Map from Google - Latvia

The Latvian climate is freezing in winter, and mild and quite rainy in summer. The coastal areas are a bit more temperate, but also more humid and windy, while the eastern area has a slightly more continental climate. The capital city, Riga, is located on the coast, but in the Gulf of the same name, which is closed and therefore receives a less pronounced maritime influence, so much so that in winter the sea can freeze.
Here are the average temperatures of Riga.
Average temperatures - Riga
RigaJanFebMarAprMayJunJulAugSepOctNovDec
RigaJFMAMJJASOND
Min (°C)-5-6-217111312850-3
Max (°C)-1-12815182020151030
Min (°F)232128344552555446413227
Max (°F)303036465964686859503732

Precipitation is not abundant, but it is frequent and well distributed throughout the year, and is more abundant in summer. In Riga, about 500 millimetres (20 inches) of rain or snow per year fall. Here is the average precipitation.
Average precipitation - Riga
RigaJanFebMarAprMayJunJulAugSepOctNovDecYear
RigaJFMAMJJASONDY
Prec. (mm)302020304050706060505030510
Prec.(in)1.20.80.81.21.622.82.42.4221.220.1
Days11810111213111313161411143

Winter in Latvia is cold and gray, with frequent snowfalls, though not abundant. Temperatures are not so low on the west coast: the daily average in January and February is around -2 °C (28 °F) in Liepaja, around -3 °C (27 °F) in Kolka, at the entrance of the Gulf of Riga, and -3.5 °C (25.5 °F) in Riga, while in Daugavpils and Kraslava, in the south-east, it drops to -5 °C (23 °F). However, the average temperatures are only the result of different situations: western currents of Atlantic origin are damp but less cold, and can raise the temperature to a few degrees above freezing (0 °C or 32 °F), while cold air masses from Russia or from the Arctic may lower the temperature even below -20 °C (-4 °F); in the worst moments, the temperature can plunge to about -35 °C (-31 °F).

Spring in Latvia is quite late: on early April, the country may still be under the snow. From mid-April, temperatures become usually milder. In May the weather can still be cold, especially at the beginning of the month, but then the temperature rises over the course of the month, exceeding even 20 °C (68 °F) in the warmest days.

Summer is a mild in coastal areas, with highs around 18/20 °C (64/68 °F), while it becomes slightly warmer in the interior, where the temperature reaches 20/22 °C (68/72 °F) during the day. Summer is also the rainiest season, especially in inland areas, where thunderstorms may occur in the afternoon. Sometimes, warm air masses from the south-east can raise the temperature, which can approach 30 °C (86 °F) for a few days.

In autumn, rainfall is frequent and the sky is often gray. The temperature gradually decreases: in October it's already cold and in November the first snowfalls occur.

Riga

For swimming, the Baltic Sea is cold: in the Gulf of Riga the sea temperature varies from 1 °C (34 °F) in February and March, to 16 °C (61 °F) in July and to 17 °C (63 °F) in August, while on the west coast it is just warmer.
Sea temperature - Riga
RigaJanFebMarAprMayJunJulAugSepOctNovDec
RigaJFMAMJJASOND
Sea (°C)21135101617151174
Sea (°F)363434374150616359524539

When to go

The best season to visit Latvia is summer, from June to August. Temperatures are mild during the day, and lend themselves to outdoor activities, while nights remain cool. There's no shortage of rainfall, which occurs on average one day out of three.
If you want to visit the country in its winter look, February is preferable to December, because, although it is even colder, it has longer days.

What to pack

In winter: very warm clothes, down jacket, hat, gloves, scarf.
In summer: clothes for spring and autumn, being ready to add or remove the outer layer, t-shirt, but also long pants, jacket and sweatshirt; raincoat or umbrella.