Weather
The term ‘weather’ refers to the local, short-term characteristics of the atmosphere of a particular place. Weather events occur regularly on Earth and include wind, rain, thunderstorms, hail, snow and fog. These events are the result of different masses of air coming together. When air masses meet, differences in temperature, pressure and the amount of moisture in the air might lead to the formation of clouds and precipitation in the form of rain, hail or snow. Average weather conditions tell us about the climate of a particular place.
The term ‘climate’ refers to the long-term or prevailing weather conditions in a particular place. For example, a desert location that has no rain for most of the year might be described as having an arid (dry) climate, even if the weather on a certain day is wet or rainy. To know about the climate of a particular place, we have to observe and record the weather over a long period of time; this gives us an idea of the most common weather conditions, and what to expect in the future. The science of studying the atmosphere and predicting the weather is known as meteorology.
A very important part of meteorology is recording atmospheric data over long periods. Meteorologists use this data to detect patterns in the weather and climate trends. Long-term records not only enable scientists to know about the past, but also help them to better predict weather patterns in the future.
Meteorologists use a wide variety of instruments for observing the weather, including:
- thermometers to measure air temperature
- barometers to measure air pressure
- anemometers to measure wind speed
- hygrometers to measure air humidity—how much moisture (water vapour) is in the air
- weather radars to detect approaching rainfall
- weather satellites to monitor cloud cover, surface temperatures of land and sea and other data about the atmosphere.
Clouds
Clouds are formed when warm moist air rises to where it is cooler and the pressure in the atmosphere is lower. As the warm moist air cools (all gases cool when they expand under low pressure), some of the water vapour in the air changes from a gas to a liquid (condenses), forming tiny water droplets. A mass of billions of these tiny suspended water droplets is visible as a cloud.
"Cumulus humilis clouds" by Toby Hudson (Wikimedia Commons) licensed under CC BY SA 3.0
Rain develops when water droplets join together and become too heavy to remain in the cloud, suspended in air currents. Gravity pulls the droplets towards the surface of the Earth, where they fall as precipitation.
When estimating cloud cover, meteorologists divide the sky into eighths. If they estimate that eight-eighths are covered by clouds, they describe the sky as having total cloud cover. If they estimate that zero parts are covered, they describe the sky as being clear. In weather forecasts, the terminology used is simpler: clear (no cloud), sunny (little chance of the Sun being obscured by cloud), cloudy (more cloud than clear sky) and overcast (total cloud cover).
Temperature
Temperature is the degree of hotness. It is a measure of the intensity of heat rather than the amount of heat. When meteorologists discuss temperature, they are specifically referring to air temperature.
There are three temperature scales used to accurately measure and describe temperature. Australia and most other countries use the Celsius (sometimes called centigrade) scale. In this scale, zero degrees Celsius (0ºC) is the freezing point of water, and 100 degrees Celsius (100ºC) is the boiling point of water. Points below freezing are quoted as negative numbers.
In the Fahrenheit scale (still used in the United States), 32 degrees Fahrenheit is the freezing point of water and 212 degrees Fahrenheit is the boiling point of water.
In the Kelvin (also known as the ‘absolute’) scale, zero degrees is ‘absolute zero’, which is the point at which there is no heat whatsoever in the object being measured and that is as low as temperature can get. In this scale, 273 degrees Kelvin is the freezing point of water and 373 degrees Kelvin is the boiling point. There are no negative absolute temperatures.
The Kelvin scale is most often used by physicists and chemists because certain relationships are simpler to describe when absolute temperature is used. The Celsius and Fahrenheit scales are commonly used to measure everyday temperature.
Wind
Wind is the movement of air over the surface of the Earth. Winds are driven by the heat from the Sun, which warms the air and causes it to rise. Where warm air rises, cool air flows in to take its place. This motion is felt on the Earth’s surface as wind. Wind is described in terms of its strength or speed and the compass direction from which it is blowing. Wind speed can be expressed in kilometres per hour, metres per second, knots, or as a force on the Beaufort Scale.
When weather forecasters describe the wind they usually include information about both its strength and direction. For example, a fresh south-westerly wind is a wind blowing from the south-west at an average speed of 30–39 kilometres per hour (the speed of ‘fresh breeze’ on the Beaufort Scale). The term ‘gusty’ is often used in weather reports to describe winds that have sudden increases above the average speed for short periods of time.
Knowledge of wind strength is useful for recreational activities like sailing, the safety of people working in high places or at sea and for forecasting the movement of pollution and smoke from bushfires to populated areas.