The weather can be extremely volatile, conditions on the hill tops can rapidly deteriorate and there can also be vast extremes between conditions in the valleys and on the fells.
Before you set out check the weather forecast for the full day (preferably by using the Met Office website at MetOfficeWeather ), watch the sky for adverse changes and turn back if conditions are against you, even if this spoils your plans.
As a rough guide, air temperature reduces by approximately 3º centigrade for every 1,000 feet gained, with increased wind strengths accentuating the risk of hypothermia.
Weather on the fells can catch you out even in the height of summer - respect it at all times.
If you are walking in Swaledale this interesting web site - LocalSwaledaleWeather - will keep you up to date with local conditions.