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The Challenge: Predict the Weather

Why do meteorologists measure humidity?

Relative humidity is the ratio of water vapor in the air at a given temperature to the maximum amount that could exist at that temperature. On hot summer days, the higher the relative humidity, the greater the discomfort. It is harder to sweat and stay cool as perspiration evaporates less readily and the body feels more hot and sticky. If the humidity is high there is more moisture in the air and so more chance of clouds forming and rain falling if the temperature drops.

The forecaster bases the possibility of rain on current weather patterns, including wind and humidity, as well as the effect of terrain and long-term weather statistics.

Humidity meterHow can we make a humidity meter?

We used human hair to make our hygrometer. Hair is composed of many millions of cells. The cell walls are sensitive to moisture and will change shape and size as the moisture content in the air changes. Hair can therefore indicate the level of moisture in the air by expanding and contracting.

Our meter was made by simply attaching one end of a long strand of hair (15cm, or about 6 inches) to the base (bottom left hand corner but invisible in the picture) and the other end to the end of a stick pointer which was pivoted near to this point. To make the meter perform correctly a weight was attached just the other side of this pivot (string and nut). When the air is moist, the hair will expand and lengthen, making the pointer pivot down. When the air is dry, the hair will contract and shorten, making the pointer pivot up. When we made our regular hygrometer observations, we made a mark to indicate where the pointer rested. Over time, you'll be able to see the humidity patterns that will help you forecast the weather.