There are four seasons in the year in the UK, but what causes them and how do they affect us?
What causes the seasons?
The Earth is like a a slightly squished ball that orbits around the Sun, taking one year to make a complete orbit. Imagine a rod sticking through the Earth going from the North Pole to the South Pole. This is its axis. Each day, the Earth spins around on its axis once - this is what gives us night and day.
But the Earth doesn't spin upright, its axis is tilted, like it has fallen over a little on one side. This tilt of 23.5 degrees is what causes our seasons. As we pass round the sun, some parts of the Earth are tilted towards the Sun they get more of its heat, while others are tilted away, and get less of the Sun's heat. This is not because they are nearer to or further away from the Sun, but because the heat from the Sun arrives in waves, which can travel through the atmosphere more directly in the areas titled towards the Sun.
Because of this the UK has four seasons:
- Summer when the northern hemisphere, where the UK is, is tilted towards the Sun
- Autumn as the northern hemisphere begins to tilt away from the Sun
- Winter when the northern hemisphere is fully tilted away from the Sun
- Spring when the northern hemisphere begins to tilt back towards the Sun again.
What kind of weather does each season give us?
The amount of heat coming from the Sun causes different patterns in the weather; the more heat the warmer it is, the less heat the cooler it is, but there are also changes in wind, cloud and types of precipitation with the seasons.
Summer is the season with the hottest temperatures and sunniest days. You may think it is also the driest season, and sometimes it is, but rainfall varies a lot in the summer months and can be when the UK sees a lot of flooding. It starts in June in the UK and finishes at the end of August.
Autumn starts in September and finishes in November, during which time the temperatures get cooler, the days get shorter and quite often the weather gets stormier.
Winter runs from December to February; these are the coldest months of the year with the shortest days. There can be quite a big split in winter weather in the UK; most winters see the storminess of autumn continue with lots of wet and windy weather, in fact the UK often sees some of its strongest winds of the year during winter. Other winters are much colder and calmer with lots of fog, frost and even snow. Some winters have a mix of the two, particularly depending on where you live. People living in the south of the UK or nearer the coasts will likely have less cold winters than those in the north of the UK and away from the coasts.
Spring begins in March and ends in May and can be one of the most easy to notice as it changes from winter - the days get longer and warmer. The weather in spring is often quite calm and dry, though there can be big differences in temperature between day and night as the ground hasn't had a chance to store up any heat from the Sun yet.
What is the solstice?
There are in fact two solstices - the summer solstice and the winter solstice.
The summer solstice occurs when the northern hemisphere is fully tilted towards the Sun. This occurs on the 21 or 22 June each year and gives us our longest day and shortest night.
The winter solstice occurs when the northern hemisphere is fully tilted away from the Sun. This occurs on the 21 or 22 December each year and gives us our shortest day and longest night.
What is the equinox?
There are two equinox and they occur half way between the two solstices, marking the start of spring and autumn and is when the northern hemisphere is not tilted towards or away from the Sun.
Technically the equinox marks an astronomical change in season, which is slightly different to the meteorological seasons described, mostly above due to the latitude of the UK.
Do other countries have the same seasons?
Yes and no; the seasons are caused by the Earth's tilt, but the depending on where you are on the Earth will depend on how much the tilt affects the weather. So most countries at a similar latitude (the same distance away from the equator) to the UK will generally have the same four seasons, but other parts of the world may only have two seasons - a winter and a summer, or a wet and a dry season. Some places only have one season as there is little change to their weather throughout the year.
Things that affect the seasons: El Niño and La Niña
The tropical Pacific Ocean has a warming and cooling cycle. This cycle is a completely natural event and usually lasts between three to seven years.
When the waters become warmer it is called El Niño, and when they become cooler it is called La Niña. During the cycle, the temperature of the ocean can change by around 3 °C between the warmest and coolest times.
People who fish off the South American coast have known about this natural event for hundreds of years. With an El Niño, they see a huge fall in the numbers of fish caught. In the UK during a La Niña year, the weather may milder, wetter and windier than normal, but this is not always true. Scientists are only just beginning to understand how the event affects Earth's weather and climate, though they do know that it causes the circulation of air to move from their normal patterns, which will cause expected weather to change during El Niño or La Niña years, and that the stronger the change in the temperatures the bigger the changes in the weather.
There are other cycles around the world that also cause changes to the weather we would normally expect, such as the Madden-Julien Oscillation, Pacific Decadal Oscillation, Atlantic Meridional Overturning Circulation and the North Atlantic Oscillation.