Rocket reads the official forecast aloud from the Station Rooftop screen. "Forty percent chance of rain."
"So it rains for forty percent of the day?" he asks. "Or on forty percent of the town?"
Raven shakes her head. "What do you notice about the word chance? It is about how sure they are."
Nova hovers over the rain gauge and drops a single pretend raindrop of light into it. "How much rain counts as rain?" she asks.
"Any?" Rocket guesses. "Even a drop?"
"The National Weather Service picked a number," Raven says. "A very small one."
Rocket checks the gauge's tiniest mark. "One hundredth of an inch," he reads. "That is barely a wet sidewalk."
The National Weather Service explains probability of precipitation, or PoP, on its Peachtree City office page.
A 40 percent chance of rain means one thing. There is a 40 percent probability that your spot gets at least 0.01 inch of rain.
It is not the share of the day it will rain. It is not the share of the area that gets wet.
It is the forecaster's confidence that your spot will get measurable rain in the forecast period.
The Binghamton office adds that measurable precipitation means 0.01 inch or more, and the period is normally 12 hours.
| Chance of rain | Forecast word (NWS Binghamton) |
|---|---|
| 10 percent | Isolated or few |
| 20 percent | Slight chance |
| 30, 40 or 50 percent | Chance |
| 60 or 70 percent | Likely |
| 80, 90 or 100 percent | Occasional, or periods of |
Forecasts also use set words for temperature and wind. These come from the same Binghamton forecast terms page.
Lower 50s means 50 to 54. Mid 50s means 53 to 57. Upper 50s means 56 to 59. Around 50 means 48 to 52.
The wind is described by the direction it blows from, with speeds in miles per hour.
| Sustained wind | Forecast word |
|---|---|
| 0 to 5 mph | Light, or light and variable |
| 15 to 25 mph | Breezy in mild weather, brisk or blustery in cold weather |
| 20 to 30 mph | Windy |
| 30 to 40 mph | Very windy |
| 40 mph or more | Strong, dangerous or high (High Wind Warning level) |
Tomorrow you will write a forecast for the next day using your station log. Here is the shape it takes.
High temperature: look at the last three days and the trend. Use a range word, such as upper 60s.
Sky: use the eighths table from Monday and your cloud log. Use what week 6 taught you about which clouds come before rain.
Wind: use your wind flag and compass. Say the direction it blows from and a speed word.
Chance of rain: use your pressure trend, clouds and wind together. Week 3 and week 8 taught you which trends point toward rain.
Then pick a percent and the matching word. If you are unsure, say so with a middle number, as forecasters do.
A chance of rain is a way of being honest. Forecasters cannot know for certain, so they say how likely rain is.
The National Weather Service explains that communicating uncertainty is as important as the forecast itself.
If your log gives mixed signals, say 50 percent. If every sign points one way, move your number toward 10 or 90.
A forecast that says 20 percent and then gets rain is not a failure. Twenty percent means rain happens one time in five.
| Statement | True or false? |
|---|---|
| A 70 percent chance of rain is called likely. | ? |
| A chance of rain tells you how long it will rain. | ? |
| Winds are described by the direction they blow from. | ? |
| Upper 50s means 56 to 59. | ? |
| The usual forecast period for a chance of rain is one hour. | ? |
Well done. Tomorrow is Forecast Day at Station Rooftop.