Rocket has taped four postcards to the Station Rooftop wall: New York, Los Angeles, Seattle and Denver.
"Same country, four different climates," he says. "I bet I can rank them by how far north they are."
Raven lays a printout beside the cards. It is a table of 30-year averages from NOAA.
"What do you notice?" she asks. "Seattle is the farthest north, but its January is warmer than New York's."
Rocket blinks. "That breaks my rule."
Nova floats beside the table and brightens two columns at once. "Latitude is one factor," she says. "Try reading two columns together."
Rocket traces a finger from the latitude column to the elevation column. "Denver," he says. "Denver is way up high."
NOAA's National Centers for Environmental Information calculate climate normals for almost 15,000 weather stations across the United States.
A normal is the 30-year average of one measurement, such as the average January temperature at one station.
The current normals cover the years 1991 to 2020. NOAA produces a new set every 10 years.
NOAA says normals act like a ruler. You hold today's weather or tomorrow's forecast against them to see what is usual.
The table below comes from NOAA's 1991 to 2020 monthly climate normals for four airport and park weather stations.
The stations are New York Central Park, Los Angeles International Airport, Seattle Tacoma International Airport and Denver International Airport.
Temperatures are monthly averages in degrees Fahrenheit. Precipitation is the monthly total in inches. Elevation is in meters above sea level.
| City | Latitude (degrees north) | Elevation (meters) | January average (F) | July average (F) | January precipitation (inches) | July precipitation (inches) |
|---|---|---|---|---|---|---|
| New York | 40.8 | 40 | 33.7 | 77.5 | 3.64 | 4.60 |
| Los Angeles | 33.9 | 30 | 57.9 | 69.6 | 2.86 | 0.04 |
| Seattle | 47.4 | 113 | 42.8 | 67.1 | 5.78 | 0.60 |
| Denver | 39.8 | 1,650 | 31.7 | 75.1 | 0.38 | 2.14 |
Regional climate depends on sunlight, height above sea level, the shape of the land and closeness to the ocean.
Latitude controls sunlight. Los Angeles, the farthest south, has the warmest January.
Altitude matters too. Denver and New York are at almost the same latitude, yet Denver's January is colder and far drier.
The ocean matters most of all. Water stores heat and gives it back slowly, so coasts facing the ocean wind get steadier temperatures.
Seattle and Los Angeles face the Pacific, where the westerlies blow in from over the water. Their summers are cooler than New York's or Denver's.
New York is on a coast too, but the westerlies reach it after crossing a whole continent of land.
| Statement | True or false? |
|---|---|
| A climate normal is a 30-year average. | ? |
| The current normals cover 1991 to 2020. | ? |
| The city farthest north always has the coldest winter. | ? |
| Denver and New York are at almost the same latitude. | ? |
| Los Angeles gets most of its rain in summer. | ? |
The normals also list the average daily high and low. In January, Denver's average high is 44.6 and its average low is 18.7.
New York's January high is 39.5 and its low is 27.9. Denver swings far more between day and night.
Denver is high, dry and far from any ocean. New York sits beside the Atlantic. Which factors from the list could explain the difference?
| City | January average high (F) | January average low (F) | Swing (high minus low) |
|---|---|---|---|
| New York | 39.5 | 27.9 | 11.6 |
| Denver | 44.6 | 18.7 | 25.9 |
Sharp reading. Tomorrow you follow one warm current across an ocean.