What is diurnal swing?
If you've read anything about natural cooling, passive design, or whether opening your windows actually saves energy, you've probably seen the term "diurnal swing." Or maybe "diurnal temperature range," abbreviated DTR. Both mean the same thing.
Diurnal swing = today's daytime high minus tonight's overnight low.
That's it. One subtraction.
It sounds trivial, but it's the single most important number for predicting whether window-cooling will work in your climate. Here's why.
Why diurnal swing matters
Window-cooling works by replacing AC hours with cool outdoor air during the time outdoor is cooler than indoor. If outdoor never gets meaningfully cooler than indoor, there's nothing to replace. The cooler the night gets relative to the day, the more "free cool" is available to bank.
A few examples:
- Phoenix, Arizona, June afternoon at 105°F, overnight low 72°F: 33°F swing. Massive banking opportunity overnight.
- Houston, Texas, August afternoon at 95°F, overnight low 82°F: 13°F swing. Some banking opportunity, but humidity often cancels it.
- Honolulu, Hawaii, July afternoon at 87°F, overnight low 78°F: 9°F swing. Effectively no banking opportunity.
- Denver, Colorado, May afternoon at 75°F, overnight low 43°F: 32°F swing. Best possible banking conditions — shoulder season in the high-elevation arid west is the natural-cooling jackpot.
How diurnal swing varies by climate
Some rough numbers for typical summer days in different US climate zones:
| Climate / city | Typical summer swing |
|---|---|
| High-elevation arid (Denver, Albuquerque, Flagstaff) | 25-40°F |
| Hot-dry desert (Phoenix, Las Vegas, Palm Springs) | 25-35°F |
| Mountain West temperate (Salt Lake City, Boise) | 25-35°F |
| Mediterranean (coastal CA: Bay Area, LA basin) | 15-25°F |
| Continental temperate (Chicago, Philly, DC) | 15-22°F |
| Marine PNW (Seattle, Portland) | 15-22°F |
| Hot-humid south (Houston, Tampa, New Orleans) | 8-15°F |
| Tropical (Honolulu, Miami in summer) | 5-10°F |
Two patterns drive the variation:
1. Humidity dampens swing
Water vapor in the air is the biggest single insulator between Earth's surface and outer space. Humid nights stay warm because the moisture traps the heat the surface radiates upward. Dry nights — desert, high-elevation, far from the ocean — let the heat escape, and overnight temperatures drop fast.
The same daytime high can produce a 25°F swing in dry Phoenix and a 10°F swing in humid Tampa. The difference is entirely in the air's water vapor content.
2. Elevation amplifies swing
Higher altitude = thinner atmosphere = less of a heat blanket. Mountain West cities at 3,000-7,000 feet routinely see 30+°F swings even when humidity is moderate, because the air column above them is thinner than at sea-level cities.
This is why a hike at 9,000 feet in Colorado at 90°F in the afternoon can be 35°F at sunrise the next morning — a 55°F swing — while a Florida day at the same latitude rarely swings more than 15°F.
Why this matters for your house
The math of window-cooling is roughly:
banking_opportunity = (indoor_temp - outdoor_overnight_low)
× hours_below_indoor
The bigger the diurnal swing, the deeper indoor_temp - outdoor_overnight_low gets. The more hours overnight that outdoor stays below indoor, the more banking opportunity.
A house in Phoenix on a 100°F day with a 70°F overnight low has 8-10 hours of useful overnight cooling and 30°F of delta to work with. A house in Houston on the same 100°F day with an 80°F overnight low has 4-5 hours and 20°F.
The Phoenix house can dump 30%+ of its AC hours onto the atmosphere. The Houston house can dump maybe 10%.
"What's the cutoff where window-cooling stops being worth the effort?"
Subjective, but reasonable rules:
- Diurnal swing ≥ 20°F: window-cooling is a high-value habit. Free $100-300/yr if you nail the timing.
- Diurnal swing 12-20°F: window-cooling is worth doing in shoulder seasons but marginal in peak summer. Probably $50-120/yr.
- Diurnal swing 8-12°F: occasional windows during spring/fall shoulder. Maybe $30-80/yr.
- Diurnal swing < 8°F: not worth the behavioral overhead. Stick with the AC + a smart thermostat.
A check you can do today: look up your zip code's average August daily high and average August daily low (NOAA's "climate normals" pages are free; so is Wikipedia's city-climate table). Subtract. That's your typical peak- summer swing.
Shoulder seasons amplify swing
One thing many people miss: the diurnal swing PEAKS in shoulder seasons (spring + fall), not in peak summer or peak winter.
Why: in peak summer, the daytime sun has its longest overlap with hot surface air; the surface stays warm well into the evening. In peak winter, the daytime sun has the shortest overlap; even the warmest hour of the day doesn't warm the surface much. Both extremes squeeze the high-minus-low.
In shoulder seasons (April-May, September-October):
- Daytime sun is strong enough to warm the surface to 60-80°F in many climates
- Overnight surface radiation has long, clear nights to drop temperature 30°F+
Result: the best window-cooling weeks of the year aren't in July and August. They're in May and September.
Phoenix shoulder-season swing: routinely 35°F. Even Denver in May regularly swings 40°F. Even Houston in October swings 20°F (better than its summer 10-13°F).
This is why a thoughtful natural-cooling tool emphasizes shoulder-season strategy — it's where most of the year's savings live.
Climate change effects
Climate-change models predict slightly NARROWING diurnal swings in most of the US, because nighttime temperatures are rising faster than daytime temperatures. The mechanism: increased atmospheric water vapor + greenhouse-gas forcing both impede the overnight radiative cooling that historically drove the swing.
What this means for window-cooling:
- Climates currently at 25-30°F swings will likely drift to 22-27°F over the next 30 years. Still very workable.
- Climates currently at 15-20°F swings will drift to 12-17°F. Still useful in shoulder seasons.
- Climates currently at 8-12°F swings will drift to 6-9°F. Likely below the "worth it" cutoff.
The window-cooling math doesn't disappear, but the geographic range where it's dominant shrinks somewhat. The high-value zones (arid West, Mountain West, Mediterranean coast) keep working; the marginal zones (humid south) get more marginal.
How WindowWise uses diurnal swing
The app pulls the next 24 hours of forecast outdoor temperature and computes:
- Maximum upcoming swing window
- Hours that will be available below your comfort range
- Optimal open-time and close-time given your schedule
- Confidence that opening now AND closing at the right moment will net positive savings vs. leaving the AC on
It doesn't ask you what your climate's typical swing is because it can calculate it from the live + forecast data, which is more accurate (today's swing isn't always typical — heat domes, atmospheric rivers, etc. compress or expand it).
The math is in the [natural-cooling-math post]. The climate-by-climate breakdown is in the [where-natural-cooling-works post]. This post is the vocabulary anchor for both.
TL;DR
- "Diurnal swing" = today's high minus tonight's low.
- Big swing (>20°F) = window-cooling is a high-value habit. Small swing (<10°F) = window-cooling barely works.
- Humidity dampens it. Elevation amplifies it.
- Shoulder seasons (Apr-May, Sep-Oct) usually have the biggest swings of the year.
- The math doesn't disappear under climate change but the geographic range narrows slightly.
[Read: The math of natural cooling] [Read: Where natural cooling actually works] [Read: How much can I save?] [Download WindowWise — free, no account]