humidity

High humidity can be uncomfortable and may bring on a feeling of impending rain or storms. People sometimes complain: "It's not the heat, it's the humidity!" because in hot, humid weather, perspiration on your skin does not evaporate as quickly, hampering your body's efforts to cool down.

Monitoring the humidity is important because high humidity can have a great impact, particularly on sports people competing in events such as the marathon.

the instrument

The hygrometer has two thermometers mounted side-by-side. They have a temperature range of minus 5 to plus 50 degrees Celsius. One thermometer bulb is exposed to the air (the dry bulb) and the other (the wet bulb) is wrapped in a damp cloth.

The rate of evaporation and cooling depends on the humidity of the surrounding air. In a dry place, such as a desert, water evaporates quickly and cools the thermometer more than in humid places, such as in the tropics.

You can calculate the humidity by taking readings from the hygrometer, using some simple mathematics, and by referring to the table at right.

for example ...

Suppose the two readings from the hygrometer are:

• wet-bulb temperature: 10 degrees
• dry bulb temperature: 15 degrees
• the wet-bulb depression is: 15 - 10 = 5 degrees