What do percentages on weather forecast apps mean?
In: Technology
It is the percentage of times in the past that current weather conditions have led to the forecast weather. What causes any specific type of weather is incredibly complex to the point where forecasters can’t perfectly predict it. So the best they can do is look at historical weather data, and when similar conditions were present in the past, X% of the time it resulted in the forecasted weather.
This percentage is the product of two values:
1) The probability/certainty of a weather event occurring, and
2) The fraction of land affected in the forecast area.
For example, 50% chance of rain over 50% of the forecast area results in a forecast of 25% rain, while a 100% chance of rain over 25% of the forecast area also results in a forecast of 25% rain.
It could be the humidity index – how saturated the air is with evaporated water. The actual volume of water that is per a given volume of air changes a lot depending on the temperature of the air. Cold air cannot hold nearly as much water as warm air, which is why people’s lips get cracked in cold weather; they are losing moisture to the air. So it’s based on how much water the air can actually hold.
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