The Atmosphere: Weather

From the sun shining to the rain pouring, the weather affects our day to day lives. In some cases, even our mood depends on the weather! An overcast day is associated with feeling miserable whilst blue skies and sunshine can leave us feeling positive. This blog post will focus on the science that causes our weather.

Clouds are formed when liquid water from the surface of the Earth is evaporated into the atmosphere. The water then condenses as water droplets onto particles of dust in the troposphere known as cloud condensation nuclei. If temperatures drop to -20°C or less, some of the water droplets freeze into ice crystals. The water droplets are about the size of the droplets found in fog. There are 3 categories for classifying clouds;

High clouds
These clouds form at the top of the troposphere between 6km and 10km, where the air temperature is very cold (see previous blog post). Because of the low temperatures, the clouds are composed of ice crystals. They usually have a wispy appearance as a result of strong winds. 

Medium clouds
Medium clouds form between altitudes of 2km and 6km. They are made of a mixture of ice crystals and water droplets.

Low clouds
Low clouds form below 6km and are formed of water droplets due to higher temperatures near the surface of the Earth. The rising of moist air create towering cumulonimbus clouds which bring heavy rain, thunder and lightning.

Click here for a cloud fact sheet


Precipitation is the name given to any type of water that falls from clouds; it usually comes from nimbostratus and cumulonimbus. The water droplet or ice crystals fall when they grow large enough for their fall velocity (the speed at which they fall) to exceed the opposing updraft caused by the evaporation of water. Rain is precipitation that falls in the form of water droplets. If temperatures are low enough for the water droplets to freeze, the precipitation falls in the form of sleet. Snow is made of ice crystals that form a complex structure within the cloud. Many of these complex structures are what forms a snowflake; its pattern depends on the temperature and humidity of the air.

Wind is the movement of air from an area of high pressure to an area of low pressure; this drives all weather. The pressure change is caused by convection currents; air heated by the land expands, becomes less dense and rises, leaving behind an area of low pressure. When the air eventually cools, it becomes dense and falls (this is known as subsidence), creating an area of high pressure. Areas of high pressure are associated with calm weather as well as clear skies. Areas of lower pressure usually have high winds, warm air as well as precipitation. Still don't understand convection?

Equatorial Low Pressure Trough:
At regions near the equator at 0°-10°North and South, the pressure is low due to the heat energy from the sun causing the air to expand and rise. The ascending air produces clouds which are responsible for heavy rainfall. It is for this reason rain forests such as the Amazon experience 23cm of rainfall annually.

Subtropical High Pressure Cells:
The subtropical region is located between 20°N/S and 35°N/S contains hot, dry air which moves from the tropical region. The heavy rain at the equator removes the moisture from this region, therefore the air is dry. 

Subpolar Low Pressure Cells:
At latitudes of 60° N/S the weather is cool and wet due to the meeting of cool air masses from high latitudes and warm air masses from lower latitudes. The low pressure causes cyclonic storms.

Polar High Pressure Cells:
At polar regions, 90° N/S, the air is extremely cold and dry. 

We are able to forecast the weather by studying these pressure cells. Below is a video explaining how the climate system works.