Always pack your thermals
When I left home it was fine and sunny but I was glad I packed my thermals as on the mountain top it was freezing cold with a brisk wind, hill fog and old patches of snow on the ground!
Picws Du - Bannau Sir Gaer (2,457 feet high) with snow on top:
It was a reminder of how different the weather can be on the hills compared to the valley below. As a rule, the higher you climb, the cooler, wetter and windier it gets.
Me overlooking Llyn y Fan Fach lake:
The rate at which air cools or warms depends on the moisture content of the air.
If the air is dry, the rate of temperature change is 3°C per 1000 feet whereas if the air is saturated, the rate of temperature change is 0.6°C per 1000 feet.
So always check the weather forecast before you go walking and be properly prepared!