The Meteorological Spring

We are now officially in spring – that's if you go by the calendar which determines the meteorological seasons. Since 1780, when the Societas Meteorologica Palatina devised the system, the seasons have been split into four groups of three complete calendar months. As the UK is in the one of the temperate zones of the Northern hemisphere spring starts on 1^st March, summer begins on 1^st June, the autumn months commence on 1^st September and finally winter comes upon us on 1^st December.

It's not the case throughout the northern hemisphere that the seasons are so strictly defined. In parts of Scandinavia the seasons are actually defined by the weather or to be more precise the temperature. In Sweden and Finland the temperature needs to be above or below a certain temperature for seven consecutive days. For example spring is deemed to have started when the daily averaged temperature rises above 0°C (we are talking about areas close to the Arctic Circle!). It is said to be summer once the temperature gets to 10°C for seven consecutive days. This system has three points to it:

1. There is never a precise date when the seasons start. A balmy week in January could kick start spring whereas in other years prolonged frost may see spring delayed until April.

2. The seasons start at different times in different parts of the country due to temperature fluctuations.

3. The change in the season has already occurred after it has been declared. In some ways this is a strange concept since we mostly try to move into seasons quicker than are happening. How many times in January have you seen a green shoot and declared it a sign of spring despite it still being officially weeks away? Magazines on sale at the end of February have a cover date of April on them. On the clothes rails in shops there are vest tops and shorts despite the fact the temperature outside is around 6°C.

Many people may be surprised that the meteorological system of ordering seasons dates so far back as the astronomical system is still commonly referred to. The astronomical seasons are based on the spring and autumn equinoxes and the summer and winter solstices. The spring or vernal equinox is when the day and night are roughly 12 hours long each. The passage of the Sun means it crosses the celestial equator going northward. It you want to catch the sun at the beginning and ending of each day it will be rising exactly due east and sets exactly due west.

Although many people think the astronomical seasons start on a certain date they are determined each year by the exact times of transit of the sun and the tropics of Cancer and Capricorn for the solstices and the times of the Sun's transit over the equator for the equinoxes. Therefore this year the start of spring will be 10.45pm on 20^th March. It just so happens that there is also going to be a solar eclipse earlier on that day. I'm hoping for a clear sky!

All the photographs for this post have been taken this week. Despite the bright sunshine there has also been a few snow flurries each day and those fabled March winds. If you are feeling cold I'm afraid spring in the UK isn't known for being warm. The highest average daily temperature for the period 1^st March to 31^st May according to the Met Office is 9.2°C recorded in 2011. Back in 1962 the coldest spring was noted with a daily average temperature of 5.8°C. Best not think about packing away those scarves and hats yet...

Linking with School Run Colours at RamblingThrough Parenthood and How Does Your Garden Grow? at Mammasaurus.