Monday
Monday: How a Forecast Is Made
Rocket bursts onto Station Rooftop before breakfast. "Tomorrow will be sunny and 72," he announces. "I have decided."
Raven looks up from the log. "Decided? What do you notice in the readings from the last three days?"
Rocket checks. The pressure has been falling. The wind flag has swung to the south. The cloud log says the high clouds are thickening.
"Oh," he says. "That is not a sunny pattern."
Nova hovers over the log and lights one column at a time. "Forecasters start exactly where you are standing," she says. "With the observations."
"Then they ask how the pattern will change," Raven says. "And they say how sure they are."
Rocket picks up the pencil. "Fine. Let me start over. Properly, this time."
Step one: look at what is happening now
The National Weather Service describes its forecast process on weather.gov. The first step is a thorough review of current observations.
Forecasters use radar, satellites and data from many ground-based and airborne instruments to get a complete picture of conditions.
Computer programs then draw an analysis, which is a graphical picture of current conditions.
Your station log is your analysis. Temperature, pressure trend, wind direction, clouds and rain are all in it.
Next, forecasters use numerical models, statistical models and years of local experience to work out how conditions will change.
The models often give different answers. Then forecasters decide which model fits the situation best, or blend them.
The forecast process is roughly the same for every kind of weather, from rain to snow to hurricanes.
The National Weather Service calls uncertainty a key part of every forecast.
The atmosphere is chaotic and never fully measured, so forecasts become more uncertain at longer time ranges.
That is why forecasters communicate which parts are uncertain and what the worst case might be if the forecast changes.
Your one-day forecast this week will say how sure you are, too, as a chance of rain.
Over eleven weeks your log has recorded temperature, pressure, wind, clouds and rain. That is the same list a forecaster reviews.
Week 3 taught you what rising and falling pressure usually bring. Week 6 taught you which clouds come before rain.
Week 8 showed why forecasts give a chance of rain instead of a promise. This week you put all of it to work.
Your forecast will not be perfect. The National Weather Service says every forecast carries uncertainty, and so will yours.
- Read the question.
- Tap your answer.
Reading the sky words
A forecast uses set words for cloud cover. The National Weather Service office in Binghamton publishes this table of meanings.
The fraction is how much of the sky is covered by clouds you cannot see through.
| Forecast word | Sky covered by opaque cloud |
|---|---|
| Clear or Sunny | 1/8 or less |
| Mostly Clear or Mostly Sunny | 1/8 to 3/8 |
| Partly Cloudy or Partly Sunny | 3/8 to 5/8 |
| Mostly Cloudy | 5/8 to 7/8 |
| Cloudy | 7/8 to 8/8 |
| Statement | True or false? |
|---|---|
| Forecasters review radar, satellite and instrument data first. | ? |
| Numerical models always agree with each other. | ? |
| A forecast for seven days ahead is more certain than one for tomorrow. | ? |
| Partly Cloudy means 3/8 to 5/8 of the sky is covered. | ? |
| Your station log works like a forecaster's analysis of current conditions. | ? |
Write it in your log beside today's date. Tomorrow, check whether your word matched the official forecast.
Stay inside during a thunderstorm, flood or high wind. The sky check can wait.
A grown-up can read the official forecast for your area aloud from weather.gov. The lessons never ask where you live.
Great start. Tomorrow you learn what a 40 percent chance of rain really means.