Dragons are vulnerable to fire: well, at least some of them.
There are many types of dragons, but they fall into two major archetypes: fire dragons and frost dragons.
Fire dragons are vulnerable to frost (25% weakness) but resistant to fire (50% resistance).
Frost dragons are vulnerable to fire (25% weakness) but resistant to frost (50% resistance).
So if you're trying fire and it's not doing a lot of damage, switch to frost.
But you should be able to tell what type of dragon it is by the type of breath attack it has.
gaming.stackexchange.com/users/None/
Yes, all dragons can be hurt by fire.
After all, if think about it, they are magical creatures and
well, it's destructive magic.
Not to mention, some dragons can be more hurt by fire than others.
It's just common sense that a dragon with ice breath wouldn't like fire, whereas a fire dragon would dislike the cold.
So the above answer is pretty good and does outline which magics work hurt what types of dragons.
However, there is a second part to this question that I don't feel got answered.
"What magic is most effective against dragons?
"
While fire and ice magic does have hit power, it's not always the best approach.
Dragons can show up without notice, and skyrim has a diverse climate.
There are times where both a fire and ice dragon can appear.
At such times, it's not always practical to cast the typical fire/ice magic.
This is why I go for shock magic.
It works the same against each type, allowing me to more fully prepare for sudden attacks.
I like to temper this with healing magic, so that I don't have to pop potions every time I get low.
That way, I'm self sustaining.
Dragon shows up?
I'm ready.
Even if my magic regen is low, as long as I can get cover, I can keep healing.
It works wonders, even at lower levels.
gaming.stackexchange.com/users/14739/Maho
The other answers are all great, but it still takes too long to kill them that way.
What I use is the first level Marked for Death shout found in the dark brotherhood sancurary.
Since its recharge time is short, you can use it 3-4 times and their heal drop by a lot.
Even works on Elder and Ancient dragons.
By using a 105 bow, I was only able to take out 1mm of health, but using the shout a few times, my bow can take out an ancient dragon with only 5-6+ hits with a 105 ebony bow.
gaming.stackexchange.com/users/19002/Arctic IceFox
Definitely use Shock against dragons.
A flying creature that gets struck with lightning either dies or stops flying; then I can just start use poisons to weaken them even further (for example, weakness to fire... or shock) plus enchanted daggers and shouts (try the ice shout).
(Note Class: "Sneaky Gandalf").
Shock plus posion (weakness, Parylisis, Ebonyskin & Dragonhide) plus magicka immunity plus magicka regeneration at 250% gives you a dead Alduin on expert.
gaming.stackexchange.com/users/15354/Marp Sugar
i think that the most powerful spell against a dragon is the oppisite of what type the dragon is for example: fire dragon is most vulnerable to ice and frost
spells frost dragon is most vulnerable to fire and flames
gaming.stackexchange.com/users/18820/cobro1998
Of note is that dragons use their Magicka pool for shouts(all dragons have 150 Magicka).
If you can manage to keep their pool of Magicka continually depleted through the use of shock spells, they cannot shout at your or anyone else.
Dual Casting and Impact help with this.
Before you have damaged their pool enough to prevent them from shouting, the stagger effect from Dual Casting can also prevent them from managing to get out a shout.
Another plus for shock based spells compared to the other elements is that it hits your target instantly.
It can be very hard to hit flying dragons with fire or ice spells, but it is relatively very easy to hit them with shock spells.
gaming.stackexchange.com/users/181987/Jtenorj3
I found having a daedria lord that you can conjure can do very well when they hit the ground.
there damage is very effective, and if you have a high enough lv in conjuring you can summon 2 of them.
summoning 2 of these guys will draw the dragons attention and you just sit back with a bow and shot them... or having a 2 storm atronach for long range is also affective..
gaming.stackexchange.com/users/26666/Dylan
In Skyrim, What type of magic are Dragons most vulnerable. I know it's not fire. Is it more effective to use frost, spark, or a power other than destruction when battling Dragons?
What are Dragons most vulnerable to?
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gaming.stackexchange.com/questions/35489
gaming.stackexchange.com/users/13588/m.n.wiser
Fire dragons are vulnerable to frost but not fire. You can use Shock against dragons. Shock, Fire and Ice seem to be the most prominent options depending on the type of dragon.
Shock, Fire and Ice seem to be the most prominent options depending on the type of dragon.