Relationship Between Rain and Headache

　Today, it was raining, later snowing, and I had a headache almost all day. When it rains, I usually get a headache. It is thought that one of the most important factors of migraine is the change of the weather. When the updraft occurs, the weather becomes bad, and the atmosphere pressure drops. By the atmosphere pressure is dropping sharply, the pressure that is holding the blood vessels in the brain also drops, then the blood vessels are more likely to spread. By the blood vessels in the brain is spread, the nerves surrounding the blood vessels will stimulated and as a result, migraine will be induced. This is one of the hypotheses, but according to my experience, it's true.

• Today, it was raining, later snowing, and I had a headache almost all day.
• Today, it rained, and then later it snowed, and I had a headache almost all day.

  Or also: "Today, it rained, and then it snowed, and I had a headache for almost the whole day."

• When it rains, I usually get a headache.
• When it rains, I usually get a headache.

  This sentence is perfect! Another option: "Whenever it rains, I get a headache." or "I get a headache whenever it rains."

• It is thought that one of the most important factors of migraine is the change of the weather.
• It is thought that one of the most important causative factors of migraines is a change in the weather.

  A more conversational way of saying this: "They say that one of the biggest causes of migraines is a change in the weather.

• When the updraft occurs, the weather becomes bad, and the atmosphere pressure drops.
• When an updraft occurs, the weather becomes bad, and the atmospheric pressure drops.

• By the atmosphere pressure is dropping sharply, the pressure that is holding the blood vessels in the brain also drops, then the blood vessels are more likely to spread.
• Because the atmospheric pressure is dropping sharply, the pressure that is holding the blood vessels in the brain also drops, and so (then) the blood vessels are more likely to spread.

  Whoa!

• By the blood vessels in the brain is spread, the nerves surrounding the blood vessels will stimulated and as a result, migraine will be induced.
• Because the blood vessels in the brain are spread out, the nerves surrounding the blood vessels will be stimulated and, as a result, a migraine will be induced.

• This is one of the hypotheses, but according to my experience, it's true.
• This is one of the hypotheses, but according to my experience, it's true.

  Good! Slightly more naturally: This is one hypothesis, but in my experience, it's true.

Wow! That's really interesting. おもしろいね！I hope you can learn as much from my corrections as I learned from your post :) You use some pretty advanced grammar!

I tried to be careful to correct your post to keep your meaning clear. This is slightly off-topic, but I had a concern reading your post: What if the people who correct your post are not as well-informed as you are about the topic that you are writing about?

For example, someone could potentially correct your sentence "the pressure that is holding the blood vessels in the brain also drops" to "the pressure that is in the blood vessels in the brain also drops," which is grammatically correct, but does not mean the same thing, and could be a problem.

Just a thought! And maybe it's not such a big deal.