Weather is the state of the atmosphere in a certain place at a certain time. Weather always changes and is different all around the world. It depends on many elements. It may be warm and sunny in one place but cold, windy and rainy somewhere else.

Climate refers to the weather conditions in a certain area over a longer period of time.

Weather is important to everyone. It affects our daily lives in many ways. What we wear depends on the weather. Weather affects the way plants and crops grow. Extreme weather may lead to dangerous situations. Hurricanes and storms may even kill people and destroy houses and roads.

Our weather is made in the troposphere – the lower layer of the atmosphere. Our atmosphere consists of 78% nitrogen and about 21 % oxygen. Water vapour in the atmosphere produces clouds, rain, snow and fog.

Main Elements of the Weather

Temperature

Temperature is one of the most important elements in our weather. How warm or cold it is depends on many factors.

The temperature of the air is higher when sun rays hit the earth. Temperatures also vary from season to season. During the summer the earth’s axis is tilted towards the sun, so the rays of the sun hit us more directly. The days are longer and we get more sunlight.

Temperatures also depend on altitude. It gets colder when you are higher up in the mountains and it’s warmer near sea level. The temperature decreases about 0.7°C per 100 metres.

Places near the sea usually have more moderate climates than areas far away from the coast. There the summers are very hot and winters very cold.

In the summer, the northern half of the earth is tilted towards the sun – leading to higher temperatures
Image: Image by Przemyslaw “Blueshade” Idzkiewicz, CC BY-SA 2.0,
via Wikimedia Commons

Air Pressure

Air pressure is the weight of the air. Cold air is heavier and sinks to the ground while warm air rises. When air moves it produces wind. Air moves from high pressure areas to places where the air is not so dense.

Air moves from high to low pressure areas

Humidity

Humidity is the amount of moisture that is in the air. Warm air expands and can hold more moisture than cold air.

When the air can hold no more moisture it forms small droplets called clouds.

Meteorologists often measure the relative humidity in the air. That is the amount of water in the air compared to how much water the air at that temperature can really hold. Relative humidity can reach nearly 100%. The air is completely saturated with water.

Precipitation

Precipitation is what comes down to earth from the clouds in the atmosphere. The most common forms are rain and snow. Other forms include hail, sleet, drizzle.

The amount of precipitation that a place gets is different and depends on many factors. Areas near the coast get more rain than places in inner regions. Sometimes high mountains stop wet air from getting to other places.

Climate Zones

Every place on Earth has a different climate, but there are four main climate zones that extend from the equator to the poles.

Tropical Climate

Tropical climates are found around the equator. Temperatures are high the whole year and there is little difference between the hottest and coldest month. The air is very humid and rainfall is at least 200 cm a year.

Near the equator it rains almost every day in the afternoon. Humid air rises and forms clouds that lead to thunderstorms . There are usually two rainy seasons a year, with drier periods in between. The world’s largest rainforests are in this climate zone – the Amazon and Congo River basin.

As you move farther away from the equator rainfall becomes less and the dry season becomes longer. Rainforests turn into savannahs .

Dry Climate

Dry climates are located between about 20° and 40° north and south of the equator. Because of high pressure areas the skies are clear and often cloudless . Deserts stretch across large parts of Africa, Arabia and Australia. In some areas it hasn’t rained in years.

Temperate Climate

Large parts of Europe, North America and Asia have seasons with warm summers and cold winters. Rain falls throughout the year, especially in coastal regions. Farther away from the sea the climate becomes continental with extreme summer and winter temperatures and little rainfall.

The Mediterranean climate is a special temperate zone found in southern Europe, northern Africa, California , South Africa and southern Australia. Summers are typically hot and dry while winters are cool and rainy.

Polar Climate

The polar region begins north and south of about 60° . In the tundras of North America, Europe and Asia winters are extremely cold and long. A short summer lets few plants grow in this treeless land.

The Arctic and Antarctic regions are covered with snow and ice.

Climate Zones
Image (modified): Skimel, CC0,
via Wikimedia Commons

Wind Systems

Because the earth rotates around its axis air does not move directly from high to low pressure areas. The Coriolis effect makes winds shift . There are three big global wind systems:

• Trade winds blow near the equator between 30° north and 30° south latitude. The trade winds north of the equator blow from the northeast, those south of the equator from the southeast. When they meet at the equator they rise. Rising air leads to the formation of clouds and therefore it starts to rain. A system of clouds and rainy weather is always around the equator but it moves its position depending on the position of the sun.
• Westerlies blow in the middle latitudes between 30° and 60 ° north and south of the equator. These winds are especially strong in higher regions. This area of western winds is also called the jet stream. Airplanes traveling from west to east benefit from strong tailwinds. They need much less fuel and can travel faster.
• Polar winds are easterly winds that blow in the Arctic and Antarctic region. Fronts develop where polar winds and westerlies meet. Storms and cyclones arise around this area where warm and cold air meets.

Trade winds blow from the northeast and southeast towards the equator.
Image (modified) : Martin23230, CC BY-SA 3.0,
via Wikimedia Commons

Cyclone forming near Iceland
Source : https://visibleearth.nasa.gov/images/68992/low-off-iceland

Air Masses

Air can be cold or warm, wet or dry. When different kinds of air masses meet a front forms. When a warm front approaches , air starts to rise and forms clouds . Light rain usually comes with a warm front.

About a day later a cold front follows. Cold fronts move faster than warm fronts and often catch up with them. The cold air moves under the warm front and pushes the warm air up. Clouds and rainfall are the result. Sometimes such a front can even cause short showers and thunderstorms.

Weather Forecasting

Scientists who study what goes on in our atmosphere are called meteorologists. They can tell us what the weather will be like in the next few days. They gather information from all sorts of instruments.

Today, weather satellites collect data from around the world. This data is entered into computers which produce weather patterns for the future.

Ten thousand weather stations on all continents observe the weather worldwide. They use many kinds of instruments to measure the condition of the atmosphere. The thermometer, for example, measures the air temperature, a barometer measures the weight of the air and a hygrometer shows how much moisture is in the atmosphere.

Weather stations use weather radar to monitor areas of rain or snow, track storms or follow the paths of hurricanes.

Weather balloons are sent into higher regions of the atmosphere and measure changes in temperature, winds and other elements A radio transmits data back to earth.

Weather forecasts are not always accurate, partly because humans make mistakes when they enter data, partly because the condition of the atmosphere always changes and is unpredictable.

Meteorologists can therefore predict weather over a short period of time fairly well, however long term weather predictions are very inaccurate.

A meteorologist studying weather maps
Image: NOAA, Public domain, via Wikimedia Commons

Extreme Weather

Although weather experts rely on average temperatures over a longer period of time to predict the weather, there are often extreme weather conditions.

The lowest temperature ever recorded on earth was at Vostok, a station in Antarctica. –89° C was measured in 1983 . The highest temperature was recorded in the Libyan desert in 1922 (57°C). The highest annual rainfall was measured in Cherrapunji, India , at the foot of the Himalayan mountain range. 26 metres of rain fell over a one year period.

Sign in Cherrapunji – the wettest place on the planet
Image: No machine-readable author provided. RMehra~commonswiki assumed (based on copyright claims).,
CC BY-SA 3.0, via Wikimedia Commons

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