1. What is weather? Chapter 2: Section 1

2. Weather Defined Weather refers to the state of the atmosphere at a specific time and place. Weather describes conditions such as air pressure, wind, temperature, and the amount of moisture in the air.

3. Air Temperature Temperature is the measure of the average amount of motion of molecules. When the temperature is high, the molecules in air move rapidly. When the temperature is low, the molecules in air move less rapidly. Air temperature is measured by a thermometer in Fahrenheit or Celsius.

4. Wind Air moving in a specific direction is called wind. Wind results because air moves from areas of high pressure to areas of low pressure. Wind direction can be measured using a wind vane. Wind speed can be measured using an anemometer in miles or kilometers per hour.

5. Wind Instruments Wind Vane Anemometer

6. Humidity The amount of water vapor present in the air is called humidity. More water vapor can be present when the air is warm than when it is cool. When enough water vapor is present in air for condensation to take place, the air is saturated. Relative humidity is the measure of the amount of water vapor present in the air compared the amount needed for saturation at a specific temperature.

7. Measuring Humidity Humidity is measured using a psychrometer and is represented by a percentage.

8. Dew Point The temperature at which air is saturated and condensation forms is the dew point. The dew point changes with the amount of water in the air. Frost may form when temperatures are near 0°C.

9. Pressure Atmospheric pressure is an indication of weather systems. Low pressure areas are windy and can be associated with stormy weather. High pressure areas are associated with fair weather. A barometer measures atmospheric pressure in Millibars.