It’s fun to drive to work in Albuquerque the first week in October: it’s time for the Albuquerque International Balloon Fiesta. Looking north from my house, there are hundreds of black dots, like a flock of birds. As I get closer on my commute downtown, the dots assume all colors imaginable, and some rather odd shapes. There are often a few stray balloons scattered around the city, but most of them manage to stay in a certain area around the Balloon Field, 800-900 of them on a mass ascension day. How do they do that?

Balloon pilots have no directional controls. They can turn on their propane burners to go up, and vent hot air to go down, but otherwise are entirely at the mercy of winds. Albuquerque, however, in the fall, has a meteorological pattern which gives pilots more control. It’s called the “Albuquerque Box”.

In stable conditions, with clear skies, low humidity and light winds, the air near the ground cools at night. Air cooled during the night is denser and runs downhill like water, and collects in the Rio Grande Valley near Albuquerque, where it forms an inversion layer: a river of cold air that gently flows downstream from higher to lower elevations. This southward flowing airmass is only a few hundred feet high. Above the inversion layer, warmer air flows according to the prevailing winds in the region: from the south or southwest.

The cold, relatively stable inversion layer is perfect for launching hot air balloons by the hundreds. They then drift south in the river of air, until the pilot takes the balloon up to catch the prevailing winds, and head back over the Balloon Fiesta Field and to the north, where the pilot can then drop back down into inversion layer and return to the Field.

By midmorning, the air near the ground heats up and the inversion layer disappears. Thus, all the ballooning activity in the Albuquerque International Balloon Festival takes place in the early morning.