You’re planning a trip to the southernmost point in the continental U.S., and you’re probably thinking: "It’s Florida, right? It’s basically summer all year."
Well, kinda.
Key West is a different beast entirely. It’s the only city in the lower 48 that has never seen frost. Seriously. The record low is 41°F, set way back in 1886. But if you think you’re getting 85-degree beach weather in January, you might be in for a surprise. Key West average temperatures by month tell a story of two distinct worlds: the dry, breezy winter and the "soul-melting" tropical summer.
Most people look at a chart, see a "75" in January, and pack only a swimsuit. Don't do that. Here is the actual, boots-on-the-ground reality of what it's like to stand on Duval Street throughout the year.
The "Winter" That Isn't (December to February)
Honestly, this is the gold standard for weather. While the rest of the country is digging out of snow, Key West is basically living in a climate-controlled bubble.
January is the coldest month here. The average high sits right around 74°F (23°C), and the lows hover at 64°F (18°C).
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It sounds perfect, but there’s a catch. Cold fronts from the north frequently dip down. One day you're in shorts; the next, a 20-mph wind is whipping off the Atlantic and you’re hunting for a hoodie. If you’re a swimmer, be warned: the water temperature in January and February is usually around 72°F. It’s refreshing for some, but many locals won't touch the ocean until April.
February is a carbon copy of January but with a tiny bit more sun. The high jumps to 76°F. It’s the driest part of the year, so you don't have to worry about afternoon thunderstorms ruining your sunset cruise at Mallory Square.
Spring: The Sweet Spot (March to May)
If you ask anyone who lives on the island when the "best" time to visit is, they’ll say April. No question.
March starts to heat up. The average high hits 78°F, but the humidity stays low. This is "convertible weather." By April, the mercury reaches 81°F. This is the month where the ocean finally starts to feel like a bath. The water temperature hits a comfortable 77°F, making it the prime time for snorkeling at Sand Key or hitting the backcountry in a kayak.
May is the transition month. It's when the "tropical savanna" climate starts to show its teeth.
- Average High: 84°F
- Average Low: 76°F
- Rainfall: It jumps from about 2 inches in April to over 3 inches in May.
This is the start of the rainy season. It’s not a wash-out, though. Usually, it’s just a 20-minute downpour at 4:00 PM that smells like wet pavement and tropical flowers, then the sun comes right back out.
The Humidity Wall (June to September)
This is the part most travel brochures gloss over. Summer in Key West is intense. It’s not just the heat; it’s the moisture.
From June through August, the average high is 89°F or 90°F. That doesn't sound too bad compared to Arizona, right? Wrong. The humidity levels are often above 75%, meaning the "feels like" temperature—the heat index—can easily reach 105°F by noon.
August is statistically the hottest month. The lows don't even drop below 80°F. You’ll wake up at 7:00 AM, walk outside, and immediately feel like you’re wearing a warm, wet blanket. This is also the peak of hurricane season.
September is the wettest month on the calendar, averaging over 6.7 inches of rain. It's also the time when the island is the quietest. If you can handle the "steam room" vibe, you can find some of the best hotel deals of the year. Just keep a very close eye on the National Hurricane Center (NHC) updates.
The Great Cooldown (October and November)
October is a wild card. It starts hot and muggy, but usually, by the end of the month—right around Fantasy Fest—the first real "cool" breeze of the season arrives. The average high drops to 85°F, which feels like a godsend after August.
November is the unsung hero of Key West weather. The humidity vanishes. The highs fall to 80°F, and the rainy season officially ends. It’s arguably the best time for fishing; the mahi-mahi are active, and the water is still a manageable 76°F.
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Seasonal Temperature Reality Check
To help you pack, here is the quick breakdown of what to expect from the mercury:
The Warmest Months (Highs of 88°F - 90°F):
- June, July, August, September.
- Expect high humidity and daily thunderstorms.
The Moderate Months (Highs of 78°F - 84°F):
- March, April, May, October, November.
- These are your "active" months for biking and walking.
The Coolest Months (Highs of 74°F - 76°F):
- December, January, February.
- Bring a light jacket for the evenings when the wind kicks up.
Actionable Tips for Your Trip
Don't just look at the numbers; plan for the environment.
If you are visiting between June and October, schedule your outdoor activities for before 10:00 AM. Key West is small, but walking from one end of Duval to the other in July can lead to heat exhaustion if you aren't hydrated.
For those heading down in the winter, check the "marine forecast" rather than just the air temperature. A 75-degree day with a 25-knot wind feels a lot colder when you're on a boat.
Lastly, always pack a high-quality polarized pair of sunglasses. The sun reflecting off the white sand and turquoise water is significantly more intense than in the northern states, regardless of what the thermometer says.
Next Steps for Your Trip:
- Check the current NOAA tide charts if you plan on visiting Smathers Beach; low tide can make the water quite shallow and grassy.
- Look at the National Hurricane Center "Atlantic Outlook" if your trip is between August and October.
- Book your water excursions (diving/fishing) for the April-May window if you want the best balance of calm seas and warm water.