According to the abstract to their paper, they looked at “levels of temperature, barometric pressure, humidity, fine particulate matter, black carbon, and nitrogen and sulfur dioxides … with corresponding levels on the remaining occurrences of that day of the week in a given month, using local meteorologic and pollutant monitors.”
In fact, for every nine degrees (Fahrenheit) of increase in the outside temperature, the researchers found that the risk of a headache increased by 7.5%.
Higher temperatures were indeed the largest factor (within their study) in increasing the risk of headaches. However, that impact may not be very much at all.
They state, “This magnitude of excess risk is obviously modest and may not be an important factor in the clinical management of individual patients, given the many other potential triggers of migraine that patients face.” [WebMD: “Warm Weather May Trigger Migraines”]
The researchers also discovered that barometric air pressure was not linked to migraine headaches. They did find, on the other hand, that lower air pressures did increase the risk for non-migraine headaches in a small way during the period of 48 to 72 hours before their ER visit.
The researchers also found that air pollution was not “strongly associated with an increased risk for migraine or non-migraine headaches.” [Abstract] However, they did see a “borderline” effect between non-migraine headaches and the pollutant nitrogen dioxide, which comes out of the exhausts of cars.
Page three concludes with comments from one of the authors, along with sources on headache information.