Before launching, pilots will call a weather service to find out about climate and wind conditions in an area. Cautious pilots only fly when the weather is close to ideal -- when skies are clear and wind conditions are normal. Storms are extremely hazardous for hot air balloons, because of the danger of a lightning strike. Even rain is a problem, because it decreases visibility and damages the balloon material (of course, it's not much fun to fly around in wet weather anyway). And while you need a nice wind current to have a good flight, very strong winds could easily wreck the balloon.
Pilots also call the weather service to get a rough idea of which way the balloon will travel, and how they should maneuver once they're in the air. Additionally, a pilot might send up a piball (short for pilot balloon). A piball is just a balloon filled with helium that the pilot releases to see the exact direction of the wind at a prospective launch site. If it looks like the wind would take the balloon into prohibited air space, the crew needs to find a new launch spot.
In the air, the pilot will use an onboard altimeter, variometer and their own observations to find the right altitude. Reaching the right altitude is pretty tricky because there is at least a 30-second delay between blasting the burners and the balloon actually lifting. Balloon pilots have to operate the appropriate controls just a little bit before they want to rise, and shut them off a little bit before they want to stop rising. Inexperienced pilots often overshoot, rising too high before leveling off. Controlled operation comes only with many hours of ballooning experience.
Now that we've seen how a hot air balloon flies through the air, let's look at the forces that make this possible. As it turns out, hot air balloons are a remarkable demonstration of some of the most fundamental forces on earth.