1.
The rule of thumb is to be in five layers for a temperature
that goes down to -5°C or lower. With every subsequent increase in temperature
by 10 degrees (say from -5°C to 5°C), you can strip off one layer. But keep in
mind that during the day when the sun is out and you are trekking, it actually
gets very warm with the layers of clothes you have on so don’t over-layer
yourself and be prepared to peel off layers depending on the heat.
An example...
