Weather is the state of the atmosphere at a certain place and time. On Earth, weather occurs in the troposphere.

• Atmosphere: the gases surrounding a planet

• Troposphere: the lowest part of the atmosphere

Air Temperature

Air temperature is the temperature (how hot or cold) the air is. It is measured in Celsius (°C) or Fahrenheit (°F). Hotter temperature causes gas molecules to move faster, and in colder temperatures, the molecules move slower.

The differences in air temperature can cause air pressure and differences in densities (warm is less dense than cold air). This results in wind and convection currents.

Wind

Wind - the movement of air

Winds are caused when there is a difference in air pressure in one area and another. Air moves from an area with high air pressure to low air pressure; this movement is called wind.

Convection Currents

Convection Currents -

Humidity

Humidity - the amount of moisture in the air

Relative humidity - the amount of humidity in the air compared to how much the air can hold

• The relative humidity is affected by the air temperature.

• For example, when air is at 100 percent relative humidity, the air molecules are saturated, which means they cannot hold any more water vapour. Warm air holds more water than cold air, so 50 percent water humidity for warm air has more water vapour than 50 percent cold air.

Clouds

Clouds - are formed when the water vapour in the air condenses into visible water droplets

They are identified by their shape and the altitude they are in.

Some types of clouds are:

• Cirrus Clouds

• They are formed in very high altitudes where the temperatures are really cold.

• They look like feathers.

• Cumulus Clouds

• They form in mid/low altitudes.

• They look puffy

• Are typically seen on sunny days, but if the clouds are very big and grey looking, it can lead to thunderstorms.

• Stratus Clouds

• They form in low altitudes

• Look flat

• Fog is an example of a stratus cloud that is on the ground

Precipitation

Precipitation: Water droplets that fall from clouds

The type of precipitation (whether it is rain or snow etc) is dependent on the temperature and some other conditions.

Forms of precipitation:

• Rain

• Snow

• Hail

• Ice pellets

• Freezing rain

The Water Cycle

The water cycle is the way that water gets recycled in the atmosphere and land.

The water cycle consists of multiple stages:

1. Evaporation: when water (from lakes, ponds, oceans etc) turns into vapour through heat (when water turns into a gas).

2. Condensation: When the water vapour turns into a liquid as it cools down, forming clouds.

3. Precipitation: when water in the clouds becomes heavy and fall down back onto Earth as rain or snow or something similar.

4. The precipitation typically goes back into oceans and bodies of water. However, water can flow above or below ground. Water flowing above ground is called runoff and water flowing below the surface is called groundwater.

Severe Weather

Thunderstorms

Thunderstorms form when the air is unstable. Air is unstable when warm and moist air is below colder, drier air. When the warm air rises, it starts to cool and negative electric charges form on the bottom of the cloud and positive charges form at the top. These positive and negative charges are attracted towards each other and lightning is formed.

• Lightning: the bolt of electricity that forms during storms.

• When lightning forms, the air surrounding the bolt starts to expand creating thunder.

• Thunder: the sound produced when air expands faster than the speed of sound.

Tornadoes

Tornadoes are formed when warm air runs into cold, dry air. The updraft resulted from the collision will create a funnel-shaped cloud if the winds change speed or direction and produce a tornado. Tornadoes typically last only a short amount of time, less than 10 minutes, and don’t travel very far, however they still cause a lot of destruction.

Hurricanes

Hurricanes are storms that form over warm, low-pressure waters in tropical waters. They are also referred to as cyclones or typhoons depending on the region. When the warm air rises, it can cause powerful winds. Due to the earth’s rotation, it will result in the winds moving in a counter-clockwise direction (in the northern hemisphere).

The centre of the hurricane is called the eye of the storm. This area is calmer than the outside of the storm.