Learn how Ephemeris turns clouds, moisture, wind, visibility, transparency, seeing, and forecast confidence into practical observing guidance.
Instead of reading weather values one by one, use the report as a sequence of questions. Each layer helps answer whether the night is worth observing, imaging, adjusting, or skipping.
Clouds decide whether the session is possible at all.
Jump to Clouds → 02 Can I keep optics dry?Dew point spread and humidity determine moisture risk.
Jump to Moisture → 03 Can my rig stay stable?Wind and gusts affect vibration, guiding, and safety.
Jump to Wind → 04 How good is the sky?Visibility, transparency, and seeing decide quality.
Jump to Sky Quality → 05 Can I trust it?Confidence tells you how much the forecast may change.
Jump to Confidence →The score is not just a weather number. It is a shortcut for session planning. A high score means the major astronomy risks are low. A low score means one or more factors are likely to limit the night.
When you are short on time, use this order. Clouds first. Moisture second. Wind third. Sky quality fourth. Confidence last.
Probably skip or wait for a clear window.
Bring dew shield, heater strap, controller, and power.
Avoid long focal lengths and high magnification.
Favor Moon, planets, double stars, or brighter targets.
Check again closer to sunset before committing.
Cloud cover is the first thing to check because it determines whether astronomy is possible. If clouds block the sky, good seeing, low wind, and dry air will not matter much.
Ephemeris separates cloud cover into low, mid, and high layers because each layer affects observing differently. Total cloud cover alone can be misleading.
Most disruptive. These can completely block the sky and often bring fog or moisture.
Patchy and inconsistent. These can interrupt imaging runs and create uneven sky quality.
Thin but important. These often leave stars visible while reducing contrast and transparency.
Low clouds are usually the most damaging cloud type for astronomy. They sit closest to the ground, block large portions of the sky, and are commonly connected to fog, haze, damp air, and changing weather.
When low cloud cover rises, observing windows can disappear quickly. Even bright planets and the Moon may become difficult if the cloud deck is thick enough.
Mid and high clouds often allow parts of the sky to remain visible, but they reduce contrast, scatter light, and make faint targets much harder to see.
Bright stars, planets, and the Moon may still look fine. Galaxies, nebulae, and long-exposure images usually suffer first.
Humidity and dew point spread work together, but they are not the same thing. Humidity tells you how moisture-heavy the air is. Dew point spread tells you how close that moisture is to condensing on your equipment.
Explains how moisture-heavy the air is and whether haze may reduce contrast.
Explains whether that moisture is likely to form dew on glass, metal, cables, and gear.
Dew point spread is the difference between air temperature and dew point. The smaller the gap, the closer the air is to releasing moisture onto your optics.
This is one of the most important values for astrophotography because a session can begin clear and slowly fail as dew forms on lenses, corrector plates, guide scopes, filters, or camera windows.
High humidity can soften contrast, increase haze, and raise the odds that dew becomes a problem. It matters most during long sessions where equipment is exposed for hours.
Humidity alone does not guarantee dew, but high humidity combined with a small dew point spread is a strong warning sign.
Wind affects telescope stability, guiding, vibration, comfort, and safety. Gusts are especially important because a short burst can ruin frames even when the average wind speed looks acceptable.
Long focal length rigs, large dew shields, lightweight mounts, and high magnification views are more sensitive to wind.
Visibility measures how clear the lower atmosphere is. Smoke, haze, fog, dust, and moisture can reduce visibility and make the sky look washed out.
This does not measure how far into space you can see. It measures how transparent the air is near the ground, which can still affect contrast and deep-sky performance.
These two terms are easy to confuse. Transparency is about how clear the sky is. Seeing is about how steady the atmosphere is.
Matters most for galaxies, nebulae, clusters, Milky Way detail, and faint targets. Poor transparency makes the sky look washed out even when cloud cover is low.
Matters most for planets, lunar detail, double stars, and high magnification. Poor seeing makes stars shimmer and fine detail blur.
A bad night for galaxies may still be useful for the Moon. A poor planetary night may still be fine for wide-field imaging. Match the forecast to what you actually plan to do.
Reality: Dew, wind, haze, and poor seeing can still ruin an otherwise clear night.
Reality: Dew risk depends heavily on dew point spread, not humidity alone.
Reality: Visibility measures lower-atmosphere clarity, not the distance of space objects.
Confidence tells you how much trust to place in the recommendation. It is higher when sources agree and conditions are stable. It drops when cloud models disagree or the weather is changing quickly.
Good for planning a full observing or imaging session.
Conditions may work, but check again closer to dark.
Expect the forecast to change before or during the night.
