1 Lesson 02: Humidity: Water in the Air Pages

2 Humidity A measure of the amount of water vapor in the air. Measured using a hygrometer

3 Relative Humidity Measure of the amount of water vapor in the air compared to the maximum amount of water vapor the air can hold. Measured using a psychrometer

4 Factor affecting humidity: Temperature The humidity of an air mass depends a great deal on the temperature of the air. The chart below shows us the maximum amount of water that air can hold at different temperatures.

5 Humidity Chart

6 From this chart we can see that warm air can hold much more water vapor than cold air. Reason: warm air molecules are more spread, more space for water vapor to fit in between. As the air mass cools, the spaces between the air molecules begins to shrink reducing the amount of space for the water vapor.

7 What does humidity have to do with our level of comfort? Your body tries to maintain a constant temperature of 37°C at all times. In hot weather you sweat. The sweat then evaporates. When water evaporates it has a cooling effect. So it is not the sweating that cools your body but the evaporation of the sweat.

8 What does humidity have to do with our level of comfort? If humidity levels are high (high moisture content) sweat does not evaporate as easily. This is because there is already a great deal of water vapor in the air. The sweat remains on your body leaving you feeling hot and sticky. Sweat evaporation stops when relative humidity reaches about 90 percent. The body temperature rises and may cause illness.

9 What does humidity have to do with our level of comfort? At low humidity levels the air is not holding much water. The dry air can soak up more water vapour and your sweat evaporates more quickly. This makes you feel dry and comfortable.

10 What happens when the air is cooled? Warm air has more room between its air molecules and hold more water vapor than cold air. As the air cools the air molecules begins to contract and there is no longer enough room for all the water vapor. If the air is cooled enough eventually a temperature is reached when the water vapor is forced out and the water condenses. The temperature at which condensation occurs is called the Dew point.

11 What happens when the air is cooled? On calm clear nights, the land cools quickly causing the air touching the land to cool quickly too. If the air cools below its dew point, dew will form on the ground. During the winter, the same effect can occur but this time the ground cools below freezing and frost forms. When this frost forms on the roads we call it "black-ice".

12 What does humidity have to do with the weather? When the air reaches its maximum amount of water that it can hold (saturation point) the water vapor will start to condense forming clouds and precipitation. In other words, the higher the humidity, the greater the chance of rain or snow.