Hat and gloves time. Brecon Beacons, 2008

Temperature decrease

Sitting at the desk, looking out the window, the sun is beginning to break through and leaves are budding on the trees, the daffodils are a riot of colour and the seasons are turning.

While it wasn’t the hardest of winters, the frozen rain, snow and lazy winds (too lazy to go round you so they go through you instead) all seem a long time ago. However there is still a bite in the wind.

It is worth reminding myself of just how easy it is to become cold as a result of wind chill, increase in elevation and any combination of these. There can be few amongst us that are unaware of the concept of wind chill, however a closer look at the table I have included below shows just how easy it is to experience a drop in temperature in quite moderate breezes. Just to pull out a couple of examples: Eight degrees Celsius is a quite nice temperature for hiking, especially in the valley bottom, however stick your head up next to a Trig point on a top in a 40 km/hr wind (which is only 25 mph), and the wind chill knocks the temperature down to minus four degrees Celsius.

Walking on a cold day, hovering around the freezing mark, throw in a 20 km/hr breeze (only 12.5 mph) and the temperature plummets to minus ten Celsius.

This is the reason I now carry a light windshirt on just about any walk of decent length. The one I use is an XL Montane Lightspeed which is only 193g in its tiny stuffsack. A light pair of Extremities Windpro gloves is also an essential. Throw a light merino hat or Buff in as well and that usually suffices for the summer and shoulder seasons. Expected adverse conditions and winter walking demands better preparation, knowledge and equipment.

The story doesn’t stop there either. Relative humidity and elevation also have a large effect on temperature. Lapse Rate is the rate at which air temperature decreases with increase in altitude. Under ‘normal’ conditions this equates to a decrease in temperature of 6.4°C/1000m. This is allowing for a degree of moisture saturation to the air as relative humidity also comes into play. Rarely is the air ‘dry’, instead it is often raining or snowing, being in cloud also makes a difference. If it is dry conditions (please) then temperature decreases by around 9.8°C/1000m. If the air is saturated (100% relative humidity) this is virtually halved: 5.5°C/1000m. Combine this loss with any wind chill and it can be seen how the temperature can easily plummet between dry valley bottom and a breezy wet peak or ridge.

All interesting stuff init? Simply a bit of knowledge to file away in the memory bank, another essential addition to the ‘skills’ armoury. Just need to remember how prepared I am when considering a bit of hill climbing. Not necessarily to worry, just to be aware…