Which Trewartha climate group uses the letter E?

The Trewartha climate group that uses the letter E is the Boreal group.

Trewartha’s system modifies the Köppen climate classification by dividing the middle latitudes according to how many months average at least 10°C. Group E covers climates with only one to three such months, giving them short, cool summers and long winters. These climates are commonly described as subarctic or boreal.

Group E is not the same as Group F, which represents polar climates. In the Trewartha system, polar climates have no month averaging 10°C or higher. Later versions sometimes subdivide Group E into maritime subarctic and continental subarctic types, but the group letter itself remains E.