Will the Sky Be Clear? Air Masses and a Star Party

Waiting for a Clear Night

1Every October, the Prairie Hills Astronomy Club holds a free star party at a small county observatory in Kansas. Club members set up telescopes on the lawn so visitors can look at the Moon, planets and star clusters. For the best views, the club needs a night with no clouds or haze and with dry air. When the air near the ground is humid, dew forms on cold telescope lenses and blurs the view. This year, club members Ayesha Siddiqui and Rafael Montoya kept an observing log for a week to help choose the date.

2An air mass is a huge body of air, often more than 1,000 km across, that has about the same temperature and humidity throughout. Air that stays over one region for days takes on the temperature and moisture of the surface below it. Over warm ocean water, air gains heat and water vapor. Over cold land, it loses heat and gains little water vapor. Over hot, dry land, it heats up but gains little moisture. Winds can carry an air mass hundreds of kilometers from where it formed.

3Kansas lies far from any ocean, between three very different regions. The warm Gulf of Mexico is to the south. The cold plains of central Canada are to the north. The hot deserts of the American Southwest are to the southwest.

4On Nights 1 and 2, the club's lenses were covered with dew within an hour, even though no rain fell. Before sunrise on October 5, a line of thunderstorms crossed the county. On Nights 3 and 4, the lenses stayed dry. Each night at 9 p.m., Rafael counted the stars he could see inside one patch of sky near the bright star Vega.