Weather and Humidity 1

What do you think weather is?

Weather is the condition of the atmosphere at a certain time and place. This condition is affected by: Temperature Humidity Precipitation Wind 3

Remember: To understand weather, you must know the water cycle. This is the continuous cycle of water movement from sources on Earth’s surface 4

Humidity The moisture in the air is called humidity. The air is like a sponge soaking up moisture, except that the moisture in the air is mainly invisible water vapor. Warm air can hold more moisture than cold air. 6

Relative Humidity The amount of moisture in the air compared to the maximum amount the air is able to hold is relative humidity. Tells us the percentage of the air that is full of water vapor. 8

Saturated air has a relative humidity of 100%. When air holds all of the water it can at a given temperature, it is said to be saturated. Saturated air has a relative humidity of 100%. 9

Example: At 32 degrees Celsius, air can hold 4 cubic meters of moisture. However, there are currently only 2 cubic meters in the air. Therefore, the relative humidity is 50%. 10

Measuring Relative Humidity A psychrometer or hygrometer is used to measure relative humidity. Consists of: 2 thermometers 1 is a wet bulb thermometer that is covered with a wet cloth. 1 is a dry bulb thermometer. 11

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The difference in temperatures shows the amount of water vapor in the air. The larger the difference between the two readings is, the less water vapor the air contains and thus the lower the humidity is. 13

Evidence of water in the air If you have ever left a cold drink on the counter for about 30 minutes, you have probably seen water droplets form on the outside of the container. What causes these water droplets to form? 14

Condensation! When the air cannot hold any more moisture, we get condensation. The air surrounding the cold drink is cooler. Therefore, the water vapor saturates the air and causes water droplets to form on the outside of the container. 15

Other examples: Think about the bathroom when you get out of the shower. Think about cars that fog up in the evening.

Dew Point Dew point is the temperature at which a gas condenses into a liquid. At its dew point, the air is saturated.

The coldness of the drink causes the air surrounding the glass to cool to its dew point. Before water vapor can condense, though, it must have a surface to condense on. 18