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The weatherman's spring starts today

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John Hammond Met Office Forecaster John Hammond Met Office Forecaster | 11:40 UK time, Tuesday, 1 March 2011

d ~ 154'368'000 km: day 60

During February, the longest winter nights have passed in the northern hemisphere for another year as the day length increases ahead of next months equinox.

Meteorologically, spring begins on 1st March and ends on 31st May, which is for statistical purposes and historical records. For many people the season changeover is when the oscillating date of the spring equinox occurs, which this year is 20th March.

Although spring can be determined by calendar dates, the natural variability within the atmosphere means that, as far as the weather is concerned, year to year differences vary widely. These changeable weather patterns can have big impacts on how flora and fauna respond through the season.

Old Key West flora

Image courtesy of George Thomas, Flickr

The increasing height of the Sun above the horizon by day gradually brings increasing warmth to the ground and the first signs of spring begin to appear as nature wakes up. However, the influence of the Sun is fairly limited early in spring.

Following the winter months, the ground has been able to cool down and it takes time for solar radiation to warm it once more. This is because air is a poor conductor of heat, so the Sun's energy is not entirely absorbed by the ground to warm it up. This leads to a delay in air temperatures increasing through the spring.

This can be illustrated when comparing the 30 year UK mean temperature for the equinox months of March and September, which effectively receive a similar amount of solar radiation. In March, the figure is 5.2 Celsius, however, in September the value is more than double at 12.3 Celsius.

In addition to day length increasing, different types of weather patterns have a big effect on how quickly the ground warms. These in turn have a direct impact on the plant and animal worlds.

Of all the seasons, some of the biggest differences between day and night temperatures occur during spring. As early as March, maximum temperatures can exceed 20 Celsius, especially in southern parts off the UK, and in May they can reach more than 30 Celsius.

In sheltered areas throughout the season, minimum temperatures can fall well below freezing and as low as minus 10 early on. Snow will be a regular feature across many parts in the first half of the season. In fact statistics show that snow is more likely to fall at Easter than at Christmas.

Looking beyond the year to year differences we see that our climate is changing. For many the first evidence of this can already be seen in gardens across the UK. Seasons are already changing with spring having advanced 2-6 days per decade in the UK. Those with gardens need to start cutting their lawns almost two weeks earlier than they did in 2001.

Research undertaken by the Met Office Hadley Centre has also confirmed that the growing season of plants is likely to increase by around 40 days by 2080, due to the earlier start to spring and later end to autumn.



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