Bremen has a moderate oceanic climate due to its proximity to the North Sea coast and temperate maritime air masses that move in with the predominantly westerly winds from the Atlantic Ocean. However, periods in which continental air masses predominate may occur at any time of the year and can lead to heat waves in the summer and prolonged periods of frost in the winter. In general though, extremes are rare in Bremen and temperatures below −15 °C (5.0 °F) and above 35 °C (95.0 °F) occur only once every couple of years. The record high temperature was 37.6 °C (99.7 °F) on 9 August 1992, while the official record low temperature was −23.6 °C (−10.5 °F) on 13 February 1940. On 13 October 2018, Bremen recorded its warmest October day on record with 28.6 °C (83.4 °F). However, the astronomer Heinrich Wilhelm Matthias Olbers reported to have measured −27,3 °C on 23. January 1823. Being at some distance from the main North Sea, Bremen still has a somewhat wider temperature range than Bremerhaven that is located on the mouth of Weser.
Average temperatures have risen continually over the last decades, leading to a 0.6 °C (1.1 °F) rise in the mean annual temperature between 1961–90 and 1981–2010 reference periods. As in most parts of Germany, the year 2014 has been the warmest year on record averaging 11.1 °C (52.0 °F), making Bremen the second-warmest German state after Berlin in 2014. While Bremen is located in the comparatively cloudy northwestern part of Germany, there has been a significant increase in average sunshine hours over the last decades, especially in the months of April, May, and July, causing the annual mean to rise by 62 hours between the two reference periods mentioned above. This trend has continued over the last 10 years, which average 1614 hours of sunshine, a good 130 hours more than in the international reference period of 1961–90. Nevertheless, especially the winters remain extremely gloomy by international standards with December averaging hardly more than one hour of sunshine (out of 7 astronomically possible) per day, a feature that Bremen shares with most of Germany and its neighbouring countries, though.
Precipitation is distributed fairly even around the year with a small peak in summer mainly due to convective precipitation, i.e. showers and thunderstorms. Snowfall and the period of snow cover are variable; whereas in some years, hardly any snow accumulation occurs, there has recently been a series of unusually snowy winters, peaking in the record year 2010 counting 84 days with a snow cover. Nevertheless, snow accumulation of more than 20 centimetres (8 in) remains exceptional, the record being 68 centimetres (26.8 in) of snow on 18 February 1979.
The warmest months in Bremen are June, July, and August, with average high temperatures of 20.2 to 22.6 °C (68.4 to 72.7 °F). The coldest are December, January, and February, with average low temperatures of −1.1 to 0.3 °C (30.0 to 32.5 °F). Typical of its maritime location, autumn tends to remain mild well into October, while spring arrives later than in the southwestern parts of the country.