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The Webfooted Astronomer - May 2000


How Do I Dew It?

By Todd Gross

After Jeff Renner's talk on Northwest weather at the April meeting, SAS member Bruce Weertman found this article on dew by Todd Gross, an amateur astronomer and meteorologist at a Boston TV station. —Editor.
Please note, that while I may speak authoritatively, I am just an amateur astronomer, like you, and all the information above reflects my personal opinion based on my experiences.

Dew, the bane of many amateur astronomers, is easy for the layperson to forecast for the night or nights ahead. Face it, most of us, even Newtonian reflector owners, will at one point or another have to deal with dew. Some of us, like myself, have the worst possible scenario: A moist climate, and a Schmidt-Cassegrain scope, both perfect for gathering dew or frost.

Then why is it that I almost never worry about dew? Why do I leave my scope out at 10 p.m., allowing it to cool down, until my planned observing session at 4 a.m., on some nights without even a dew shield? Dew, or the lack thereof, you see, is fortunately one of the easiest types of weather to predict!

What produces dew (or frost)? Basically, on clear or partly clear, low-wind nights, the temperature falls to a temperature called the dew point. The dew point is the lowest the temperature can fall. The only way it can fall lower, is if the dew point itself changes. (In actuality the dew point often rises somewhat as the temperature drops, but not on clear, calm nights).

What is important about the temperature falling to the dew point is what happens when it does. Either fog forms, rain or snow falls, or dew or frost forms. On a clear, calm night, dew or frost forms. The dew accumulates on your optics, and can render an observing session useless. Reflectors, and to a lesser degree, refractors both are naturally shielded from the sky, and thus the dew. But as long as the glass is open to the sky at all (especially if you are viewing towards the zenith), you will eventually dew up. That is because your optics lose heat even faster than the surrounding air when they face the sky.

Opposite of how you feel warm in the sun on a sunny day, space, with it's near absolute zero temperature, actually draws heat from surfaces, such as your telescope's objective, It cools you down faster than the air, just like the sun warms you up! Thus, when skies are mostly clear, and winds are light, especially if the dew point is high, you will have problems, no doubt, on any surface that faces the sky. And the more directly the surface faces the sky, the worse the problem because the temperature of the glass falls to the dew point, condensing moisture from the surrounding air.

Dew and frost are more common in some parts of the country and world, than others. Areas that are typically affected by large bodies of water, such as the Gulf of Mexico, and the Pacific and Atlantic Oceans, will have more of a problem than the desert Southwest. You will find that all areas will have at least some dew from time to time. There are a few places such as southern Florida that are only very rarely dew-free.

What is the secret to saving the trouble of piling on the dew shield, dew heating equipment, tarps, umbrellas, and so forth? The answer is in the weather systems themselves. While I am the first to admit that there can be situations where it is a tough call, there are a couple of very concrete weather set ups that will be easy for you to identify. This method doesn't apply to the tropics, however.

The important dew-less weather scenario that is easiest to recognize occurs after a cold front passes by. This may be preceded by rain or snow, but is followed by either clearing, or some lingering clouds. If it does clear, the wind often stays fairly brisk for one or more days. On a weather map, you will be between a receding cold front or storm center (low pressure system), and an incoming fair weather cell (high pressure system). No dew or very limited dew will form in most places in this circumstance.

To make things even better, especially in the upper Midwest of the United States and in the Northeastern United States these systems often stall in place, (Low pressure to the Northeast; high pressure to the Southwest of you) allowing the wind to keep up for several days. The catch is that if you live in cities or towns near a mountain range, such as the Appalachians, you can stay cloudy for days in addition to being windy. This is the case in places like Charleston, West Virginia, and Rochester, N.Y. On the other hand, many cities in the Northeast corridor, benefit from the mountain range to the west, and stay mostly clear for several days in this same weather setup, along with the wind. Thus, dewless nights can persist for many days straight prior to a high pressure, fair weather system finally moving on in.

Once a high pressure cell, or any very weak weather maker with very little wind, does finally arrive, the reverse holds true. While it may be clear, dew will likely form as clear skies and light winds allow for radiational cooling, the culprit that produces the dew or frost. These conditions will persist until the High Pressure cell, or area of very weak weather systems (low or high pressure nearby) moves on.

One very interesting irony is that the very conditions that produce dew or frost to form, will also often produce good stability to the atmosphere. In other words, on a night with lots of dew, the planet seeing should be good! (But don't forget to take precautions again the dew!) This means that on a night when the stars do not show much twinkle, you can expect that dew will form. A night when stars twinkle a lot will be an unsteady night, and one where dew will likely not form.

While all of the this can be applied worldwide, especially the part about looking for twinkling stars on a dew-less night, the rest works best in the Eastern 2/3 of the US and Canada. Trying to apply the previous weather scenarios gets more difficult in the mountains (such as the Rockies), and the above examples do not take place as often in California, or the South of France. Generally speaking though, it will work outside of the tropics.

Todd Gross, Channel 7 Meteorologist, Boston, Email address: and website

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