May Weather in Boston Massachusetts, United States
Daily high temperatures increase by 10°F, from 61°F to 71°F, rarely falling below 50°F or exceeding 84°F.
Daily low temperatures increase by 9°F, from 46°F to 55°F, rarely falling below 40°F or exceeding 63°F.
For reference, on July 21, the hottest day of the year, temperatures in Boston typically range from 67°F to 82°F, while on January 29, the coldest day of the year, they range from 23°F to 36°F.
Average High and Low Temperature in May in Boston
The figure below shows you a compact characterization of the hourly average temperatures for the quarter of the year centered on May. The horizontal axis is the day, the vertical axis is the hour of the day, and the color is the average temperature for that hour and day.
Average Hourly Temperature in May in Boston
frigid 15°F freezing 32°F very cold 45°F cold 55°F cool 65°F comfortable 75°F warm 85°F hot 95°F sweltering
The month of May in Boston experiences essentially constant cloud cover, with the percentage of time that the sky is overcast or mostly cloudy remaining about 54% throughout the month. The highest chance of overcast or mostly cloudy conditions is 54% on May 24.
The clearest day of the month is May 6, with clear, mostly clear, or partly cloudy conditions 47% of the time.
For reference, on May 24, the cloudiest day of the year, the chance of overcast or mostly cloudy conditions is 54%, while on September 6, the clearest day of the year, the chance of clear, mostly clear, or partly cloudy skies is 64%.
Cloud Cover Categories in May in Boston
0% clear 20% mostly clear 40% partly cloudy 60% mostly cloudy 80% overcast 100%
A wet day is one with at least 0.04 inches of liquid or liquid-equivalent precipitation. In Boston, the chance of a wet day over the course of May is increasing, starting the month at 28% and ending it at 32%.
For reference, the year's highest daily chance of a wet day is 33% on May 24, and its lowest chance is 23% on September 4.
Probability of Precipitation in May in Boston
To show variation within the month and not just the monthly total, we show the rainfall accumulated over a sliding 31-day period centered around each day.
The average sliding 31-day rainfall during May in Boston is gradually increasing, starting the month at 3.3 inches, when it rarely exceeds 5.5 inches or falls below 1.4 inches, and ending the month at 3.5 inches, when it rarely exceeds 7.1 inches or falls below 1.3 inches.
Average Monthly Rainfall in May in Boston
Over the course of May in Boston, the length of the day is increasing. From the start to the end of the month, the length of the day increases by 59 minutes, implying an average daily increase of 1 minute, 57 seconds, and weekly increase of 13 minutes, 40 seconds.
The shortest day of the month is May 1, with 14 hours, 5 minutes of daylight and the longest day is May 31, with 15 hours, 4 minutes of daylight.
Hours of Daylight and Twilight in May in Boston
The latest sunrise of the month in Boston is 5:39 AM on May 1 and the earliest sunrise is 29 minutes earlier at 5:10 AM on May 31.
The earliest sunset is 7:44 PM on May 1 and the latest sunset is 30 minutes later at 8:14 PM on May 31.
Daylight saving time is observed in Boston during 2021, but it neither starts nor ends during May, so the entire month is in daylight saving time.
For reference, on June 21, the longest day of the year, the Sun rises at 5:07 AM and sets 15 hours, 17 minutes later, at 8:24 PM, while on December 21, the shortest day of the year, it rises at 7:10 AM and sets 9 hours, 5 minutes later, at 4:14 PM.
Sunrise & Sunset with Twilight in May in Boston
The figure below presents a compact representation of key lunar data for May 2021. The horizontal axis is the day, the vertical axis is the hour of the day, and the colored areas indicate when the moon is above the horizon. The vertical gray bars (new Moons) and blue bars (full Moons) indicate key Moon phases. The label associated with each bar indicates the date and time that the phase is obtained, and the companion time labels indicate the rise and set times of the Moon for the nearest time interval in which the moon is above the horizon.
Moon Rise, Set & Phases in May in Boston
We base the humidity comfort level on the dew point, as it determines whether perspiration will evaporate from the skin, thereby cooling the body. Lower dew points feel drier and higher dew points feel more humid. Unlike temperature, which typically varies significantly between night and day, dew point tends to change more slowly, so while the temperature may drop at night, a muggy day is typically followed by a muggy night.
The chance that a given day will be muggy in Boston is gradually increasing during May, rising from 0% to 4% over the course of the month.
For reference, on August 2, the muggiest day of the year, there are muggy conditions 39% of the time, while on January 1, the least muggy day of the year, there are muggy conditions 0% of the time.
Humidity Comfort Levels in May in Boston
dry 55°F comfortable 60°F humid 65°F muggy 70°F oppressive 75°F miserable
This section discusses the wide-area hourly average wind vector (speed and direction) at 10 meters above the ground. The wind experienced at any given location is highly dependent on local topography and other factors, and instantaneous wind speed and direction vary more widely than hourly averages.
The average hourly wind speed in Boston is gradually decreasing during May, decreasing from 6.6 miles per hour to 5.7 miles per hour over the course of the month.
For reference, on February 20, the windiest day of the year, the daily average wind speed is 9.1 miles per hour, while on July 31, the calmest day of the year, the daily average wind speed is 4.8 miles per hour.
Average Wind Speed in May in Boston
The hourly average wind direction in Boston throughout May is predominantly from the west, with a peak proportion of 33% on May 31.
Wind Direction in May in Boston
Boston is located near a large body of water (e.g., ocean, sea, or large lake). This section reports on the wide-area average surface temperature of that water.
The average surface water temperature in Boston is rapidly increasing during May, rising by 8°F, from 46°F to 53°F, over the course of the month.
Average Water Temperature in May in Boston
Definitions of the growing season vary throughout the world, but for the purposes of this report, we define it as the longest continuous period of non-freezing temperatures (≥ 32°F) in the year (the calendar year in the Northern Hemisphere, or from July 1 until June 30 in the Southern Hemisphere).
The growing season in Boston typically lasts for 7.2 months (220 days), from around April 4 to around November 11, rarely starting before March 18 or after April 23, and rarely ending before October 26 or after November 26.
The month of May in Boston is very likely fully within the growing season, with the chance that a given day is in the growing season gradually increasing from 96% to 100% over the course of the month.
Time Spent in Various Temperature Bands and the Growing Season in May in Boston
frigid 15°F freezing 32°F very cold 45°F cold 55°F cool 65°F comfortable 75°F warm 85°F hot 95°F sweltering
Growing degree days are a measure of yearly heat accumulation used to predict plant and animal development, and defined as the integral of warmth above a base temperature, discarding any excess above a maximum temperature. In this report, we use a base of 50°F and a cap of 86°F.
The average accumulated growing degree days in Boston are increasing during May, increasing by 262°F, from 126°F to 389°F, over the course of the month.
Growing Degree Days in May in Boston
This section discusses the total daily incident shortwave solar energy reaching the surface of the ground over a wide area, taking full account of seasonal variations in the length of the day, the elevation of the Sun above the horizon, and absorption by clouds and other atmospheric constituents. Shortwave radiation includes visible light and ultraviolet radiation.
The average daily incident shortwave solar energy in Boston is gradually increasing during May, rising by 0.6 kWh, from 5.7 kWh to 6.3 kWh, over the course of the month.
Average Daily Incident Shortwave Solar Energy in May in Boston
For the purposes of this report, the geographical coordinates of Boston are 42.358 deg latitude, -71.060 deg longitude, and 39 ft elevation.
The topography within 2 miles of Boston contains only modest variations in elevation, with a maximum elevation change of 118 feet and an average elevation above sea level of 10 feet. Within 10 miles contains only modest variations in elevation (502 feet). Within 50 miles contains significant variations in elevation (2,037 feet).
The area within 2 miles of Boston is covered by artificial surfaces (77%) and water (23%), within 10 miles by artificial surfaces (65%) and water (29%), and within 50 miles by trees (39%) and water (33%).
This report illustrates the typical weather in Boston, based on a statistical analysis of historical hourly weather reports and model reconstructions from January 1, 1980 to December 31, 2016.
Temperature and Dew Point
There are 3 weather stations near enough to contribute to our estimation of the temperature and dew point in Boston.
For each station, the records are corrected for the elevation difference between that station and Boston according to the International Standard Atmosphere , and by the relative change present in the MERRA-2 satellite-era reanalysis between the two locations.
The estimated value at Boston is computed as the weighted average of the individual contributions from each station, with weights proportional to the inverse of the distance between Boston and a given station.
The stations contributing to this reconstruction are:
To get a sense of how much these sources agree with each other, you can view a comparison of Boston and the stations that contribute to our estimates of its temperature history and climate. Please note that each source's contribution is adjusted for elevation and the relative change present in the MERRA-2 data.
All data relating to the Sun's position (e.g., sunrise and sunset) are computed using astronomical formulas from the book, Astronomical Algorithms 2nd Edition , by Jean Meeus.
All other weather data, including cloud cover, precipitation, wind speed and direction, and solar flux, come from NASA's MERRA-2 Modern-Era Retrospective Analysis . This reanalysis combines a variety of wide-area measurements in a state-of-the-art global meteorological model to reconstruct the hourly history of weather throughout the world on a 50-kilometer grid.
Land Use data comes from the Global Land Cover SHARE database , published by the Food and Agriculture Organization of the United Nations.
Elevation data comes from the Shuttle Radar Topography Mission (SRTM) , published by NASA's Jet Propulsion Laboratory.
Names, locations, and time zones of places and some airports come from the GeoNames Geographical Database .
Time zones for airports and weather stations are provided by AskGeo.com .
Maps are © Esri, with data from National Geographic, Esri, DeLorme, NAVTEQ, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, and iPC.
The information on this site is provided as is, without any assurances as to its accuracy or suitability for any purpose. Weather data is prone to errors, outages, and other defects. We assume no responsibility for any decisions made on the basis of the content presented on this site.
We draw particular cautious attention to our reliance on the MERRA-2 model-based reconstructions for a number of important data series. While having the tremendous advantages of temporal and spatial completeness, these reconstructions: (1) are based on computer models that may have model-based errors, (2) are coarsely sampled on a 50 km grid and are therefore unable to reconstruct the local variations of many microclimates, and (3) have particular difficulty with the weather in some coastal areas, especially small islands.
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