This month is the most popular time for stargazing: Vacations and summer breaks free up the late hours, and nights are invitingly warm. By an unfortunate conspiracy of nature, July is also North America’s month of maximum humidity. As a result, the year’s most familiar night skies are also the year’s wettest, offering prime examples of how water dims, tints, and yet sometimes clarifies, our view of the universe.
In general, humidity is bad news for astronomy. Water absorbs light, especially light at the red end of the spectrum, coloring our view of the world. Ocean water sops up the red so effectively that 100 feet below the sea surface only the bluish components of sunlight penetrate; a sunken ship’s crimson lower keel appears black to divers since nothing can look red in the absence of red light. Likewise, water vapor in the air takes a little bit of the red out of starlight before it reaches the ground.
Liquid water in the atmosphere causes more obvious viewing problems. When the humidity is high, water vapor condenses around aerosols, forming tiny suspended droplets that spread around all wavelengths of sunlight equally—a fancy way of saying that water whitens the sky. That is why Louisiana skies look milky in July, while dry desert and mountain states have skies of cobalt. At night, such scattering spreads the sky-spoiling effects of a bright moon or nearby city lights. (Many people do not realize that water vapor by itself is transparent. The white mist gushing from a teapot, commonly called steam, is actually microscopic droplets of hot, condensed water. The true steam is in the clear zone closest to the spout.)
These phenomena peak in the summer because the amount of water in the air depends strongly on temperature. At 95 degrees Fahrenheit, air can hold 30 times as much water as it can at 0°F. When air reaches saturation, also known as the dew point, light-scattering droplets form readily. This is a particular problem in the humid southeastern states, where warm summer night air is frequently right at the dew point. Under those conditions, sky gazers typically miss about 60 percent of the incoming starlight, enough to wipe away the Milky Way. This is one big reason why no major observatories have been built east of the Mississippi for the past century.
Observers in Montana and Arizona have such dry air that their summer nights closely simulate the view of the heavens from outer space. Dry, haze-free air is virtually transparent to visible wavelengths, dimming stars by just 30 percent. Veteran astronaut Andy Thomas, a crew member on this year’s inaugural flight of the reengineered space shuttle, says that stars from dry, unpolluted earthly locations look as good as or better than they do through the shuttle’s glass view port.
Despite all these negatives, humidity does have one major upside: It steadies the “seeing,” the astronomer’s descriptive and remarkably untechnical term for the blurriness caused by Earth’s atmosphere.
In this case, the source of the blurring is not water vapor but the atmosphere as a whole. Air bends light like a lens—with cool air, which is relatively dense, having a greater impact than warm air. As light passes through air layers of disparate temperatures, the rays bend one way and then another. Each twist and turn adds an element of distortion. When seeing is poor, stars twinkle wildly and telescopes show planets as hopeless smudges.
Humid air holds its heat more effectively after sundown, which tends to improve seeing. Air cannot cool below its dew point, so water-saturated air all tends to settle at about the same temperature. The result is relatively homogeneous air layers that produce little blurring and hence yield breathtakingly sharp images. The dimming effect of humidity still blots out faint objects such as galaxies and nebulas. As a trade-off, however, bright objects such as Jupiter (now blazing in the west after sunset) and the moon often look crisper from muggy areas than from dry ones.
For those seeking the best view of the stars, the desert is still your best bet. For planet lovers, however, this might be a good time to start whistling Dixie.
What’s up in the July sky
July 5: Earth reaches aphelion, its annual far point from the sun. At a distance of 94,512,036 miles, the sun appears 7 percent dimmer than it did in January, softening summer’s heat.
July 8: Grayish Mercury and dazzling-white Venus meet just beneath the crescent moon. See them low in the west, 40 minutes after sunset.
July 12 and 13: The moon passes near Jupiter. This is the planet’s final good month of evening viewing in 2005; soon it slips into the sun’s glare.
July 25: Venus, Mercury, and Saturn gather together low in the west 40 minutes after sunset.
July 27 The gibbous moon and Mars, now growing rapidly brighter, rise together at midnight.
All month: M13, a globular cluster containing millions of stars, sits almost directly overhead at the end of twilight. Binoculars show it as a dimly shimmering ball of light.
