May 21 is generally a great time to visit Boston. While there is a slight chance of rain (25%), showers are usually brief. Expect comfortable 20°C weather. As it's peak season for Boston, we recommend booking your stay well in advance to secure the best rates.
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Essential Travel Info for Boston
Currency
USD
$ United States dollar
Language
English
Driving
Right side
Connect
Greater Boston uses10-digit dialing. Th...
Safety
Moderate Safety
GPI: 2.44
Boston Weather Forecast for May 21
"Mild & Comfortable. Expect average highs of 19.8°C. 25% chance of rain."
What to Wear in Boston in May
Based on historical data for May 21, Boston typically experiences temperatures between 10.3°C and 19.8°C. We recommend packing light jacket, long pants to stay comfortable. Trends suggest it will stay mostly dry (25% rain probability), but checking the forecast before heading out is always wise.
Complete Packing Guide
Weekly breakdown & essential items for Boston
Smart Suitcase
- Light Jacket
- Long Pants
Average High
19.8°C
Daytime Peak
Average Low
10.3°C
Nighttime Low
Feels Like
19.8°C
🌤️ Feels accurate
Rain Chance
25%
Avg 1.6mm if it rains
Cloud Cover
64%
Overcast
Pressure
1016.2 hPa
Medium volatility
Boston Weather Conditions & UV Index
Weather Confidence
30-year variance analysis
Based on 10 years of data
~ Weather is generally predictable, but pack for minor variations. A light backup layer is recommended.
Sun Exposure
UV Index & Safety Guide
Protection Needed
SPF 30 recommended
04:00 - 10:00
16:00 - 19:00
Type I-II
🔴 17 min to burn
Type III-IV
🟡 26 min to burn
Type V-VI
🟢 43 min to burn
UV estimate based on latitude, season & cloud cover. Actual UV may vary.
Outdoor Comfort
How pleasant is it outside?
Dew Point
14.6°C
Pleasant
Heat Stress
Low
No heat warning
😊 Best outdoor time:
Any time during daylight hours
Marine Conditions
Marine Life
No special warnings. Enjoy the swim!
Swim Index
❄️ Cold Water Hazard
Travel & Logistics Insights
Local Calendar
Business Day
Standard hours in United States
Flight Traffic
High
95/100 pressure index
Health Advisory
Healthy
Routine only • View Details →
Landing Slots
~643
Daily arrival options (inc. codeshare)
Sunrise, Sunset & Wind in Boston
Daylight & Sun
Solar Elevation Cycle
🌅 Sunrise
04:18
☀️ Noon
11:42
🌇 Sunset
19:05
⏱️ Daylight
14h 47m
Wind Speed
18.9 km/h
Gentle breeze
Sunshine
9.5 hrs
Direct Sun (Cloud-free)
Humidity
72%
Humid / Damp
Travel Intelligence
Exclusive flight analytics & health data for Boston
Smart Flight Estimator
Airport-to-Airport Logistics
unlock smart flight estimates
Add your home location in settings to see airport details.
Travel Health & Safety in Boston
Travel Comfort Index
Real-time tourism conditions
Weather Quality
☀️ Excellent
Tourist Crowds
🟡 Busy
Price Level
💰💰💰 Expensive
Health & Wellbeing
Meteo-Health Forecast
Moderate fluctuations. Some sensitivity possible.
Weather unlikely to affect joint pain.
Moderate fish activity expected.
Air quality is excellent. Perfect for outdoor activities.
Regional Safety Profile
Seismic Activity
Zero seismic activity detected in 30 years.
Atlantic Hurricane Risk
Season: Jun - Nov
"According to 30YearWeather's analysis of 30 years of NASA satellite data, Boston on May 21 averages 19.8°C with a 25% precipitation probability. Wedding/Outdoor Event Score: 94/100."
— Source: 30YearWeather.com|Data: NASA POWER (1991-2021)|View Methodology
Boston Weather History (2010–2024)
Temperature History
Hall of Fame
Records from last 10 years
Better Alternatives Nearby
Consider these dates within ±7 days
May 25
21.3°C • 21% rain
Score
96
May 26
21.6°C • 21% rain
Score
96
May 28
22.2°C • 22% rain
Score
96
💡 Moving your event by a few days could significantly improve weather conditions
Compare Other Cities
Same date (May 21) in other destinations
🌍 Explore the same date in different destinations around the world
📍 Is Boston Worth Visiting on May 21?
Planning a trip to Boston on May 21? Our historical weather analysis based on 30 years of NASA satellite data shows that this time of year is characterized by pleasant mild weather averaging 19.8°C with a 25% historical precipitation risk.
✈️ Travel Logistics: Approximately 643 landing slots available daily during this period, indicating peak tourist activity.
🌊 Coastal Conditions: Water temperature averages 13.2°C (polar plunge). Wave conditions are generally fun waves with heights around 0.8m.
📊 Historical Extremes: The hottest May 21 recorded in the last 30 years saw temperatures hit 29.2°C in 2024, while the coldest dropped to 4.9°C in 2020.