Hygrometer
Three things to read, in order
A dew point graph carries more information than a temperature graph, and almost all of it is in relationships rather than in the numbers themselves.
- Level — how high the dew point line sits. This is how much water is in the air, and it maps straight onto how the day will feel.
- Slope — whether it is rising or falling. A moving dew point means a different air mass is arriving. A flat one means nothing is happening except the sun.
- Gap — the vertical distance to the temperature line. This is the dew point spread, and it is what predicts anything visible.
Most people read only the first. The third is the useful one.
The shape you will see most
On an ordinary day the temperature line traces a broad arc — low before dawn, peaking mid-afternoon — while the dew point line runs nearly horizontal underneath it.
That flat line is the point. It means the air over you contains the same water at 4am as at 4pm, and every bit of the relative humidity swing you saw in the forecast was the temperature moving, not the water. It is the clearest possible demonstration of why humidity climbs at night without a drop being added.
The shapes that mean something
| Pattern | What is happening | What to expect |
|---|---|---|
| Flat dew point | Stable air mass | An ordinary day. Dampness at dawn, drying through the morning. |
| Dew point rising | Humid air moving in | Heavy evening, warm night, slow drying. Finishes may not cure. |
| Dew point falling sharply | Drier air behind a front | Good drying, big temperature swings, static a day or two later. |
| Lines converging overnight | Approaching saturation | Heavy dew, then mist or fog before sunrise. |
| Lines already touching | Saturated | Fog now. Nothing outdoors will dry. |
| Afternoon dew point dip | Daytime mixing | Normal. Expect it to recover after sunset. |
The gap, in numbers
The spread between the temperature and the dew point is worth learning as a set of rough thresholds:
- Under 2°F — the air is effectively saturated. Fog, mist, and dew on everything.
- 2–5°F — very little margin. Cold surfaces will run with water.
- 5–9°F — cold glass and metal will fog; most other things stay dry.
- Over 9°F — a wide margin. Condensation needs something genuinely cold.
This is the calculation behind will it be foggy, and it is what pilots mean when they talk about the temperature-dew point spread before a morning departure.
The overnight floor
One prediction the graph makes reliably: the dew point sets a floor under the overnight temperature.
As the air cools towards its dew point, the water starting to condense releases latent heat, which slows the cooling to a crawl. So the temperature line tends to flatten out as it approaches the dew point line rather than crossing it.
Practically, that means tonight's dew point is a good first guess at tonight's low. It is the basis for deciding whether there will be a frost when the forecast low is borderline — a dry night falls much further than a damp one.
Where to see one
The humidity forecast draws the hourly dew point and relative humidity for the next two days at any location, with each hour banded by comfort. Watching the two rows diverge overnight makes the whole idea concrete in a way no explanation quite manages.