Calculator

Humidity forecast

Choose a location to see the current outdoor reading.

Readings come from Open-Meteo, an open weather API — not from Apple Weather, which is what the iPhone app uses. The two model and interpolate differently, so expect small differences between this page and the app. Your coordinates are sent to Open-Meteo to fetch the reading and are not stored here.

The figures above are the answer for the values this page loaded with. Changing the controls needs JavaScript — without it the calculator cannot recalculate, so read them as one worked example rather than a live result.

What will the humidity be later, or tomorrow?

Choose a location above for an hour-by-hour outdoor humidity and dew point forecast covering the rest of today and tomorrow. Watch the two rows separately: the percentage will climb steeply overnight and fall through the morning, while the dew point underneath barely moves — because only one of them is actually about the water in the air.

Reading the two rows

Each hour carries two figures, and the whole value of the strip is that they disagree.

The relative humidity on top is the number people quote. It will swing enormously — 95% before dawn, 45% by mid-afternoon — and almost none of that swing is water arriving or leaving.

The dew point underneath is the water. Watch it across the same 24 hours and it usually moves a few degrees at most, unless a genuinely different air mass arrives. When it does jump, something real has happened: a front, a wind shift, air off the sea instead of off the land.

What each pattern means

Flat dew point, swinging humidity — an ordinary day. The air is not changing; the thermometer is. Nothing to plan around beyond the usual dawn dampness.
Dew point climbing through the day — humid air is moving in. Evenings will be heavier than the forecast high suggests, and overnight lows will not fall far.
Dew point and temperature converging overnight — the air is heading for saturation. That is dew on every surface, and if they meet, fog.
Dew point falling sharply — drier air arriving, usually behind a front. Good drying, big temperature swings, and static indoors a day or two later.

The overnight floor

The most practical thing in the strip is that the dew point sets a floor under the overnight temperature. Air cannot cool much past the point where it saturates, because condensing water releases heat almost as fast as the ground loses it.

So if tomorrow's dew point is 68°F, the night will not be a cool one, whatever the sky does. And if it is 45°F on a clear calm night, the low can go a long way down — which is the basis for working out whether there will be a frost when the forecast low is borderline.

Where the forecast comes from

Open-Meteo, read directly from your browser. The derived figures — the comfort bands, the condensation margin — are computed here with the same implementation the Hygrometer app uses, though the app's underlying observations come from Apple Weather rather than this source. Small differences between the two are expected and are about the weather model, not about the arithmetic.

Common questions

What is the humidity forecast for tomorrow?

Pick a location above — the strip covers the rest of today and all of tomorrow, hour by hour. Each hour shows the relative humidity, the dew point beneath it, and a bar coloured by which comfort band the humidity falls in.

Why does the forecast humidity peak overnight?

Because relative humidity is measured against the saturation point, and that falls as the air cools. The water content usually barely changes overnight — but the air's capacity to hold it drops, so the same water reads as a much higher percentage. Peak humidity is typically just before dawn, at the coldest moment of the night.

How far ahead is a humidity forecast reliable?

Dew point is generally well forecast a day or two out, because it tracks the air mass and air masses move predictably. Relative humidity inherits the uncertainty of the temperature forecast on top of that, so it is the less dependable of the two at range — another reason to read the dew point row.

How do I use this to predict fog?

Look for hours where the dew point and the air temperature converge. When the gap closes to about a degree the air is saturating and mist or fog becomes likely, which is usually in the hours before sunrise on a clear, calm night. There is a fuller explanation in the guide on reading the dew point spread.

Is this the same forecast as the app?

Similar, not identical. This page reads Open-Meteo; the iPhone app reads Apple Weather. They run different models, so expect small differences in the numbers even though the physics applied on top of them is the same code.

Sources

Open-Meteo — open weather API — Source of the forecast on this page

Published 31 July 2026