When does it most likely rain in Tallinn?
Because Tallinn has a humid continental climate (in the transitional zone between maritime and continental climate), rain is likely all year round and is quite evenly distributed throughout the year, except for a two-week spell of drought or excessive rainfall.
From May to September, and especially in August, there are quite a lot of showers. Thunderstorms can also occur (most commonly during the second half of the day). At other times of the year thunderstorms and downpours are very rare, steady rain or snow is more common and then there is no clear pattern of rainfall.
Does Tallinn have snow?
Since the middle of the 20th century, the average temperature in Estonia has risen by more than the global average (0.2 – 0.3 °C per decade). The number of days with snow has decreased (between 1961–2002, the average decrease has been 25.9 days) and the amount of rain has increased (5–15%). Considering global warming (climate change), these trends are predicted to continue, except with June becoming colder. In spite of this, snow between November and April will remain common. There will be 0–5 days of snow in October and April.
This all means that, according to current trends, in the year 2100 the winter will be clearly cyclonic: stormy, rainy and warm, the sea will no longer freeze, though cold spells and snowstorms will not disappear. The summer will be clearly anticyclonic: less windy, hot and with torrential showers.
Windy and snowy winter
Winter has very changeable weather, and there is usually snow between December and March. In warm winters the snow arrives at the end of December or the start of January (with maybe the odd snowy period earlier), occurring and melting rapidly. In a cold winter the snow can be seen at the end of October or in November and stay until April.
Lake-effect snow is most likely in December and January. This happens when a cold air mass moves across long expanses of warmer lake or sea water. In Tallinn lake-effect snow occurs when the cold air mass moves from the northeast to the southwest over open, ice-free water on Gulf of Finland, sometimes causing heavy snowfall (more than 10 cm of snow can fall in a single day). In some rarer cases lake-effect could bring blizzard-like conditions or snowsqualls. During a warm winter, lake-effect snow can occur in February or even in March because the sea is not frozen.
Winter is often windy and the temperature depends on the synoptic situation, which means irregularity and that there is no significant daily pattern like in summer. The arrival of a warm air mass can see the morning temperature rise to above 0 °C or a cold air mass arriving can cause it to fall below –10 °C during the middle of day. From November to January, the sun has no clear effect on air temperatures, except on very cold and calm days.
The air temperature between November and March varies between +5 and –25 °C, although on some occasions it is possible to rise to as much as 15 °C in March or fall to –25 or –30 °C in January or February. The lowest temperature has been –34.4 °C in February and the highest 15.8 °C in March.
The most common temperature in winter is 0 to –10 °C, though in February and March the sun’s warmth is sometimes noticeable.
In spring, the uneven warming of the sea and inland areas have an influence on the weather. The effect of the breeze is particularly noticeable, especially in May. The breeze, which is comprised of land- and sea-breezes, occurs usually alongside settled and sunny weather, when the sea is still cold but inland it is warming up considerably.
The breeze is felt more strongly during the day – a calm and sunny noon can suddenly become chilly and windy as the breeze brings cool air from the sea. The temperature can sometimes even fall by 10 °C over the course of a few minutes. The impact of the breeze is especially noticeable if it is warmer than 15 °C in April or 20 °C in May.
Depending on the year, March can be either winter or spring month. April is often the most changeable, where on the one hand there can be a heatwave and on the other hand, freezing temperature and snowstorms. Over the last 20 years it has been both 28 °C and –10 °C.
Both April and May regularly have overnight frosts. In an exceptional year, there can even be snow in the first half of May.
The risk of heatwaves increases in May: the first time the temperature will reach 25 to 30 °C is at some point in May. For example in 2014, the heatwave in May was unbearable – it caused health problems and complaints, temperature rose up to 32 °C already on May 19th and it was humid.
The first thunderstorm usually can be seen in May, very rarely in June and equally unusually in April.
Gentle summer spiced with lightning and thunderstorms
Summer is variable but mild. This means that it rarely climbs above 30 °C (the record for both July and August is 34.4 °C), usually it is 15–20 °C, in a warm summer 22–27 °C. Overnight frosts are rare and can occur only in June (both on the ground and in the air). Very rarely overnight frosts may occur in early July (only on the ground) and at the end of August. Almost in all cases, overnight frosts occur somewhere in the suburbs of Tallinn, not directly next to the sea or in the city centre.
At the beginning of summer (in May or June) the promenade can be buried under a dense fog, especially when the air mass is warm and humid, and there is a sea breeze. In 2013 during hot weather in May, fog settled over the sea and some parts of the city for weeks.
The breeze, which can unexpectedly make the day’s weather chilly, is common until July and, if there is hot and calm weather, sometimes even in August.
There are 10 to 15 thunderstorm days. The month with the most thunderstorms is July, although if it is a cool summer, most thunderstorms will be seen in August.
Sometimes, most commonly towards the end of summer, there can occur hail or ice pellets. Large hail (hailstones) is very unusual. Tornadoes can rarely occur over the sea in the second half of the summer – this is called a waterspout. Only one tornado, in 1967, is known in mainland of Tallinn and even this is not officially confirmed.
Heavy rain and downpours sometimes occur either with lightning (cumulonimbus clouds present, accompanied heavy showers, even hail) or without (cyclone-related torrential but steady rain). Downpours can cause localised flooding, sometimes even a flash flood, especially when this happens in downbursts (originated from cumulonimbus clouds).
Cloudy autumn skies
Autumn is usually cloudy, dull and windy. Usually there are rain periods with steady rain, which can last for days. The first overnight frosts in the suburbs of Tallinn usually arrive in October, and in colder cases during the second half or late of September.
Sometimes September and October have longer spells of clear, settled, warm weather (mornings can be foggy and with overnight frost), called an Indian Summer, in which case the daily maximum is at least 17 °C.
The last thunderstorms usually occur in September but are often restricted to the sea due to much warmer and humid surface, although sometimes there are thunderstorms in October or even November. In this case, thundersnow can be seen, with the air temperature below 0 °C.
There can also be spells of overcast weather at the same time as dry and calm periods, which are most likely in October and November. These are related to anticyclones and therefore called anticyclonic gloom.
Usually the first snowfall happens in October and in November the first wintry days arrive, although in the recent past, winter has even started as early as October.
If the autumn is warm, then warm, snow-free, sometimes rainy weather can last until December or even into the New Year. This could become more common in the future due to climate change. Despite of warm autumn, several snowfalls or wintry periods are common as well.
The first snowstorms usually come in October or November. A snowstorm strictly speaking means weather with sustained wind at least 21 m/s, if not so strict, then wind gusts at least 21 m/s. In the recent past there have been many settled and warm (snow-free) autumns, when the first stronger wind (gale, extratropical cyclone) comes at the end of November or December.