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

What is weather?

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.

Temperature

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.

Humidity

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 evaporationadds water vapor to the air, or when the airis cooled to its dew point.

A psychrometer measures relative humidity.

Relative Humidity

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.

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Unsaturated Saturated Dew Point

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

Precipitation

Sleet vs. Freezing Rain

Sleet Freezing Rain

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 pressureMore dense

Air Pressure

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

Wind

What is visibility?

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

Visibility

Thermometer – Measures temperature

Barometer – Measures air pressure

Anemometer – Measures wind speed

Psychrometer – Measures relative humidity

Instruments for Measuring Weather

Wind Vane Shows wind direction