Firestorm

Perhaps the most feared area effect skill in the Witch's arsenal, the mighty Firestorm skill is legendary for its power and cruelty. The incantation tears a rift in the heavens that showers a wide swath of ground in a rain of searing flames. These bolts pound down without mercy, burning everything they touch, then exploding violently to cause even more harm.

Description on the official Path of Exile website

Skill Functions and Interactions

Area of Effect: Increased AoE will increase both the area in which the fireballs randomly fall and also the radius of the individual fireball damage. Full spell damage is done at any point within the AoE of a fireball, however due to the fact that enemies take up an area, and not simply a point less spell damage is dealt to any given enemy as area increases. This is why, contrary to popular belief, increasing the AoE of Firestorm decreases the average number of hits to single target.

Duration: Increasing duration with either supports or passives will increase the total number of fireballs that fall before the spell ends.

Damage Effectiveness: Damage from secondary sources like 'added damage' supports or heralds applies at only 30% of their listed value.

Damage per Second: Due to the complexity of estimating actual DPS of the skill including all possible factors, ingame tooltip shows only average damage per hit of one fireball, with actual DPS of the skill being considerably higher. The closest currently available estimate can be obtained by using the damage calculator.

Gem level progression

Level			Mana Cost	Fire Damage	Exp.	Total Exp.
1	28	67	13	N/A	0	0
2	31	73	14	N/A	199345	199345
3	34	79	15	N/A	285815	485160
4	37	85	16	N/A	401344	886504
5	40	92	16	N/A	554379	1440883
6	42	96	17	N/A	477437	1918320
7	44	100	18	N/A	583786	2502106
8	46	104	18	N/A	710359	3212465
9	48	109	19	N/A	1355511	4567976
10	50	113	19	N/A	1138877	5706853
11	52	117	20	N/A	1368233	7075086
12	54	121	20	N/A	1638338	8713424
13	56	125	21	N/A	1956648	10670072
14	58	130	21	N/A	3655184	14325256
15	60	134	22	N/A	3017327	17342583
16	62	138	22	N/A	7759995	25102578
17	64	142	23	N/A	15138193	40240771
18	66	146	24	N/A	26083825	66324596
19	68	151	24	N/A	62620247	128944843
20	70	155	25	N/A	211708088	340652931
21	72	159	25	N/A	N/A	N/A
22	74	N/A	26	N/A	N/A	N/A
23	76	N/A	26	N/A	N/A	N/A
24	78	N/A	27	N/A	N/A	N/A
25	80	N/A	27	N/A	N/A	N/A
26	82	N/A	28	N/A	N/A	N/A
27	84	N/A	28	N/A	N/A	N/A
28	86	N/A	29	N/A	N/A	N/A
29	88	N/A	30	N/A	N/A	N/A
30	90	N/A	30	N/A	N/A	N/A
31	91	N/A	30	N/A	N/A	N/A
32	92	N/A	31	N/A	N/A	N/A
33	93	N/A	31	N/A	N/A	N/A
34	94	N/A	31	N/A	N/A	N/A
35	95	N/A	31	N/A	N/A	N/A
36	96	N/A	32	N/A	N/A	N/A
37	97	N/A	32	N/A	N/A	N/A
38	98	N/A	32	N/A	N/A	N/A
39	99	N/A	33	N/A	N/A	N/A
40	100	N/A	33	N/A	N/A	N/A

Alternate skill effects

Name	Cost	Description
Siege Firestorm EffectSiege Firestorm EffectSkill Gem EffectYour Firestorm becomes a flaming siege assault	25	Replaces the standard projectile effects on a gem with burning cages of spiked metal. Preview

Version history

Version	Changes
2.1.0	Damage increased by around 20% higher at level 20.
2.0.0	Firestorm: Had a very slight damage buff. Area of effect of explosions increased by 11% (9 to 10). It now grants 1% area damage per 1% quality instead of reduced delay between fireballs. The Duration of Firestorm has been increased from 1.3 seconds to 2 seconds at all levels. Base Cast Speed has been increased from a cast time of 1 second to 900ms. Critical Strike Chance has been increased from 5% to 6%.
1.0.4	Firestorm: The radius of the area that fireballs land in has been increased by 39%. The delay between strikes has been reduced from 0.15 seconds to 0.10 seconds. Damage has been increased by 35%. The quality bonus has been reduced by 33.33% to match the reduced projectile delay.
1.0.0	Reduced mana cost.
0.11.2	The damage done by the Firestorm spell when used by all versions of Ambrosia and Merciless Fire Fury has been increased. Merciless Gravicius's Firestorm has been very slightly increased in damage.
0.11.1	Spells with Damage Effectiveness no longer have their own damage affected by Damage Effectiveness. Damage values on these skills have been updated to reflect this.
0.9.12qq	Fixed a bug where monsters in high level areas would not do the correct Firestorm damage.
0.9.11i	Fixed the randomness when it was used with Traps.
0.9.11f	Increased the damage and made its cluster tighter.
0.9.9	Fixed a bug where the damage from Firestorm would not display on the client at the right time.
0.9.6	Changed Firestorm to be affected less by additional damage from support gems. Fixed a bug where Firestorm could hit monsters through walls.
0.9.5	Modified Firestorm's behaviour so that the explosions have a larger radius and are more likely to fall near the centre of the affected area. Doubled the number of fireballs that fall and rebalanced damage around these changes.
0.9.3c	Raised the overall damage and its high level damage scaling.
0.9.3	Firestorm has been added to the game.