The Winter Window Light Index

The 50 most populous U.S. cities, three public measures each. Sunlight from the NASA POWER climatology for 2001 to 2025; population from the Census Bureau’s 2023 estimates.

December sunlight, city by city

Ranked by how much sunlight reaches the ground on an average December day. The second column is the day’s total energy on a flat, open surface; the third is sunrise to sunset on the shortest day; the last is how much of September’s sunlight is gone by December.

• Darkest Decembers: Seattle (0.85), Portland (0.98), Milwaukee (1.16), Detroit (1.24), Minneapolis (1.31).
• Brightest: Miami (3.45), El Paso (3.40), Tampa (3.29), Tucson (3.25), Phoenix (3.08).
• Shortest day: Seattle, 8h 25m of daylight on 21 December; the longest of the fifty is Miami, 10h 32m.
• Median city: 2.27 kWh/m² a day in December.

Why the ranking is not just a map of latitude

Day length is pure latitude: the further north, the shorter 21 December. Sunlight is latitude plus weather. A city with a long December day under a grey sky can sit below a city further north with clear winters, which is why the daylight column and the sunlight column do not line up row for row. The drop since September mixes both: the sun gets lower everywhere, and in some cities the cloud arrives at the same time.

What it means for a plant on a windowsill

A houseplant indoors lives on a slice of this. Glass, the direction the window faces, the eaves above it and the distance from the pane all cut the outdoor number further, and none of those are visible to a ranking of cities. What the index does tell you is how much the whole supply shrinks in winter where you live, which is the reason a plant that coped in September can stall by the holidays without anything else changing.

The part a city average cannot give you is your own sill. A phone held where the pot sits, at the same hour on a clear and a cloudy day, gets you that number. How to measure light for plants walks through it, why plants struggle in winter covers what changes besides the light, and the light meter in the GrowSpot app is free to use on iPhone and Android.

Method

• Cities: the 50 most populous incorporated places in the Census Bureau’s Vintage 2023 estimates. Indianapolis, Louisville and Nashville are ranked by their balance, as the Bureau does.
• Where each city is: the internal point in the Census 2023 Gazetteer. San Francisco is the exception: its internal point falls offshore because the city includes the Farallon Islands, so City Hall is used.
• Sunlight: NASA POWER monthly climatology, all-sky surface shortwave downward irradiance, averaged over January 2001 to December 2025, in kilowatt-hours per square metre per day, read at each city’s point. The source grid is about one degree, so cities close together can share a cell.
• Daylight: computed from latitude for 21 December, counting sunrise and sunset the way almanacs do, with the sun’s centre 0.833° below the horizon. Terrain and buildings are ignored.
• Drop since September: (September − December) ÷ September, from the same sunlight series.

All of it is reproducible from public sources. The files above carry every input next to every result.

What it is honestly not

It is not a measurement of any window. The sunlight figure is for an open, level surface outdoors; a vertical pane facing south in winter can catch more of a low sun than that surface does, and one facing north catches far less. It is also a long-run average, so a particular December can be brighter or darker than its row.

License and attribution

The index is published under Creative Commons Attribution 4.0. Use it, chart it or reprint it with credit:

Sunlight data courtesy of the NASA Langley Research Center POWER Project. Population and place coordinates from the U.S. Census Bureau.

The other open files

We also publish light-meter readings from real rooms, what published sources say about indoor light and the light tier we file each species under.