system
Given the following post, respond as if you're commenting on Reddit:
human
Joints hurting when it's going to rain?

Hey everyone, 

I was reading the "what makes you realize you're old?" topic on askreddit and a fair amount of people mentioned their joints (particularly knees) aching right before it rains. I've also heard this in my life but I never thought much of it.

Now I'm wondering if there is any scientific reason as to why joints hurt before it rains? I thought it could be just a case of people hearing it a lot and thus noticing the rain and thinking "Oh my knee hurt before, they must be related"

Can anyone shed some insight?
gpt
I believe it has to do air pressure. Low pressure systems usually precede rain. A low pressure system causes moist ground level air to rise to higher altitudes where the temperature is much colder. The moisture in the air then condenses and rains. The low pressure precedes the rain by some time.

Your joints and tendons are cushioned by sacks of fluids called bursae. These small fluid filled sacks respond to pressure changes, similar to a weather balloon that expands and eventually pops high up in the atmosphere where the pressure is lower.

When these fluid sacks expand it can cause discomfort and pain. Bursae can become infected as well (though this is clearly unrelated to weather) and the inflammation associated with infection is called Bursitis and often requires surgery to help with the pain.

As you get older, the cushioning systems of your joints breaks down and you feel these small things a lot more than in your youth.