Definition
Fronts are the boundaries between two air masses of different temperature, humidity, or density. As one air mass moves into the area occupied by another, the zone where they meet is called a front, and weather changes — including shifts in wind, temperature, pressure, visibility, and precipitation — typically occur along this boundary.
Plain English
A front is the dividing line where two large bodies of air with different qualities meet. Because the air on each side is different, the weather usually changes when a front passes through.
Context Anchor
Seen on surface weather maps, weather briefings, and preflight planning products when checking what weather may affect a route.
Derivation
From military usage — a 'front' is the line where two opposing forces meet. Meteorologists borrowed the term in the early 1900s because air masses behave like opposing armies meeting along a battle line, with weather 'action' concentrated at the boundary.
Why Pilots Care
Crossing a front often brings sudden changes in wind, visibility, clouds, and turbulence that affect route planning and flight safety.
Grounding Statement
If you fly toward a front, you are flying toward a place where the air ahead may be noticeably different from the air you are in now.
Intuition Check
Fronts do not mean the front side of an airplane or simply the leading edge of any cloud. In weather, fronts are boundaries between different large bodies of air.
Example Sentence 1
The briefer warned that a cold front would cross the route by mid-afternoon, bringing gusty winds and a line of showers.
Example Sentence 2
After passing through the front the temperature dropped and the winds shifted from the southwest to the northwest.