Weather Planning

The main aim of this article is to highlight some of the weather factors which you may find useful when on the move. With a little careful planning it is possible to have a good day out in the hills in what would otherwise be a miserable day.

Lapse Rate (Changes of temprature with altitude)

As you gain altitude the ambient air temprature will drop by 1°C per 100 meters in dry air (hot or cold bright day) and 1°C per 200 meters in wet air (cloudy, foggy or humid).

As a rough guide, In the UK, for weather planning purposes you should use 1°C per 150 meters of ascent where the air is not obviously very wet or very dry. As you loose altitude the ambient air temprature will increase by 1°C per 100m.

The exception to this rule is when there is a temprature inversion, this is where there is a thin layer of the atmosphere where the decrease in temperature with height is much less than normal (or in extreme cases, the temperature increases with height). An inversion can also be referred to as a "stable" air layer. The image on the left is a good example of a temprature inversion where only the top of the tower can be seen through the cloud where the remainder of the tower is shrouded with cloud (wet air) at lower levels.

Wind Speed (Beaufort Scale)

The Beufort scale is a user friendly method of determining the wind speed and it's strength. By observing the effect of the wind on natural objects you should be able to match the effects to the scale in the table below therefore giving you the windspeed and strength.

Scale Descriptive Term Speed (kph) Effect
0 Calm 0 Smoke rises vertically
1 Light air 3 - 6 Smoke and loose snow may drift a little
2 Light breeze 7 - 12 Leaves rustle
3 Gentle breeze 13 - 19 Leaves, rushes & grass move. Flags extended
4 Moderate breeze 20 - 29 Raises dust & paper
5 Fresh breeze 30 - 39 Crested wavelets on lakes, small trees sway
6 Strong breeze 40 - 50 Large waves and spray on lakes. Winds whistle in crags
7 Near gale 51 - 60 Difficult to walk against the wind. Risk of frost nip on exposed flesh
8 Gale 62 - 74 Very hard walking, energy output doubled. Shroud of blowing snow up to 50m high.
9 Severe gale 75 - 87 Almost impossible to walk. Some damage to buildings. Streams blown back. Trailing ropes will be horizontal off the ground
10 Storm 88 - 101 Difficulty standing. Safer on hands and knees. Seek shelter. Energy output trebled. Whirlwinds of blowing snow on crests, plumes and streamers
11 Violent storm 102 - 132 Impossible to stand. Widespread damage. Spindrift forms a suspended shroud of up to 100m high. Exposed flesh freezes
12 Hurricane 132 + Wind may lift and carry you across the ground. Breathing difficult when facing into the wind

 

Wind Chill

The wind chill factor is a measure of how much colder moving air is from still air for example if the air temprature behind sheltered ground is 10°C and the air temprature in open ground is with the wind blowing at force 4 (20 - 29 kph), the temprature will drop to 2°C