Yesterday’s weather is an Extreme Programming (XP) term to keep teams from over committing during sprints and iterations.

The story goes something like this. Once upon a time there was a government spent a boatload of money on a weather prediction satellite. It took years. Cost of millions of dollars. But they were finally able to launch a satellite that was able to accurately predict the weather about 70% of the time. Not bad.

It was then that someone realized if they simply said today's weather will be the same as yesterday's weather they'd also be accurate 70% of the time.

XP uses this concept to keep teams from over committing during sprints/iterations. It reminds us that a good predictor of the future is what we’ve done in the past.

Read more about Yesterday's Weather in Planning Extreme Programming by Martin Fowler and Kent Beck.