Climate and weather: Which is which
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Short-term conditions, such as the temperature or amount of rainfall on a particular day, are weather events. Even unusually hot or cold – or wet or dry – conditions can be put down to weather variability. In contrast, climate determines both the average weather and the range of typical conditions. So how often a particular place sees an unusually hot or unusually wet day depends on the local climate.
The climate of a particular place describes both the average and the range of weather conditions over a period of at least a decade and generally 20 to 30 years. In addition, climate determines how often the weather conditions are close to average and how often they’re close to one extreme of the climate range. For example, London’s daytime summer temperatures average about 20 °C, dropping to about 13 °C at night. The Sahara Desert, as well as having higher average temperatures, has a larger daily temperature range – sometimes varying from 50 °C during the day to freezing temperatures at night. In contrast, the Sahara is closer to the Equator than London, so experiences smaller seasonal variations.
Scientists often describe the climate in terms of statistics, such as the average number of rainy days each year, or how often the temperatures are above or below a specific threshold. This is similar to the statistics of a coin toss – it’s impossible to predict which side up the coin will fall each time, but tossing the coin 100 times will always give about 50 heads and 50 tails. Like each coin toss, individual weather events are highly variable and unpredictable, but – like the average number of heads and tails – climate statistics hold true over the long term. However, a change in the average statistics, such as an unusually high number of hot or cold days over many years, can indicate a shift in the overall climate conditions.
Although by definition average weather conditions occur more often than extreme conditions, events can occur that lie outside the normal range of weather for a particular location. That’s why it’s so difficult to attribute an individual weather event to the state of the climate. This is similar to a pair of loaded dice that are more likely to roll sixes. No individual six can be attributed to the loading, because normal dice occasionally roll sixes too. However, the average number of sixes can be attributed to the loading of the dice. Likewise, scientists can attribute the average number of extreme events to the state of the climate.