Why Humidity, Not Pressure, Seems to Affect Ankylosing Spondylitis
Ask someone with ankylosing spondylitis whether weather affects their symptoms and most will say yes without hesitating. Ask them whether it's the cold, the damp, or the pressure, and the answers get a lot less consistent. That's actually a useful clue, because when researchers have looked closely at weather and AS, the pattern that holds up isn't pressure. It's humidity.
What the research shows
Weather and pain has been studied across several rheumatic conditions, most thoroughly in osteoarthritis and rheumatoid arthritis, and more recently in axial conditions like AS. A large UK study called Cloudy with a Chance of Pain, run by the University of Manchester, tracked over 13,000 people with chronic pain conditions using a smartphone app and found a real link between weather and pain, with humidity and wind standing out more consistently than pressure alone. Studies looking specifically at ankylosing spondylitis symptom severity against local weather records have reported a similar pattern: more humid days correlate with more reported pain, with the effect often showing up one to seven days after humidity rises rather than immediately.
That lag is worth paying attention to. It means the humid day itself usually isn't when you feel it. It's the days after.
Why humidity and not pressure
It's a fair question, since barometric pressure gets most of the attention in weather-and-pain conversations generally. But when studies control for temperature alongside pressure, pressure's effect on its own tends to weaken or disappear, while humidity's effect holds up. One plausible explanation is that humidity affects the viscosity of synovial fluid and the behaviour of connective tissue around joints and entheses, the sites where tendons and ligaments attach to bone that are so often affected in AS. Cold, damp conditions are also a classic trigger reported by people with inflammatory joint conditions generally, and humidity is a big part of what makes cold feel "damp" rather than simply cold.
Why this matters for how you track symptoms
If you've been mentally filing away "bad weather" as your trigger, it's worth being more specific. A cold, dry, high-pressure day and a mild, humid, low-pressure day can feel completely different to someone with AS, even though both might get lumped together as "bad weather" in memory. And because the effect can lag by several days, connecting a flare back to the actual humid day it started with is genuinely hard to do from memory alone.
Tracking humidity with Spondy
This is why Spondy tracks local humidity specifically, rather than barometric pressure. It reads your local humidity automatically and flags it as Low (under 40%), Moderate (40 to 70%), or High (above 70%, where the research shows the strongest link to worsening symptoms). Because it's logged automatically day by day alongside your flares, enthesitis, and pain scores, you can look back and see whether your flares actually cluster in the days following high-humidity spells, rather than relying on a general sense that "the weather's been bad."
If the pattern's there, you'll have real evidence to bring to your rheumatologist, and a heads-up on the days worth planning around. If it's not, you'll know to look elsewhere for what's actually driving your flares.
See whether humidity is driving your flares
Spondy tracks local humidity automatically alongside your symptoms, so you can see your own pattern instead of guessing.
This article is for general information and isn't a substitute for medical advice. Talk to a rheumatologist or your doctor about your own symptoms and treatment.