Portsmouth, where it's located

The climate of Portsmouth, a city located in Hampshire, is oceanic, with quite cold, rainy winters and mild, relatively rainy summers. However, the south coast of England, where Portsmouth is located, has a mild and sunny microclimate, compared to the rest of Great Britain.
The average temperature of the coldest month (February) is of 5.5 °C (42 °F), that of the warmest month (August) is of 17.9 °C (64 °F). The lowest record is -8 °C (17.5 °F), the highest is 35 °C (95 °F). Here are the average temperatures.
Portsmouth - Average temperatures
Min (°C)33469121414121064
Max (°C)881013171921211916129
Min (°F)373739434854575754504339
Max (°F)464650556366707066615448

Precipitation amounts to 700 millimeters (27.6 inches) per year: so, it is at an intermediate level. It ranges from 40 mm (1.6 in) in the driest months (April, June) to 85 mm (3.3 in) in the wettest ones (October, November, December). Here is the average precipitation.
Portsmouth - Average precipitation

On average, there are around 1,920 sunshine hours per year: we are at the highest levels in Britain. Here are the average sunshine hours per day.
Portsmouth - Sunshine

The sea temperature ranges between 8 °C (46 °F) and 17 °C (63 °F). So, it never becomes warm for swimming. Here are the average sea temperatures.
Portsmouth - Sea temperature
Temp (°C)98891113151717161311
Temp (°F)484646485255596363615552


Best Time

The best time to visit Portsmouth is the summer, from June to August, since it is the warmest of the year, although even in this season, it's better to bring a sweatshirt, a light jacket and an umbrella. The month of June is a bit cooler, so you may prefer July and August. September is a good month as well: it's a bit rainier than the summer months, but the temperatures are still pleasant, especially in the first part.
As previously mentioned, the sea is very cool even in summer, however, on sunny days, when the breeze is not too cool, you can sunbathe.