What does a chance of rain really mean?
The chance of rain is one of the most misunderstood elements of a weather forecast.
When there is a 50% chance of rain predicted for your neighborhood, you may think that meteorologists are saying there are coin-flip odds that rain develops or not.
But the chance of rain actually means the percentage of land area that meteorologists expect to get wet. So instead of a 50-50 chance of rain developing that day, meteorologists are saying they expect 50% of the area to see rain.
Predicting rain for population instead of geographic area gets trickier.
The First Alert Weather team forecasts for a 16-county area in Northern Illinois and Northwest Indiana. While the science of weather forecasting is advanced enough to accurately predict the portion of that 16-county area that will see rain each day, it is not yet advanced enough to predict what percentage of the population will see rain.
Since more than half of the area's 9.7 million people are concentrated in the small geographic footprint of Cook County, an isolated storm that only affects Chicago has an oversized impact on the proportion of people who see rain.