If you are unaware of the climate of North Wales, as those who may be travelling here from afar may be, then it is generally damp and cool as it is strongly influenced by moist air from the Atlantic. Summer temperatures can vary from 10 to 25° Celsius, but about 20°C seems a good average figure. Summer temperatures can be warm enough to trek in shorts and t-shirt, but the mountain tops can experience conditions that are sub zero well into summer if you take wind chill into account.
The spring and autumn (fall) can bring all sorts of weather, including snow. The first snow can fall on the hills at the end of October, but it is often patchy and there are many more days without snow lying than there are with. The snow cover has been very sparse in some recent years, with the winter of 2007/08 having only a handful of days where snow lay on the highest summits, but with the winters since then seeing some decent snow falls. Just be prepared for strong winds and heavy rain all year round, and appreciate that the conditions do change appreciably in a matter of hours.
Remember that the mountains cause their own weather. Rainfall is much higher on the mountains and they are often in cloud when the coast is basking in sunshine. The wind is twice as strong as it is at the base and can be exceptionally strong in some localised area where the lay of the land channels the wind. Be careful in any ‘bwlch’ or col as the wind can be strong here, as it will be over narrow ridges. That’s why routes such as Crib Goch or Bwlch Main on the South Ridge and Rhyd Ddu path are not recommended in windy conditions. The temperature will also drop as you ascend the mountain. This is known as the lapse rate and varies depending on conditions, with a maximum drop in temperature of 1° Celsius per 100m. That could mean that if it’s 10°C at sea level, then it might be around freezing at the summit! Add wind chill onto that, and you’re talking about really serious conditions.
And finally, always check the weather forecasts! By their nature, general forecasts aren’t particularly useful as they don’t take into account the weather the mountains themselves cause. Instead visit the Met Office website who provide a daily mountain forecast for Snowdonia – with the conditions at different altitudes stated clearly.
MWIS also provide an excellent forecast, but it is only for the Weekends. If you have an internet enabled phone, then bookmark them. Some places print these forecasts and pin them up outside and you can usually depend on a Youth Hostel or Park Warden office to provide the latest mountain forecast.