Friday, March 02, 2007

Here Comes Spring

A day off today, but no lie-in. Clear, star-filled skies overnight brought frosts this morning, returning the garden to its winter appearance, and even the edges of the pond froze. The terraces and steps were white and crunchy with that delicious sparkliness that a hard frost brings. In cold weather the temperatures in rural areas are often colder than urban or more populated areas.

On calm clear nights, the air near the ground cools as heat radiates back into space until just after dawn. This results in a temperature inversion where temperatures increase with height above the ground. Air over the ground on hill and mountain sides is cooler than the air at the same altitude above the surrounding valley. This causes the cold dense air to flow down the hill side to the valley floor (cold air drainage) resulting in a katabatic wind. Katabatic winds are generally light, and often blow unnoticed. However, some areas in the world see katabatic winds of incredible velocities. Outflowing winds in Antarctica can reach speeds of 120mph.

As a result of katabatic winds, temperatures in valleys and low lying areas can be significantly lower than on the surrounding higher ground. They can be low enough for ground frost to form while the surrounding hill sides remain frost free, or for frost to be more severe than in nearby areas at higher altitude. These frost prone areas are known as frost hollows. This effect can be enhanced when the cold air drainage is obstructed by obstacles such as hedges or railway embankments. Temperature differences can be as much as 8°C between a valley bottom and land at 200m (700ft). Ground frost can form even when air temperatures (which are measured in Stevenson Screens 1.2m above the ground) are as high as 4°C.
Large temperature ranges can also be recorded in frost hollows. Rickmansworth, in Hertfordshire a very well know frost hollow, recorded the largest daily temperature range in England when, on 29th August 1936, the temperature climbed from 1.1°C at dawn to 24.9°C within 9 hours! Other well-known frost hollows in the UK are the Welsh Marches, the Glens of Scotland, the Pennine Valleys, the Vale of Evesham, Shrewsbury and Redhill. Frosts are often seen here earlier in the autumn and later in the spring than on the surrounding higher land.

After breakfast we drove across to the orthodontist, then to Sainsburys for brufen and a bottle of mineral water, then up to school for just after nine. And all the time the blue, blue skies were calling to us to abandon our day and stay out in the sunshine and play. Even the girl at the checkout hoped the weather would hold until the end of her shift.

Driving home over the mountains, I called in at the farm and bought a bag of potatoes. All week I've been thinking of baked potatoes, mashed potatoes, roast potatoes. I've driven past this farm for nearly a decade on the school run, and each spring the snowdrops appear, lining the dry stone walls that mark out Derbyshire as the most beautiful county in England. Potatoes bought and stowed in the boot, I told the farmer this, and we realised our children are at the same school. Rather cheekily I asked if I could buy a clump of his snowdrops, to bring home to my garden a permanent reminder of the school run. And here they are, a permanent reminder of clear blue skies; of overnight frosts; of the absolute joy of children and parenthood; and the relentless passage of time, as winter makes way for spring, and the new year is underway.

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