People in large cities often notice their "humidity today near me" reading sits 5–15% above the suburbs a short drive away, and it is rarely an instrument error. Concrete, asphalt and masonry absorb heat all day and release it slowly through the evening, creating what meteorologists call an urban heat island. Warmer air holds more water vapour than cool air, so a downtown block can stay measurably muggier than the countryside around it even when there is no water anywhere nearby.
The built environment adds moisture of its own on top of that. Air conditioning condensers dump warm, damp exhaust into narrow streets, vehicle engines release water vapour as they burn fuel, and every square metre of paving is a square metre that is no longer a park, a lawn, or a tree canopy able to cool the air through shade and evaporation. Tall buildings then trap the result: air moves slowly through a street canyon, so heat and humidity accumulate instead of dispersing.
The practical consequence is that a city-wide forecast tends to describe conditions at an airport on the edge of town — open, exposed, and several degrees cooler than the centre. Reading humidity from your own GPS position instead picks up the station that actually shares your surroundings, which is why the number here can differ noticeably from a national weather app. If you are deciding whether to run, cycle, or leave a window open, that difference is the part that matters.