1. Weather!

2. What is weather?
Weather is the condition of the Earth’s atmosphere at a certain place and time.
The elements of weather:
Temperature
Humidity
Precipitation
Air Pressure
Wind
Visibility

3. Temperature What is temperature?
Temperature is a measure of how hot or cold something is.
A thermometer measures temperature.
In science, we use degrees Celsius.
A liquid, usually alcohol or mercury, is sealed
in a glass tube. When its heated, it expands,
(thermal expansion) rising higher in the
tube. When it cools, it contracts, filling less
of the tube.

4. Humidity What is humidity?
Humidity is the amount of water vapor in the air.
As water evaporates, it changes from liquid to gas, from liquid water to water vapor. That amount of water vapor in the air is humidity. As more water evaporates and turns to water vapor, humidity increases.
Warm air can “hold” more water vapor than
cold air.

5. What is relative humidity?
Relative humidity is the amount of water vapor in the air compared to the total amount of water vapor needed to reach saturation.
Saturation occurs when the rates of evaporation and condensation are equal. Relative humidity is 100%.
Air can become saturated when evaporation
adds water vapor to the air, or when the air
is cooled to its dew point.
A psychrometer measures relative humidity.

6. The condensation can form dew, fog and clouds.
Dew Point
What is dew point?
Dew point is the temperature at which more condensation occurs than evaporation in air.
The condensation can form dew, fog and clouds.
condensation
condensation
condensation
evaporation
evaporation
evaporation
Unsaturated Saturated Dew Point

7. Precipitation What is Precipitation?
Precipitation is any form of water that falls to the Earth’s surface from clouds.
Types of Precipitation:
Rain – water droplets; the most common type of precipitation
Sleet – rain falling through cold air forms ice pellets
Hail – layered lumps of ice
Snow – ice crystals in the form of flakes
Freezing Rain – liquid droplets that land and freeze when they hit surface temperatures of less than 0°C

8. Sleet vs. Freezing Rain
Sleet Freezing Rain

9. Air Pressure What is Air pressure?
Air pressure is the force of air molecules pushing on an area.
Air pressure and density decrease as altitude increases.
Lower pressure
Less dense
Higher pressure
More dense

10. Wind
What is Wind?
Wind is air that is moving horizontally, parallel to the ground.
It is caused by the uneven heating of the Earth’s surface by the Sun
This causes pressure differences in different areas
Air moves from high to low pressure

11. Visibility What is visibility?
Visibility is a measure of the transparency of the atmosphere. It describes how far we can see.

12. Instruments for Measuring Weather
Thermometer – Measures temperature Barometer – Measures air pressure Anemometer – Measures wind speed Psychrometer – Measures relative humidity

13. Wind Vane
Shows wind direction