Season Rating
One Season
Suitable for use for use during a typical UK summer, when overnight temperatures are normally at their highest.
Two Season
Late Spring to Early Autumn, overnight temperatures unlikely to fall below 0°C.
Three Season
For use in all but winter conditions, with heavy overnight frosts.
Four Season
All year use. However some may find they are too warm in warmer months.
Five Season
High altitude and expedition use.
Four and Five Season bags may ne too hot for use during the summer months.
All the above are based on the "average person", for UK valley use. If you feel colder in general, the best advice would be to go for a bag with a higher seasonal rating.
Consider also the construction of the bag. Cheaper bags will be stitched straight through(seams will match on the inside & outside). Leaving cold spots.
How many layers are used in the construction. Single layer compresses the filling where stitched. Double offset is used to elimate these cold spots by overlapping the stitched areas with a thicker piece of the second layer.