3.5 Map CR help

[AkazilliaDeNaro]

So I recently read through a book, and in said book there was an area that I have based a map on.

The area was an Ice cave of sorts, that would smash areas at certain intervals, and the deeper you went into the cave the colder it got, to the point where a freezer would feel warm in comparison.

The map I made is 90' x 90', hails in random spots at random points, all of the map is difficult terrain, and there are a few(2-3) Ice Mephits inside (CR 3).

I don't know what to make the hail damage, the cold environment damage, and what level i should hit the players with this map.

[Strigon]

It sounds like standard fare for a level 4 party, or an easy level 5. Level 3's might run into some issues, but if you give the right treasure at the end, those can often be the most fun!

Honestly, it seems as though this is quite simple for a party who does their research, or plans in advance; cold isn't the hardest thing to get around (Though if they don't have a spellcaster, it becomes a bit more annoying - be sure to repay them in kind). I think this is something good for level 4's, and it lets them know monsters aren't always the main threat, which can come in useful later!

[Gwachitallemall]

Hail damage is pretty minor by itself, looking at the DMG, (Not Frostburn, because I don't have it with me) Hail does 1 lethal damage, per storm if it's large enough. (5% chance.)

Your cold damage is where you'll get your party. There's the Cold weather (Below 40F), which is fort save per hour, so that one won't probably matter.

Then there's Severe cold, (Below 0 F), which is fort save every 10 minutes.

Then there's extreme cold. (Below -20F), which is no save, every minute you take 1d6 lethal. In addition, you must make a fort save dc 15, or take 1d4 non lethal. In addition, if any of your party is wearing metal, they are under the effect of the chill metal spell.

All of these items are in the DMG/d20SRD.

[AkazilliaDeNaro]

Hail damage is pretty minor by itself, looking at the DMG, (Not Frostburn, because I don't have it with me) Hail does 1 lethal damage, per storm if it's large enough. (5% chance.)

Your cold damage is where you'll get your party. There's the Cold weather (Below 40F), which is fort save per hour, so that one won't probably matter.

Then there's Severe cold, (Below 0 F), which is fort save every 10 minutes.

Then there's extreme cold. (Below -20F), which is no save, every minute you take 1d6 lethal. In addition, you must make a fort save dc 15, or take 1d4 non lethal. In addition, if any of your party is wearing metal, they are under the effect of the chill metal spell.

All of these items are in the DMG/d20SRD.

The areas where it is hailing are supposed to be super dangerous. so i was thinking 1d4 cold/bludgeoning per round inside.

Also i was thinking a middle ground between severe and extreme, fort save every 5 rounds.

[Surpriser]

In addition you should introduce various obstacles (of the terrain variety, not monsters) that have to be overcome. Examples would be ice crevasses, ledges, cave-ins, pillars, slides, sinkholes,...
Otherwise the whole adventure becomes a series of (eventually boring) Fort saves and the question: Who has the highest movement speed?
In addition, this helps if the players decide to spend some ressources to get Endure Elements for everyone.

Depending on how long you want to keep your players inside, they should not make more than one to maybe five saves versus the cold, with another few at the coldest zones. Otherwise you will soon reach the "Another Fort save? How thrilling..." state. However, you can ask for additional saves when they fail at some of the obstacles (for example, falling into a crevasse or being buried in ice).

Also, I would propose to leave the damage from the cold at rather minor levels - thats what the obstacles and monsters are for. Instead, play up the additional effect of severe cold: Hypothermia. According to the SRD, anyone who takes damage from environmental cold becomes fatigued. While by RAW this does not escalate, you might change that to increase to first exhaustion and then unconsciousness on additional failed saves - e.g.:
1 failed save: fatigued
3 failed saves: exhausted
5 failed saves: unconscious