The seasons, we've all got a favourite. But confusion can often arise around the date that each season starts and this is because meteorological seasons are different from astronomical ones.
The seasons occur due to the earth's tilt on its axis (23.5º to be precise) as it rotates around the sun. Both the northern hemisphere and southern hemisphere receive the same amount of incoming solar radiation (insolation) over the course of the year. In astronomy, its the position of the Earth as it orbits the sun determines when the seasons start.
In the northern hemisphere, the summer solstice (20-22 June, depending on the year) denotes the start of astronomical summer - the point at which the northern hemisphere is pointing directly towards the sun. This means longer days, shorter nights and more solar radiation reaching the northern hemisphere, compared to the southern hemisphere. Hence for us, we see warmer days.
Three months later, the Autumn equinox in the northern hemisphere (21-23 September) takes place, as the sun is directly above the equator. This provides equal solar radiation to both hemispheres. In the southern hemisphere, they experience their Spring or vernal equinox.
The winter solstice (December 20-22) occurs when the northern hemisphere points away from the sun, meaning shorter days, longer nights and colder weather. Finally, the northern hemisphere Spring Equinox (March 20-22) takes place the sun is directly above the equator again.
Since the seasons vary in length, the start date of a new season can land on different days each year. This makes yearly comparisons for statistics rather difficult.
In meteorology, the seasons are defined as:
Spring - March, April, May
Summer - June, July, August
Autumn - September, October, November
Winter - December, January, February
These are based on the annual temperature cycle, as well as coinciding with the start of the calendar month, to make a clear transition between the seasons.
They're still split into four groups of three months each. This makes it easier for meteorologists and climatologists to compare seasonal and monthly statistics. In meteorology, Spring always starts on the 1st of March and runs to 31st May.