Thursday, 26 June 2014

Passing Storm

We waited all afternoon for a storm that didn't come.

High, white towers of cauliflower, cumulo-nimbus clouds would build up, form an anvil at the top and then just collapse on themselves, leaving a deep blue void. There was obviously a huge amount of energy up there but down at ground level there was no movement and no sound. The hot, heavy, moisture-laden air sat beneath the sky and it stifled all noise. Even the sound of the nearby motorway was muffled. Birds did not sing and insects did not hum. Every door and window in the house was open.

It was like living under water. Every step was laboured and slow. Even time slowed down and the hours dragged.

There was a sudden change around 4 pm when the light faded to purple dusk. We turned on the lights. (This was on the day of the summer solstice, with about 6 hours before sunset.) A cool breeze blew through the house from the east and the trees began to shake themselves. This meant that a storm was getting close and it was to the east of us. The dark sky meant that we were right under a cloud-tower.

Insects and garden birds ran for cover, but a sparrow hawk rejoiced in the mobile air and dozens of swifts chased the passing front to mop up helpless flies. The biggest swift in the flock turned out to be hobby; a falcon that specialises in dragonflies.

As a child I noticed that thunder storms travelled against the wind, but I did not understand why. Now I do.

As a storm cloud builds upwards towards the cold stratosphere, it sucks in the heated stagnant air from ground level so that we perceive that the wind as travelling towards the storm. At a higher altitude the prevailing wind pushes the clouds along, but as it hits the jetstream the top gets blown off to form a flat anvil.

Meanwhile inside the cloud, heavy moisture-laden air is travelling upwards and cooling at a fantastic rate. Huge rain drops form around dust particles, but the updraft is so strong that they do not fall. They may keep ascending and freeze, thousands of feet above the ground. At some point, the weight of ice outweights the upward force of the vortex and hailstones as big as marbles cascade earthwards. They might fall out of the cloud and smash your green-house, or they might melt on the way down and just fall as big drops of rain.

That is exactly what happened next. Large drops of rain fell about a foot apart and left splat-marks in the dust; and that was that. Storm over. The air was fresh and cool because it had been sucked downwards to replace the warm air that had been sent skywards.

Later, another cloud moved in and we had a really heavy downpour with two flashes of lightening and two loud bangs. Then the clouds tore themselves apart to reveal glimpses of a golden heaven bathed in sunlight. Lit up from beneath, pink candyfloss castles sailed into the sunset.

The warm tarmac steamed for an hour afterwards.