In Estonia, the climate is Baltic, ie slightly continental, with freezing winters and cool summers, which are also moderately rainy. Estonia is the northernmost of the Baltic republics, and it is also the coldest. The climate is fairly uniform, but the western coastal areas, west of Tallinn, are a bit milder because they receive a greater maritime influence. This influence is far less pronounced along the northern coast, overlooking the Gulf of Finland, being a narrow branch of sea that can freeze in winter. In winter, the mitigating influence is virtually absent even in the vicinity of Lake Peipus, located in the east on the border with Russia, because in this season it's almost always frozen.
Here are the average temperatures of the capital Tallinn, located on the norh coast.
Tallinn - Average temperatures
Min (°C)-7-8-40510121273-1-5
Max (°C)-1-2281519212015930
Min (°F)191825324150545445373023
Max (°F)302836465966706859483732

Precipitation in Estonia has a pattern similar to that of the other Baltic countries, being distributed fairly evenly throughout the year, with a relative minimum between February and April, and a maximum in summer, but is slightly more abundant than in Latvia and Lithuania, at least in the north, where it exceeds 700 millimetres (27.5 in) per year.
Here is the average precipitation in Tallinn.
Tallinn - Average precipitation

Winter in Estonia is very cold: the temperature remains below freezing (0 °C or 32 °F) even during the day for most of the time. The average temperature in January and February ranges from -2 °C (28 °F) in the two main islands (Hiumaa, Saaremaa), to -4 °C (25 °F) in the coast of the Gulf of Riga, to -5 °C (23 °F) in Tallinn and on the north coast, and finally it goes down to -6.5 °C (20.5 °F) in the inland north-eastern areas.
The Atlantic currents, which sometimes succeed in penetrating into the southern part of Scandinavia and in Central Europe, have some difficulty in getting to Estonia. However, Atlantic weather fronts can arrive here, giving rise to frequent snowfall, more likely than rainfall. More often, cold air masses coming from the Arctic or from Russia move over the country, lowering the temperature to about -20 °C (-4 °F). The cold records are respectable: in the worst moments the temperature can plunge to -40 °C (-40 °F). December is the month when the sun is lowest above the horizon, and the days are shortest, while the coldest month is February, as is often the case in Nordic countries.

Tallinn in winter

In spring, the days get longer, and the temperature rises slowly; in general the thaw occurs at the beginning of April, even though between late April and early May, the return of cold weather, even with snow showers, is possible. April is a month with great contrasts, so it's possible to still see some freezing periods, alternating with the first warm days, more likely in the second half of the month. In the middle of May, the temperature becomes generally acceptable.

Summer in Estonia is a pleasant season, since the maximum temperatures hover around 20/22 °C (68/72 °F), which means that they are suitable for outdoor activities. Nights are cool, with minimum temperatures around 11/12 °C (52/54 °F), which rise to 14 °C (57 °F) on the west coast.
However, summer is quite rainy: it rains on average one day out of three, but it is not impossible to see the sun coming out. In inland areas, there's some chance of thunderstorms in the afternoon.
While Atlantic fronts can bring some cool and rainy days even in summer, warm air masses from the south-east can bring some hot days, and in these situations the temperature may reach as high as 32/33 °C (90/91 °F), but these periods usually don't last long.

Autumn is a dull and rainy season; if September can still have acceptable temperatures, it quickly becomes colder, so much so that in late October the first snowfalls are possible. Compared with spring, autumn is darker, because the days are much shorter, a difference which occurs everywhere apart from the Equator, but that is more remarkable in the Nordic countries.

The sun in Estonia is rarely seen from November to February, while from May to August it shines for just over half the time (the days are very long, so the total sunshine is not low).
Tallinn - Sunshine

The sea in Estonia is cold, and in winter it can freeze across the country, but especially in the Gulf of Finland. In summer, it reaches 17 °C (63 °F), so it's not advisable for swimming, if not for those who are brave enough.
Tallinn - Sea temperature
Temp (°C)21124101517141073
Temp (°F)363434363950596357504537

When to go

The best time to visit Estonia is summer, from June to August. The temperatures are mild during the day and cool at night. Typically August is a bit rainier than June and July.
In the months of November, December and January, the days are very short and the sun sets early.
In March, usually the country has still a wintery appearance, with snowy landscapes, but the days are much longer than in the previous months.
At Easter, you can still find snow, especially in the years when it comes early.

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.