Dew Heaters and the Air-Conditioned Room
In Guyana the cold place is indoors. A lens carried out of an air-conditioned room fogs at the door, and glass left under a clear, humid sky gets wet within hours. This lesson shows why, when a dew heater is worth running, and how to read both from tonight's forecast.
Dew Risk Calculator · Dew and Monsoon Playbook
Most advice about dew and condensation is written for cold countries, where people carry warm gear out into cold air. In the tropics the problem runs the other way, and the night air is wetter. Two numbers from the forecast answer both problems: the air temperature and the dew point.
Why glass gets wet
The dew point is the temperature at which the water vapour in the air starts to condense. Any surface colder than the dew point gets wet, even when the air around it is warmer.
- On a humid Georgetown night, air at 26 °C and 85% humidity has a dew point of about 23.3 °C.
- So anything cooler than 23.3 °C collects water, although the air is 26 °C.
Glass that faces a clear sky loses heat to it, so it ends up colder than the air. The British Astronomical Association puts it at 1 to 2 °C below the air after a couple of hours under a clear sky, and several degrees below in still air. That is why the number to watch is the spread: the air temperature minus the dew point.
- Spread of 2 °C or less, or humidity of 92% or more: bare glass will reach the dew point. The site calls this high dew risk.
- Spread of 4 °C or less, or humidity of 80% or more: it can on a still night. The site calls this moderate.
- Anything drier: low. Dew is unlikely, though a dew shield costs little.
These are the same rules Stargazing Weather, the Tonight score and the Dew Risk Calculator all use.
The site's dew rule: move the humidity and the spread and watch the risk change.
The air is 25 °C and the dew point is 23.5 °C. What dew risk does the site give?
- Low
- Moderate
- High (the right answer)
- It depends on the Moon
The spread is 25 minus 23.5, which is 1.5 °C. That is under 2 °C, so bare glass facing a clear sky will reach the dew point: high risk.
What a dew heater does
A dew heater is a strap with a heating wire inside. It wraps around the lens barrel or the dew shield, close to the front glass, and keeps the glass a little warmer than the air so it never reaches the dew point. Kendrick, a heater maker, advises keeping optics 2 to 4 degrees above the air. The British Astronomical Association adds that with a good dew shield, a fraction of a degree is enough.
- Start it at setup: before the hour the risk turns high, not after the glass has fogged.
- Add a dew shield: a tube that reaches past the front glass, so the glass sees less of the cold sky.
- Low power is enough: the aim is a few degrees, not a warm lens. Kendrick notes that 2 to 4 degrees is not enough to affect the view, and the battery lasts longer.
The Dew Risk Calculator reads tonight's forecast for your location hour by hour, and the wettest hour decides. It says "Yes: run a dew heater" when any hour reaches high risk, "Pack a dew heater" when the worst hour is moderate, and that none is needed when the whole night stays low. The wettest hour decides, not the night's average, because glass fogs at its wettest hour.
The air-conditioned room
In a cold country the warm house is the problem: cold gear brought indoors fogs. In Guyana the bedroom with the air conditioner is the cold place, and the night outside is warm and wet.
- A lens that sat in a 22 °C room is at 22 °C.
- Carried into a night with a 23.3 °C dew point, it starts 1.3 °C below the dew point, so it fogs at once.
- The water forms inside the camera or the telescope tube too, not only on the outer glass.
Sony's camera help names this exact case: bringing a camera "from an air conditioned car or room into a hot place outside". Its advice is to seal the camera in a plastic bag first, and to take it out once the air in the bag has reached the outside temperature, after about an hour.
- In the cool room, put each camera, lens and eyepiece case into a plastic bag and seal it.
- Carry the bags outside and leave them closed in the shade for about an hour.
- Open them once the gear has warmed to the air.
- On observing days, keep the gear out of the air-conditioned room altogether, in a dry box (see the next lesson).
Your room is 21 °C and tonight's dew point is 23 °C. What happens to a lens carried straight outside?
- Nothing, because the night air is warmer than the room
- It fogs, because it starts 2 °C below the dew point (the right answer)
- It fogs only after midnight
- It stays clear if the sky is cloudy
Glass takes the temperature of the room it sat in. At 21 °C it is 2 °C colder than the 23 °C dew point outside, so water condenses on it as soon as it meets the night air.
The calculator runs this check for tonight: type your room's temperature and the hour you plan to go out.
Ten years of Georgetown and Lethem nights: how often the dew rule reads high, and how humidity climbs through the night.