Dew is formed whenever falling temperature of the air passes the point where saturation occurs. The reduction of the temperature of air raises the relative humidity because of the diminished capacity to contain moisture. As the temperature declines there will come a point at which the air is saturated and any further decrease of temperature will cause supersaturation. At this point the moisture will be deposited on the cooler surfaces in the form of drops. The temperature at which dew begins to form is known as the dew-point. The sweating of cold water pipes, the dew that forms on a water glass and other relatively cold surfaces is caused by a temperature below the dew-point of the air.

The temperature at which dew forms will depend on the

Fig. 161. Swiss cottage Barometer.This device is arranged to show the condition of atmospheric humidity by the movement of the images. It is not really a barometer.

Fig. 161. - Swiss cottage " Barometer." This device is arranged to show the condition of atmospheric humidity by the movement of the images. It is not really a barometer.

Dew-point Table

Dew-point in degrees Fahrenheit, barometer pressure 29 inches

Air temp, t

Vapor press, e

Depression of wet-bulb thermometer (t-t')

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

30

0.164

27

25

22

18

14

9

+3

-5

-20

31

0.172

29

26

23

20

16

11

5

-2

-14

-50

32

0.180

30

27

24

21

17

13

8

+ 1

-9

-29

33

0.187

31

28

25

22

19

15

10

3

-5

-20

34

0.195

32

29

27

24

20

16

12

6

-2

-14

-50

35

0.203

33

30

28

25

22

18

14

8

+ 1

-8

-28

36

0.211

34

31

29

26

23

20

15

11

4

-4

-19

37

0.219

35

32

30

27

24

21

17

13

7

-1

-12

-44

38

0.228

36

33

31

28

26

23

19

14

9

+3

-7

-25

39

0.237

37

34

32

29

27

24

21

16

12

6

-3

-16

40

0.247

38

35

33

31

28

25

22

18

14

8

+ 1

-10

-35

41

0.256

39

37

34

32

29

26

23

20

16

11

4

-5

-21

42

0.266

40

38

35

33

30

28

25

21

17

13

7

-1

-13

-59

43

0.277

41

39

36

34

31

29

26

23

19

15

10

+3

-7

-28

44

0.287

42

40

38

35

32

30

27

24

21

17

12

6

-2

-17

45

0.298

43

41

39

36

34

31

29

26

22

19

14

8

+2

-9

-37

46

0.310

44

42

40

37

35

32

30

27

24

20

16

11

5

-4

-20

47

0.322

45

43

41

39

36

34

31

28

25

22

18

13

8

+0

-12

-53

48

0.334

46

44

42

40

37

35

32

30

27

23

20

15

10

+4

-6

-25

49

0.347

47

45

43

41

39

36

34

31

28

25

21

17

13

7

-2

-15

50

0.360

48

46

44

42

40

37

35

32

29

27

23

19

15

9

+2

-8

51

0.373

49

47

45

43

41

39

36

34

31

28

25

21

17

12

6

-3

52

0.387

50

48

46

44

42

40

37

35

32

29

26

23

19

11

9

+ 1

53

0.402

51

49

47

45

43

41

39

36

34

31

28

24

21

16

11

5

54

0.417

52

50

49

47

44

42

40

38

35

32

29

26

23

19

14

8

Dew-point Table (Continued)

Dew-point in degrees Fahrenheit, barometer pressure 29 inches

Air temp, t

Vapor press, e

Depression of wet-bulb thermometer (t-t')

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

55

0.432

53

52

50

48

46

43

41

39

36

34

31

28

24

21

16

11

56

0.448

54

53

51

49

47

45

43

40

38

35

32

29

26

23

19

14

57

0.465

55

54

52

50

48

46

14

42

39

36

34

31

28

24

21

16

58

0.482

56

55

53

51

49

47

45

43

40

38

35

32

29

26

22

18

59

0.499

57

56

54

52

50

48

46

44

42

39

37

34

31

28

24

20

60

0.517

58

57

55

53

51

49

47

45

43

41

38

35

32

29

26

22

61

0.536

59

58

56

54

52

51

49

46

44

42

39

37

34

31

28

24

62

0.555

60

59

57

55

54

52

50

48

46

43

41

38

35

32

30

26

63

0.575

61

60

58

56

55

53

51

49

47

45

42

40

37

34

31

28

64

0.595

62

61

59

58

56

54

52

50

48

46

44

41

38

36

33

30

65

0.616

63

62

60

59

57

55

53

51

49

47

45

43

40

37

34

31

66

0.638

64

63

61

60

58

56

54

53

51

48

46

44

42

39

36

33

67

0.661

65

64

62

61

59

57

56

54

52

50

48

45

43

40

38

35

68

0.684

67

65

63

62

60

58

57

55

53

51

49

47

44

42

39

36

69

0.707

68

66

64

63

61

60

58

56

54

52

50

84

46

43

41

38

70

0.732

69

67

66

64

62

61

59

57

55

53

51

49

47

45

42

40

71

0.757

70

68

67

65

63

62

60

58

57

55

53

51

49

46

44

41

72

0.783

71

69

68

66

65

63

61

60

58

56

54

52

50

48

45

43

73

0.810

72

70

69

67

66

64

62

61

59

57

55

53

51

49

47

44

71

0.838

73

71

70

68

67

65

64

62

60

58

56

54

53

50

48

46

75

0.866

74

72

71

69

68

66

65

63

61

60

58

56

54

52

50

47

76

0.896

75

73

72

70

69

67

66

64

62

61

59

57

55

53

51

49

77

0.926

76

74

73

71

70

68

67

65

64

62

60

58

56

54

52

50

78

0.957

77

75

74

72

71

69

68

66

65

63

61

59

58

56

54

52

79

0.989

78

76

75

73

72

70

69

67

66

64

62

61

59

57

55

53

80

1.022

79

77

76

75

73

72

70

69

67

65

64

62

60

58

56

54

amount of moisture present in the air, but with a definite humidity and air pressure it will always occur at the same temperature. If the dew-point is above freezing, the dew will form as drops of water, but if it is at or slightly below the freezing point, the dew will appear as frost. White frost is formed when the dew-point is only a few degrees below the freezing point. A Black frost occurs when the atmospheric humidity is so low that dew does not form until the temperature is much below the freezing point.