Definition
The rate at which air temperature and dew point move closer together as an unsaturated air mass cools, typically about 4.4 °F (2.5 °C) per 1,000 feet of altitude gained. When the temperature and dew point meet, the air is saturated and cloud or fog formation becomes likely.
Plain English
As air rises and cools, the temperature drops faster than the dew point, so the two numbers get closer together. Convergence rate is how quickly that gap closes with altitude — about 4.4 °F for every 1,000 feet.
Context Anchor
Seen in weather study when estimating the height where clouds may begin to form from the difference between temperature and dew point.
Derivation
Convergence comes from the Latin convergere, meaning 'to bend together' or 'to come together.' Here, the two values that are coming together are temperature and dew point.
Why Pilots Care
It lets a pilot quickly calculate the approximate altitude where clouds will form or whether fog is likely before departure.
Analogy
Think of two cars on the same road, one ahead of the other. If the gap between them keeps shrinking, they are converging. In this weather use, the shrinking gap is between temperature and dew point.
Grounding Statement
Picture two lines on a graph — temperature and dew point — slowly pinching together as you climb. When they touch, you’re at cloud base.
Intuition Check
Convergence rate does not mean how fast two aircraft are closing on each other here. In this context, it means how fast the temperature-dew point gap gets smaller as air rises.
Example Sentence 1
With a surface temperature of 70 °F and a dew point of 61 °F, the convergence rate suggests cloud bases around 2,000 feet AGL.
Example Sentence 2
A slower convergence rate means clouds will form at a higher altitude than expected.