High dewpoint means hazier skies but crisper telescope images

Kingston Point (photo by Dion Ogust)

Kingston Point (photo by Dion Ogust)

It has been like Bolivia around here. We’ve endured a very humid six weeks, June and now July, with welcome relief lasting only briefly. Meteorologists express humidity with the word “dewpoint.” Air with a 65- or 70-degree dewpoint is very humid. If the dewpoint is around 60 degrees, it’s slightly humid. A dewpoint in the 50s feels quite comfortable. And the 40s indicate crisp, autumnlike air with deep blue skies. Most of June and July, the dewpoint has been mostly stuck near 70, which is what they routinely experience in the Carolinas or the Gulf Coast.

One quick indicator of extreme humidity is when your windows fog up on the outside. This only happens when the dewpoint is higher than your air-conditioned room’s temperature. A foggy window means that the outside air must have a higher dewpoint than your 68-degree room, so the cool glass plunges the air hitting it to a temperature where it can no longer hold its moisture as vapor. Likewise, humid air chilled to its dewpoint is what makes toilet bowls and pipes sweat – which they’re been doing like crazy this season. It also guarantees that early morning lawns are wet with dew.

Humid air contains countless tiny droplets, which reflect all the Sun’s colors equally, and therefore whitens the atmosphere. Thus, on humid days the sky near the horizon is white and not blue at all. Even the overhead sky is light blue, not rich or cobalt.


At night, such conditions manifest as a sky limited in star-count. Telescopically, this isn’t a good time to look for faint galaxies or nebulae. But one good thing unfolds for backyard astronomers: On a humid night, the air loses its heat very slowly. Air can never cool below its dewpoint, which limits our nightly lows to the mid-60s. The resulting homogeneity of temperature with altitude allows light from the universe to arrive without being refracted (bent) one way and then another by varying air densities (colder air is denser). Incoming light thus takes a straighter path.

Result: Stars don’t twinkle on humid nights. And telescopically, fine detail emerges. Images are rock-steady. Point a telescope at the Moon – or Saturn, the medium-bright star one-third of the way up the sky in the southwest the first couple of hours after nightfall – and you’ll be treated to exquisite views of rare quality.

See, humidity has a bright side.