Definition
The attractive force between molecules of the same substance that holds them together. In the atmosphere, this force between air molecules is very weak, which is why air flows freely, mixes easily, and can be compressed.
Plain English
It's the pull that keeps the tiny particles of a substance stuck to each other. In air, that pull is very small, so air spreads out and moves around easily instead of holding together like water or metal.
Context Anchor
Seen in discussions of the structure of the atmosphere, air pressure, density, and why air can support flight.
Derivation
From Latin 'cohaerere,' meaning 'to stick together' ('co-' = together, 'haerere' = to cling). 'Molecular' refers to molecules — the smallest pieces of a substance. So the term literally means 'molecules sticking together,' which is exactly what it describes.
Why Pilots Care
Understanding that air molecules barely cling to each other helps explain why air can be compressed, why it changes density with altitude and temperature, and why it behaves as a fluid that wings can move through and generate lift from.
Grounding Statement
Picture water droplets clinging together to form a bead — that's strong cohesion. Now picture air, where the molecules barely pull on each other at all and just drift apart. That weak pull is molecular cohesion in air.
Intuition Check
Cohesion here does not mean teamwork or agreement. It means a physical attraction between molecules.
Example Sentence 1
Because molecular cohesion in air is weak, the atmosphere compresses under its own weight, making air denser near the surface than at altitude.
Example Sentence 2
Unlike water, which has stronger molecular cohesion, the atmosphere allows gases to mix and rise freely.