The truth about April showers

April showers did not  seem to dampen the spirits of racegoers at the Grand National meeting [GALLO/GETTY]

Drip, drip, drop
Little April shower
Beating a tune
As you fall all around

Drip, drip, drop
Little April shower
What can compare
To your beautiful sound

Do you remember this song from Bambi which plays whilst the fawn and his mother shelter in a cave?
The first two verses of the classic song from the !942 Disney animation hint at the significance of showers in the month of April.

This is not a one-off either; ‘April Showers’ was a well-known song which was written for the 1921 Broadway musical Bombo. The song, made famous by Al Jolson, had a similar theme to the later Disney song:

Life is not a highway strewn with flowers,
Still it holds a goodly share of bliss,
When the sun gives way to April showers,
Here is the point you should never miss.

Though April showers may come your way,
They bring the flowers that bloom in May.
So if it’s raining, have no regrets,
Because it isn’t raining rain, you know, (It’s raining violets,)
And where you see clouds upon the hills,
You soon will see crowds of daffodils,
So keep on looking for a blue bird, And list’ning for his song,
Whenever April showers come along.

So are there sound meteorological reasons to associate the month of April with showers? Well it would seem that for some parts of the northern hemisphere there are.

As winter fades and spring arrives, the sun rises higher in the sky. By April the elevation of the sun is great enough to produce considerable warming of the air close to the ground.

Yet whilst the air at low levels is warming, the air higher up in the atmosphere takes much longer to respond to the longer, warmer days.

Warm air near the surface and cold air aloft makes the atmosphere unstable. There is also plenty of moisture available after winter and early spring rains. So developing clouds will keep building until they reach a part of the atmosphere where the temperature is similar to their own. The result is large cumulus and cumulonimbus  – shower clouds.

If April was typically dominated by high pressure, none of the above factors would give rise to showers. But the larger scale weather patterns in the northern hemisphere are rarely benign.

During the spring months, as the warmth returns to the northern hemisphere, the jet stream begins to move northwards. This fast-moving band of wind high up in the atmosphere marks the boundary between warm and cold air.

Its more northerly spring position also encourages the development of low pressure systems which are then driven in an easterly direction across the Atlantic. Such weather systems are conducive to shower development, particularly across Western Europe.

The weather conditions that little Bambi experienced all those years ago are similar to those which will have soaked some of the runners in Sunday’s London Marathon. April showers are, indeed, as significant a threat now as they have always been.