There are four seasons; spring, summer, autumn and winter. The seasons relate to weather patterns in parts of the world such as Europe and North America where the onset of growth in plants and the reawakening of nature spring can be separated from the hot season when crops ripen summer or when trees lose their leaves autumn or fall and a final cold period winter. For most of the northern hemisphere, the spring months are usually March, April and May, and so by this definition spring starts on 1 March.
This is because m eteorologists tend to divide seasons into periods of three whole months based on average monthly temperatures, with summer as the warmest and winter as the coldest.
Astronomically, the arrangement of the planes of the orbit of the Earth and its equator are such that the planes intersect at two times, the Equinoxes, when the length of the day and night are equal. Mid-way between these are the solstices, when the Sun is at its highest and lowest in the sky at mid-day. These times can be determined very accurately and, as they occur near the times when the seasons are changing, have been used to indicate the start of each season.
In temperate latitudes such as the UK, the cycle of four seasons per year is a good representation of the weather pattern but in other parts of the world this is not so. In the polar regions, there are effectively only two seasons, winter when the Sun never rises and summer when it never sets. In the tropics, there can be two wet and two dry seasons, both hot, each year.
In the far-east the weather is dominated by the monsoons and there are three seasons, cool-dry, hot-dry and hot-wet. The cause of the seasons is that the plane of the Earth's orbit around the Sun is inclined to the plane of the Equator. This means that the direction of the Sun's rays relative to the ground and the number of daylight hours change during the year and hence the amount of solar energy received at different latitudes changes during the year.
In reality there are no hard and fast criteria to determine the start of each season; the onset of spring, for instance, could be the date on which the first daffodil flowers or the first birds make their nests.
The first is astronomical and calculates the start of spring based on the position of the Earth in relation to the sun. This is due to the It is also known as the spring or vernal equinox - when day and night are of approximately the same length. The astronomical and meteorological dates for the start of spring use different criteria to work out the seasons.
The meteorological calendar is the easier of the two dates as it splits the year into four easy seasons of three months based on the Gregorian calendar. The astronomical calendar, however, takes into account equinoxes which mark the two times in the year when the equator is the closest part of Earth to the sun, with both the northern and southern hemispheres sharing sunlight equally.
Since the seasons vary in length, the start date of a new season can fall on different days each year. This year, astronomical spring began on 20 March and ended on 21 June For upcoming years, the dates for astronomical spring will be:. Meteorological seasons are instead based on the annual temperature cycle and measure the meteorological state, as well as coinciding with the calendar to determine a clear transition between the seasons.
The meteorological seasons consist of splitting the seasons into four periods made up of three months each. These seasons are split to coincide with our Gregorian calendar, making it easier for meteorological observing and forecasting to compare seasonal and monthly statistics.
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