001


81#. http://www.srh.noaa.gov/elp/wxcalc/wxcalc.shtml

82#. see CD, appendix_data, text, WYDOT folder



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Last Modified: Fri, 17 Oct 2008