1. Dew Point and Relative Humidity
Using a Sling Psychrometer

2. Sling Psychrometer A double thermometer
One is ‘normal’ = dry bulb
One is covered with a wet piece of cloth = wet bulb
When water evaporates, it removes heat.
Therefore, the wet bulb will always record a lower temperature than the dry bulb.
Wet Bulb
Dry Bulb

3. Using the psychrometer
Wet the cloth at the end of one of the apparatus.
Carefully (!), gently (!), etc spin the psychrometer around for a few minutes.
Record the temperature of the dry bulb and the wet bulb in °C

4. Calculating Dew Point
Subtract the wet bulb temperature from the dry bulb temperature and record this value.
You should now have 3 values recorded:
Dry bulb ____ °C
Wet bulb ____ °C
Difference ____ °C

5. Calculating Dew Point, cont.
Open your ESRT to page 12.
Locate the dry bulb temperature on the vertical (left) axis of the Dew Point Temperature Chart.
Locate the difference between the wet and dry bulb temperature on the horizontal (top) of the chart.
Locate where the two values intersect: this is the dew point.

6. Calculating Relative Humidity (%)
Record and calculate the same three values as you did for dew point:
Dry bulb temp ___ °C
Wet bulb temp ___ °C
Difference ___ °C
Use the Relative Humidity chart on pg. 12 of your ESRT as you did for dew point.
The relative humidity is at the intersection of the dry bulb temperature and the difference values.