The short answer
Clouds block the sky outright, but they aren’t the only weather that matters. Thin, high cloud can dim faint stars without being obvious, and humid or hazy air scatters light, which brightens the sky and hides faint objects even on a cloudless night. High humidity also brings dew, which can fog binoculars and telescopes. For stargazing, check an hourly forecast that shows cloud cover at different heights, and watch the humidity and dew point as well as the clouds.
Not all clouds are equal
| Cloud | What it does |
|---|---|
| Low cloud and fog | Blocks the sky completely while it is overhead |
| Mid-level cloud | Usually blocks the sky; gaps can open and close quickly |
| Thin high cloud (cirrus) | Often hard to see at night; dims faint stars and adds a glow around the Moon and bright lights |
| Daytime fair-weather cumulus | Often dissolves around sunset as the ground cools |
Thin high cloud is the one that fools people. At night it can be almost invisible, but the sky looks gray rather than black, fewer stars are visible than usual, and a hazy ring may appear around the Moon. In light-polluted areas it’s often easier to spot, because it reflects city light and glows faintly.
Puffy daytime clouds that form over warm land often fade in the evening as the ground cools, so a cloudy afternoon doesn’t always mean a cloudy night. The reverse is also true: clear evenings can cloud over later, especially when a front is approaching.
Humidity and haze
Moisture in the air, along with dust, smoke and pollution, reduces the sky’s transparency: how much light gets through. Water vapor itself is invisible, but in humid air tiny particles absorb moisture and grow, scattering more light. The result is haze.
Haze hurts stargazing in two ways:
- It dims faint objects. Faint stars, the Milky Way and galaxies fade first. Stars near the horizon suffer most, because their light passes through the most air.
- It brightens the sky. Haze scatters artificial light, so light domes from towns spread and grow. A suburban sky on a humid summer night can look noticeably worse than the same sky on a dry winter night. See the Bortle scale for how sky darkness is rated.
Smoke from distant wildfires has a similar effect and can travel very long distances, turning an apparently clear sky milky. Some of the clearest skies come after a cold front or a period of rain has swept particles out of the air, though the air can be turbulent for a while afterward. Seeing and transparency explains the difference between clear air and steady air.
Dew and your equipment
On clear nights, surfaces lose heat by radiating it toward the sky and can cool below the air temperature. When a surface drops to the dew point, the temperature at which the air becomes saturated, moisture condenses on it. That is why dew forms most on clear, calm, humid nights, and why it often appears first on things pointed at the sky, like a telescope’s front lens.
- Use a dew shield. A tube extending past the front of a telescope or finder slows the lens’s heat loss to the sky.
- Consider a dew heater. A low-power heating strip keeps optics just above the dew point. Many observers in humid climates consider one essential.
- Don’t wipe dewed optics. Rubbing can scratch coatings. Let them dry naturally indoors with the caps off, then cap them once dry.
- Cover what you’re not using. Keep eyepieces and binoculars in a case or pocket between uses.
When the air temperature is forecast to fall close to the dew point, expect dew, and in calm conditions possibly fog.
Reading a forecast for stargazing
A general weather icon is built for daytime and is often wrong about the night sky. More useful:
- Hourly cloud cover, ideally split into low, mid and high cloud, so you can see when gaps are expected and whether the cloud is the thin kind or the thick kind.
- Humidity and dew point, to judge haze and dew.
- Satellite imagery, which shows where the cloud actually is and which way it’s moving. It’s a good check on the forecast in the hour before you go out.
- An astronomy-specific forecast, many of which add estimates of transparency and seeing.
The stargazing conditions tool brings several of these together. Cloud forecasts are uncertain, especially for patchy cloud, so treat them as odds rather than promises.
Making the most of a partly cloudy night
Partly cloudy nights can still be productive. Pick bright targets that show in the gaps, such as the Moon, planets and bright stars, rather than faint ones that need a long, steady view. Binoculars are ideal because they’re quick to point. And keep an eye on the sky: gaps in a breaking cloud layer can widen into a clear spell. For what’s up while the gaps last, check tonight’s sky, and for other factors that decide whether you can see something, read why can’t I see it?