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Using Temperature-Dew Point Tables

 

Most of the time, temperature and humidity data derived from field site observations will have to be computed by the meteorologist into a usable format. Field data will usually consist of wet and dry bulb readings from a Field Observer. In order for the meteorologist to convert these readings into temperature and relative humidity data, the Relative Humidity and Dew Point tables must be used. Care must also be taken to insure the correct table for the correct elevation is used. Once initial dew point and humidity calculations are made, forecast afternoon humidities can also be determined when the air mass over the fire site remains stationary or static. It becomes the responsibility of the meteorologist to solicit and verify the accuracy of forecast fire weather parameters as these are used, by the Fire Behavior Analyst, to forecast fire behavior and develop a safe and effective fire fighting plan.
The table below is an expanded portion of a relative humidity and dew point calculation table to be used in this exercise.

WET BULB TEMPERATURE
40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63
61 -23 -4 5 12 18 23 27 30 34 36 39 42 44 46 48 50 52 54 56 58 59 61
3 7 11 15 19 23 27 31 36 40 44 49 54 58 63 68 73 78 84 89 94 100
62 -44 -12 1 9 15 20 25 29 32 35 38 41 43 45 48 50 52 54 55 57 59 60 62
1 4 8 12 16 20 24 28 32 37 41 45 50 54 59 64 69 74 79 84 89 95 100
63 -23 -5 5 12 18 23 27 31 34 37 39 42 44 47 49 51 53 55 56 58 60 61 63
2 6 10 14 17 21 25 29 33 38 42 46 51 55 60 64 69 74 79 84 89 95 100
64 -50 -13 0 9 15 21 25 29 32 35 38 41 43 46 48 50 52 54 56 57 59 61 62
1 4 8 11 15 19 23 26 30 34 39 43 47 51 56 60 65 70 74 79 84 89 95
65 -25 -5 5 12 18 23 27 31 34 37 40 42 45 47 49 51 53 55 57 59 60 62
2 6 9 13 16 20 24 28 31 35 39 44 48 52 56 61 65 70 75 80 85 90
75 -25 -4 6 14 20 25 29 33 36 39 42 45 47 49 52 54 56
2 4 7 10 12 15 18 21 24 27 31 34 37 40 44 47 51
DRY BULB 83 -16 0 10 17 23 28 32 35 39 42 44 47 50
TEMPERATURE 2 4 6 9 11 13 16 18 21 23 26 29 31
Determining Relative Humidity from the Tables:

1. Select the proper table for the site elevation.

2. Secure the dry and wet bulb readings for the site.

3. Find the wet bulb temperature at the top of the table, then move down the column.

4. Find the dry bulb temperature at the left side of the table, then move horizontally to the right.

5. Find the intersection of the wet and dry bulb columns, which is a block with two numbers.

6. The lower number (blue) in the block is the relative humidity, the upper number (red) is the dew point temperature.

 
Try the following examples!
1. What is the relative humidity if the wet bulb temperature is 50 degrees F and the dry bulb temperature is 61 degrees F?
Answer
2. What is the dew point temperature for the above readings?
Answer
3. With a dry bulb temperature of 75 degrees F and a wet bulb temperature of 61 degrees F, the dew point temperature and relative humidity are?
Answer
Another use for the dew point temperature is to determine the relative humidity at various temperatures within a stable or stationary air mass. This can be used to determine the minimum relative humidity for the afternoon using the predicted high temperature for the day and the computed morning dew point temperature.

4. At 8:45 am the dry bulb temperature was 61 degrees F and the relative humidity was 44%. What will the minimum afternoon relative humidity be with a forecasted afternoon high temperature of 83 degrees F?

Answer

Please refer any questions or comments about this web site to: jim.prange@noaa.gov