Weather results from the combined effect of different parameters in the atmosphere. Largely, it is attributed to temperature and moisture (pressure) differences, which themselves can have several different causes together resulting in their variation. For instance, sunlight is straight and therefore plenty in the tropics, which is why they feature extremely hot summers. But the resulting pressure void also brings much rainfall.
Places are affected by their altitudes also. The higher one goes, the colder it becomes and starts to snow. Then there are different types of winds and storms that keep blowing regularly on the surface of the Earth.
Predicting the weather
The metrological department (the "met") does the difficult task of weather forecasting.
Impressive as it sounds, everyone knows how futile all efforts at weather forecasting have been. This is because the weather is an extremely complex system that may require computing power and analysis methods we may never be able to develop. Also, small changes in weather in one place have a ripple effect, causing unforeseen, large-scale changes. This is known as the "butterfly effect" popularly.
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