I’ve long wondered whether dew point or relative humidity is more important for good seeing. Which would be better: a cool night with lower dew point but higher humidity, or a warm night with higher dew point but lower humidity?
William Shepard
Troy, Michigan
Dew point/relative humidity questions come up fairly often and this is a good one. Water vapor in the atmosphere is pretty important for deep-sky observing and photography.
The atmosphere can only hold a finite amount of water before it becomes full, or saturated. That amount is somewhat flexible (it is possible to be oversaturated), but that definition is good for our purpose here. The problem is that the amount of water a sample of air can hold depends on temperature.
Relative humidity (RH) is a comparison between the actual amount of water in the atmosphere and how much would be needed to saturate it. If you have 80 percent RH, then the air is holding 80 percent of the water that it could — at that temperature. It’s kind of like a bucket that’s 80 percent full at one temperature, except that the bucket would grow larger as the temperature goes up and shrink when it falls. The amount of water wouldn’t change.
Dew point is the temperature at which the moisture in the air would condense and, supposedly, form dew (or frost) on a telescope. To belabor our example, it’s the point at which falling temperatures would shrink our bucket to the point where the contents overflowed. In practice, dew point is a more stable measurement of water in the atmosphere. It measures the absolute amount of water rather than the relative amount of water.
For example, a sample of air that is saturated at 40 degrees Fahrenheit (4 degrees Celsius) and one that is saturated at 78 F (26 C) both have a relative humidity of 100 percent, but the sample at the higher temperature has 3½ times as much water as the colder sample. Both are filled when saturated, but one contains much more water.
Now consider the dew point: Let’s say both samples are in same-sized buckets and have a dew point of 32 F (0 C). That’s a relative humidity of 73 percent for the cooler sample and 23 percent for the warmer. Both will reach condensation at the same temperature. And, at 32 F (0 C), air has a “moisture content” of 0.09 pound per square inch (0.006 kilogram per square centimeter), or 6.11 millibars. Those look like strange numbers, but that’s because moisture content is usually expressed as vapor pressure — the pressure that the water molecules contribute to the total atmospheric pressure. The units don’t matter; what matters is that the number is the same for both samples. In other words, they both have the same amount of water vapor, and if our two buckets were cooled to very cold temperatures, they would have the same amount of ice on the bottom.
So, dew point is a proxy measure of the actual amount of water in the air (by weight or pressure), while relative humidity is a measure of the fractional amount of vapor.
Winter and summer nights often have relative humidities that reach close to 100 percent by morning, if not well before. Yet, winter nights are often brilliantly clear while summer nights can be hazy. Dew point is better at telling you when your optics will acquire a coating of moisture, since your telescope is usually colder than the air around it and the dew condenses first where it’s least wanted. If you have a regulated dew heater, you want to measure what’s up against the telescope, not what’s in the air around you.
However, from an observational point of view, it’s not the water near the ground that’s the problem, it’s the total amount of water vapor above you. To assess that, I look at the water vapor satellite images that are available online, such as at https://weather.cod.edu/satrad, for instance. There are several choices of water vapor image, but I like the high-level one. Just check the image before you go out observing and see if there’s a dry area in the upper atmosphere that’s coming toward you. If so (and it’s otherwise clear), plan on doing your best photography or reaching for the faintest objects that night. You might also plan on using a dew heater, because low water vapor content means that the night will cool quickly. If you are a planetary observer or photographer, high relative humidity (or dew point) near the ground (in Florida, for instance) may be an indication that the air is under an inversion and stable and the seeing is superb, so it’s not always one or the other. Most observers know that nights with thin fog are often the best for teasing out detail in Mars, Jupiter, or Saturn.
So, to answer the question, it really depends on what’s going on higher up in the atmosphere, but a lower dew point is a better starting point for good observing. In reality, both cool and warm nights will reach near 100 percent RH at some point, but the colder air would contain less water.
Jay Anderson
Meteorologist, Winnipeg, Manitoba
