The Invisible Squeeze
Ask anyone what atmospheric pressure means, and you’ll usually get a shrug. Yet it’s one of the most valuable clues a weather watcher has. Pressure is simply the weight of all the air pressing down on you. Imagine the atmosphere as an ocean of gas several miles deep, resting on your shoulders. At sea level, that’s roughly a tonne of air pressing on every square metre of your body — about 15 pounds per square inch. You don’t feel it because the pressure inside you pushes back out, perfectly balanced.
But here’s the crucial bit: that weight isn’t constant. It rises and falls depending on what the air above you is doing, and those changes telegraph tomorrow’s weather with surprising reliability.
In Britain, we live in the firing line of shifting pressure systems, sitting as we do under the battleground between Atlantic maritime air and continental influences. Our local readings here in Monkton Heathfield typically range from around 980 hPa (millibars) during deep depressions to 1040 hPa in stubborn winter anticyclones — a variation of about 6% either side of the average of 1013 hPa.
What Pressure Tells You
The magic of pressure lies not in the absolute value but in the change.
High pressure means air is descending and spreading outward at the surface. Descending air warms and dries, suppressing cloud formation. The result is settled weather: clear skies in summer, crisp frosty mornings in winter, and light winds. Highs tend to move slowly and dominate for days.
Low pressure is the opposite — rising air, drawing wind inward. Rising air cools, moisture condenses, and out come the clouds and rain. Lows move faster, generally west to east across us, each bringing its own frontal rain band and blustery interlude.
Even more useful is the rate of change. A fall of more than 3 hPa in three hours — something our station monitors continuously — often means a depression is deepening and approaching, and unsettled weather will arrive within a day. A slow, steady rise suggests a ridge of high pressure building and calmer conditions returning. Mariners worked this out centuries ago with nothing but a mercury barometer; the same logic now drives the charts on this website.
So What Is Humidity?
Humidity measures something quite different: how much water vapour the air is holding compared to how much it could hold at that temperature.
Think of air as a sponge. At 15°C, that sponge can hold a certain amount of water vapour. Relative humidity of 70% means it’s carrying seven-tenths of its capacity. Crucially, warm air holds far more moisture than cold air — warm the air by ten degrees and, with the same moisture content, the relative humidity plummets. That’s why central heating makes winter air feel desert-dry indoors, and why your lips chap: the air outside was damp, but heated-up air is thirsty.
This temperature dependence is also why dew forms. As evening cools the air, its capacity to hold moisture shrinks until, at the dew point, it can hold no more. Excess vapour condenses — onto grass as dew, or in the air itself as mist and fog. Anyone who’s driven the lanes around Creech St. Michael on an autumn morning knows how abruptly that can reduce visibility.
Does Humidity Forecast Like Pressure Does?
Here’s the honest answer: not really — at least not on its own. Pressure is causative. The pressure gradient physically drives the wind and determines whether air rises or sinks, which determines cloud and rain. When pressure changes, weather follows, roughly on schedule. It’s a leading indicator. Humidity is more of a reactionary indicator. It tells you the current state of the air — how moist it is right now — but high humidity doesn’t cause weather in the way falling pressure does. A humid summer afternoon doesn’t guarantee rain; it merely shows the raw material is present if something comes along to lift the air. Where humidity earns its forecasting stripes is in combination:
Very high humidity plus falling pressure is a classic pre-rain signature. Warm, moist air arriving ahead of a front will push humidity up while pressure begins its slide. Our station saw exactly this pattern repeatedly during the wetter spells this year.
High humidity with light winds on a clear evening predicts fog or heavy dew by morning — the temperature will fall toward the dew point, and condensation follows. This is one of the most reliable short-range forecasts an amateur can make, needing nothing more than a thermometer, a hygrometer, and a glance at the sky.
A sudden drop in humidity in winter often accompanies the arrival of colder, drier continental air — which, if it persists, is the setup for frosts.
Reading the Two Together
The most skilled weather readers treat pressure and humidity as complementary instruments, a bit like reading a suspect’s motive and opportunity together. Pressure gives you the why — the engine of weather systems bearing down on us. Humidity gives you the how wet — how much moisture is available for those systems to work with. For practical guidance from this site’s live data: watch the pressure trend chart first for signals about the next 24 hours, then glance at humidity to sharpen the picture. Falling pressure with humidity climbing through 85–90% means conditions are ripening for rain. Steady or rising pressure with moderate humidity? Settle in for something drier.
Listen to the Barometer
There’s an old weather proverb that captures it nicely: “When the glass falls low, prepare for a blow.” A hundred and fifty years before satellites, coastal communities lived by their barometers because pressure genuinely foretells the weather. Humidity, by contrast, is best treated like a good supporting witness — worth listening to, but only decisive when paired with other evidence.
Next time you glance at our gauges, spare a thought for humidity and pressure. One tells you what’s coming; the other tells you if you want to take your umbrella. And that would make you your own forecaster.