The Vernal or Spring Equinox approaches

Tuesday 19 March 2013, 17:19

Derek Brockway Derek Brockway

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Tomorrow the Vernal or Spring Equinox will occur at 11:02am in the UK but remember the clocks don't go forward until the weekend after next - on Sunday, 31 March.

The equinox refers to the moment that the sun crosses directly overhead at the Earth's equator.

The word equinox comes from the Latin words aequus (equal) and nox (night). It means equal day and night but in reality, the day is slightly longer than the night at an equinox.

One reason is because the sun is not a single point of light, but appears to be a disc. So when the centre of the Sun is still below the horizon, the upper limb is already visible and emits light.

The second reason deals with refraction within the atmosphere. Refraction makes the Sun's disk appear higher than it really is. This produces more daylight time than would occur if the earth had no atmosphere.

This combination makes the day slightly longer than the night.

In Wales, day and night were almost equal on March 17 before the Spring Equinox with sunrise in Cardiff occurring 6:22am and sunset at 6:21pm.

Brecon Beacons with snow Snow covered Brecon Beacons taken on March 16, 2013 by Mike Davies.

Some people consider tomorrow to be the first day of spring in the northern hemisphere but from a weather point of view we haven't see the last of winter yet.

There has been snow in places over the past few days and later this week it will turn wet and windy.

Low pressure will bring heavy rain, strong to gale force winds and some snow. Most of the snow is likely in parts of Mid Wales and the north, especially on high ground with blizzards on the mountains.

Snow is not unusual in March or April; in fact, it is more likely to snow at Easter than it is at Christmas.

Next week looks set to bring more cold weather with easterly winds but there is hint that it may turn a bit milder by Good Friday. I will keep you posted.

Derek

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