As a science, human biometeorology studies the relationship between atmospheric conditions and people. There are of course all sorts of indisputable and obvious connections between weather and health. There are also significant but less direct connections between weather and health, such as the onset of allergies during pollen season. In such cases, the atmospheric conditions are clearly affecting health, but they are playing more of a supportive role than a primary one.
But some researchers are interested in looking at less direct potential connections between atmospheric conditions -- like temperature, barometric pressure, and humidity -- and painful conditions such as arthritis, fibromyalgia, and sinus or migraine headaches. The difference here is that the connections are not as obvious and the mechanism that would cause the symptom isn't known.
maybe because we are made of water to a large degree. as it rains humidity rises and our bodies absorb more water. perhaps that dilutes some of the needed fluids that give us ease of movement in our joints and cushioning for other areas? just a thought.