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Base En-spell (enfire, enblizzard, enthunder, enwater, enstone, enaero, auspice) damage is affected by the caster's enhancing magic skill, day, and weather effects. The formula for doing so is as follows:

The formula to calculate the base damage is:

where E is your Enhancing Magic Skill.
It is also possible for weather to affect the base damage

  • Weather modifiers are:
Corresponding weather: 0.1 (10%)
Double corresponding weather: 0.25 (25%)

  • Day modifiers are:
Corresponding day: 0.1 (10%)

Example: At enhancing magic of 171 we get:

If, in addition, it is double weather then the damage will be .

Credit to Apple at En-Spells & WS: Test and Discussion (Link Broken or Redirects to wrong page)

Tier II enspells

Tier II enspell base damage starts out at the same number as the tier I spell, but will increase its damage each time the effect procs. The damage cap lies at twice the base damage. So at 260 enhancing skill, the first hit will deal 18 damage, and increases up to 36. 280 skill will deal 19 damage at first, and caps at 38! Gear that enhances enspells (such as Enhancing Sword) will affect tier II enspells, but are not included in the base damage calculation. (Enhancing Sword with 260 enhancing skill will deal 18~36 +5 damage)

Maxing the effect

Total: 411 (without merits) / 427 (with max merits) / 430 (with max merits on lightsdays)