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May 2021 Summary – Warmer and Wetter Than Average

June 3, 2021

Blue Hill Observatory May 2021 Summary: May was generally warmer and wetter than average. The 24-hour adjusted mean temperature for the month…

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Blue Hill Observatory May 2021 Summary:

May was generally warmer and wetter than average. The 24-hour adjusted mean temperature for the month of 57.7 deg F was 2.2 degrees warmer than the 1891-2020 130-year average for May, and it was 1.0 degrees warmer than the 1981-2010 30-year average. The average maximum temperature of 68.1 deg F was 1.7 degrees warmer than the 130-year mean, and the average minimum temperature of 48.0 deg F was 1.5 degrees warmer than the 130-year mean. The highest temperature observed all month was 87F on the 22nd and 23rd, and the lowest temperature was 38F on the 1st. The month ended with a pair of cold rain storms over more than three days from the 28th to 31st, which included 63 consecutive hours of temperatures colder than 50F. This was the longest such period since there was a comparable period of 63 hours of temperatures below 50F on May 31st to June 3rd in 2015. The daily high temperature on the 30th of 46F was a new record low maximum for the date, breaking the previous record of 51F set in 1996. This reading was also the coldest high temperature this late in the season since a high of 46F was observed on June 2nd, 1946. May total precipitation was 5.60 inches, which was nearly two inches more than the long-term 130-year average and an inch and a half more than the 1981-2010 30-year mean. The largest rainfall of the month occurred during the last four days when 3.57 inches were observed during a pair of cold rain storms on May 28th to 31st. The first of these storms brought the greatest precipitation in 24 hours during May of 2.50 inches on the 28th-29th. An additional 1.07 inches of rain fell on the 30th-31st. No frozen precipitation was observed during May, and the final 2020-2021 seasonal snowfall was 76.2 inches, which was about 14 inches more than the long-term average. A thunderstorm day was observed on the 26th, which was two fewer than average for May, and it was the third thunderstorm day of the year. The mean wind speed for the month was 11.4 mph, and the prevailing wind direction was from the west-northwest. The monthly peak wind gust was 44 mph from the east-northeast during the rain storm on the 29th. May sunshine was above average with 241.3 hours, which was 55 percent of the possible bright sunshine and three percent more than the long-term average.

Blue Hill Observatory Spring (Mar-May) 2021 Summary:

Spring 2021 was generally very warm and sunnier than average. The 24-hour adjusted mean temperature for March-May of 48.0 deg F was 3.1 degrees warmer than the 1891-2020 130-year average for spring, and it was 1.9 degrees warmer than the 1981-2010 30-year average. Spring 2021 was the eighth warmest on record. The average maximum temperature of 58.5 deg F was 3.5 degrees warmer than the 130-year mean, which tied for the eighth warmest on record, and the average minimum temperature of 38.3 deg F was 1.6 degrees warmer than the 130-year mean. Spring 2021 precipitation was 12.66 inches, which was slightly more than the 130-year mean, but it was an inch and a half less than the 1981-2010 30-year average. Total snowfall during spring was 3.8 inches, which was more than 11 inches less than the long-term average. The mean wind speed for spring was 12.7 mph, which tied for the fifth lowest on record, and the prevailing direction was from the west-northwest. The peak wind gust during the season was 69 mph from the west-northwest on March 1st. Spring was much sunnier than average with a three-month total of 691.7 hours, which was 58 percent of the possible bright sunshine, making the season the sixth sunniest spring on record.

BHO Warmest Spring (March-May) 24-Hour Mean Temperature, deg F (1885-2021):

1) 51.3 in 2010
2) 50.9 in 2012
3) 49.4 in 1991
4) 49.0 in 1977
5) 48.4 in 1998
6) 48.2 in 1976
7) 48.1 in 1945
8) 48.0 in 2021
9) 47.9 in 1903
10) 47.8 in 2016

BHO Warmest Spring (March-May) Average Daily Maximum Temperature, deg F (1885-2021):

1) 61.4 in 2010
2) 61.1 in 2012
3) 60.5 in 1991
4) 60.3 in 1977
5) 59.6 in 1945
6) 59.0 in 1976
7) 58.8 in 1903
8) 58.5 in 1921
   58.5 in 2021
10) 58.4 in 1946

BHO Highest Spring (March-May) Sunshine, hours [percent] (1885-2021):

1) 750.6 in 1977 [63%]
2) 715.4 in 1964 [60%]
3) 709.6 in 1963 [60%]
4) 694.2 in 1911 [59%]
5) 693.8 in 1962 [59%]
6) 691.7 in 2021 [58%]
7) 689.3 in 1969 [58%]
8) 684.3 in 1934 [58%]

BHO Lowest Spring (Mar-May) Mean Wind Speed, mph (1885-2021):

1) 12.0 in 2012
2) 12.3 in 2016
3) 12.6 in 1995
   12.6 in 2018
5) 12.7 in 2019
   12.7 in 2021

Mike Iacono
Chief Scientist
Blue Hill Observatory

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Blue Hill Observatory & Science Center · Legal

Data Use & Privacy Policy

Please read the full Policy before accessing Observatory data · Effective January 1, 2025

Notice: By accessing, downloading, or using any Blue Hill Observatory data or services, you unconditionally accept and agree to be bound by this Policy in its entirety. If you do not agree, you may not access or use Observatory data.
Effective January 1, 2025 · Milton, Massachusetts · bluehill.org
Part I — Data Use Policy
§ 01

Ownership and Proprietary Rights

All observational records, compiled datasets, metadata, database structures, and related materials provided by the Blue Hill Observatory & Science Center are the exclusive property of the Blue Hill Observatory & Science Center. The Observatory has maintained continuous weather observations at the summit of Great Blue Hill since 1885, and the entirety of that record — its compilation, structure, curation, and organization — constitutes a proprietary database protected under applicable United States law, including copyright law and common law database protections.

All rights not expressly granted in this Policy are reserved by the Observatory. No access to or use of Observatory data shall be construed as a waiver of any rights or as a license to use data beyond the scope expressly stated herein.

§ 02

Governing Law and Jurisdiction

This Policy shall be governed by and construed in accordance with the laws of the Commonwealth of Massachusetts, without regard to its conflict of law provisions. For interactions with international users, this Policy shall apply to the maximum extent permitted by applicable local law.

Any dispute arising from or related to this Policy shall be subject to the exclusive jurisdiction of the state and federal courts located in Norfolk County, Massachusetts. By accessing Observatory data, all users consent to personal jurisdiction in Massachusetts and waive any objection to the venue or inconvenience of such forum.

§ 03

Permitted Uses

Observatory data are made available for the following limited, non-commercial purposes only:

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All other uses require prior written authorization from the Observatory.

§ 04

Attribution Requirement

Any use of Observatory data in any published, presented, or publicly distributed work must include the following verbatim acknowledgment:

“Data provided by the Blue Hill Observatory & Science Center, Milton, Massachusetts. Continuous weather observations since 1885.”

Failure to include proper attribution is a material violation of this Policy.

§ 05

Prohibited Uses

Unless explicitly authorized in writing, Users may not:

  • Resell, license, sublicense, or commercially exploit Observatory data in any form
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§ 06

Commercial Licensing

Organizations seeking to use Observatory data for commercial purposes must obtain a license prior to any such use. Licensing inquiries: data@bluehill.org.

§ 07

Automated Access

Automated scraping, bulk harvesting, or systematic downloading of Observatory data is strictly prohibited without prior written authorization.

§ 08

Enforcement and Remedies

Violation of this Policy may result in immediate revocation of data access, pursuit of legal remedies available under United States law, and referral to applicable authorities. The Observatory reserves all rights to seek injunctive relief, damages, and attorneys’ fees in connection with any unauthorized use of its data.

Part II — Privacy Policy
§ 09–10

Scope & Information Collected

This section governs the collection, use, storage, and disclosure of personal information from visitors to the Blue Hill Observatory website and users of its data services. The Observatory may collect contact information (name, email, organization) when voluntarily submitted, usage data including IP addresses and pages visited, and data download records.

§ 11–12

How We Use Information & Retention

Information is used solely for: responding to inquiries and licensing requests; monitoring and enforcing compliance with this Policy; improving website performance; and communicating Observatory news to opted-in users. The Observatory does not sell, rent, or share personal information with third parties for marketing purposes. Data access records may be retained indefinitely for compliance and enforcement purposes.

§ 13

International Users and Data Transfers

The Observatory is located in the United States. International users acknowledge their information will be processed in the United States under U.S. law. Users requiring specific data processing agreements under non-US law must contact data@bluehill.org before accessing data.

§ 14–16

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The Observatory website may use cookies for analytics and functionality. By continuing to use the site, users consent to cookie use. The Observatory implements reasonable measures to protect personal information but cannot guarantee absolute security. Observatory data services are not directed at children under 13.

Part III — General Provisions
§ 17–20

Modifications, Severability, Entire Agreement & Contact

The Observatory reserves the right to modify this Policy at any time. Updated versions will be posted at bluehill.org. Continued use of Observatory data following any modification constitutes acceptance of the revised Policy. This Policy constitutes the entire agreement between the User and the Observatory. Questions and licensing inquiries: Blue Hill Observatory & Science Center, Milton, Massachusetts — data@bluehill.org

By accessing, downloading, or using any Blue Hill Observatory data or services, you acknowledge that you have read, understood, and agree to be bound by this Policy.
Effective January 1, 2025 · Blue Hill Observatory & Science Center · Milton, Massachusetts
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