Definition
A gas law stating that, at constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature. Heating the gas causes it to expand; cooling causes it to contract.
Plain English
When you heat a gas without letting the pressure change, it takes up more space. When you cool it, it takes up less. The hotter it is, the bigger it gets.
Context Anchor
Seen in aviation weather, basic physics, and discussions of how air expands or contracts when temperature changes.
Derivation
Named after Jacques Charles, the French scientist who first observed the relationship in the 1780s while experimenting with hot-air balloons. The law that lifts a balloon is the same one that makes hot air less dense than cold air.
Why Pilots Care
Temperature-driven volume changes affect air density, engine power output, and aircraft performance calculations.
Grounding Statement
Imagine a balloon left in a hot car. As the air inside warms, the balloon stretches and grows larger, even though the same amount of air is inside.
Intuition Check
Do not read “law” here as a rule pilots must obey. Here, a law is a dependable physical relationship: at constant pressure, warmer gas takes up more space and cooler gas takes up less space.
Example Sentence 1
On a hot summer afternoon, Charles's Law helps explain why the airplane needed a longer takeoff roll than it did during a cool morning departure.
Example Sentence 2
A student pilot uses Charles's Law to understand how rising air temperature reduces air density at a given altitude.