When do temperatures start to become negative as altitude increases?

The correct answer, indicating that temperatures start to become negative around 24,000 feet, relates to how atmospheric temperature typically behaves with increasing altitude. In the troposphere, which is the lowest layer of Earth's atmosphere, temperatures generally decrease with altitude. This decrease is often roughly 3.5 degrees Fahrenheit for every 1,000 feet.

At sea level, average temperatures can range widely but as you ascend to about 24,000 feet, it is common for temperatures to reach below freezing, particularly in mid-latitude regions. By this altitude, the influence of solar radiation diminishes, and the air becomes significantly colder.

Understanding this temperature behavior is important for flight planning, weather prediction, and understanding atmospheric conditions, especially since aircraft operation often occurs at altitudes where cold temperatures are prevalent. In contrast, the other options reflect altitudes where temperatures might still be above freezing, particularly in certain seasons or geographic locations where temperature profiles can vary significantly.