Seasonal allergies really seem to contribute my migraines. Extreme heat can also trigger my migraines. Also, many summer activities such as mountain climbing/hiking or sitting/lying around in the sun for too long can trigger my migraines or make them worse. When it goes from heat to rain, that almost always triggers a migraine for me, and that can happen quite a bit around where I live. When the weather stays the same though, and we're not in an extreme heat wave, my migraines stay pretty stable though, even in the summer. :)
Seemed I could only vote for one- I have problem with heat and barometer changes, I believe. Wouldn't be surprised if some congestion plays a role too.