The climate of Finland is characterized by long and cold winters, and by short, mild and moderately rainy summers. Among the Scandinavian countries, Finland is the one with the coldest climate, due to the proximity to Russia.
Winter is long and cold throughout the country; it is almost five months long even in Helsinki, which lies in the far south. Here, the temperature remains almost constantly below freezing (0 °C or 32 °F) from mid-November to late March.
Here are the average temperatures of Helsinki.
Average temperatures - Helsinki

Helsinki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Helsinki	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	-8	-9	-5	0	5	10	13	11	7	3	-2	-6
Max (°C)	-2	-3	2	9	16	20	22	20	15	9	3	-1
Min (°F)	18	16	23	32	41	50	55	52	45	37	28	21
Max (°F)	28	27	36	48	61	68	72	68	59	48	37	30

As you head north and east, the winter gets even colder and longer. The many lakes that are located in Finland are frozen, and sometimes even the sea freezes (more often the Gulf of Bothnia, but sometimes even the port of Helsinki and the eastern part of the Gulf of Finland), and it is necessary to resort to icebreakers to allow maritime traffic to continue.
During winter, the weather can greatly vary depending on meteorological conditions: when Finland is affected by cold air masses from neighbouring Russia, the temperature drops below -20 °C (-4 °F), while when it is reached by mild winds from the Atlantic Ocean, the temperature is around the freezing point. On colder periods, the temperature can fall as low as -50 °C (-58 °F) in the far north (Lapland). In January 1987, the temperature reached -35 °C (-31 °F) even in the "southern" Helsinki.
The thaw usually occurs in April, at the beginning of the month in the south of Finland, and at the end of the month in Lapland. In autumn, normally the snow begins to fall in October in Lapland, and in November in the south.



In Lapland, the northernmost part of Finland, the climate is cold for most of the year, with a short and intense summer. Here the winter (which in the Nordic countries is the season when the temperature remains below freezing) lasts seven months, from October to April.
The days are very short from November to January, and in December you almost never see the sun, also because of the cloud cover.
May and September are two transitional months, with night temperatures around freezing, and the only months with a mild climate are the three summer months, from June to August, when it can even get hot during the day. In this season the vegetation grows rapidly, to take advantage of the brief period of mild weather, while mosquitoes literally invade the landscape. If daytime temperatures during the summer in Lapland resemble those of the rest of Finland, around 20 °C (68 °F), with peaks around 30 °C (86 °F), nights remain cool or even cold, with minimum temperatures typically below 10 °C (50 °F).
Here we can see the average temperatures of Rovaniemi, the capital of Lapland, located in the southern part of the region (therefore, the least cold).
Average temperatures - Rovaniemi

Rovaniemi	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Rovaniemi	J	F	M	A	M	J	J	A	S	O	N	D
Min (°C)	-14	-14	-9	-4	2	8	11	9	4	-1	-8	-12
Max (°C)	-8	-8	-3	4	11	17	20	16	10	3	-3	-6
Min (°F)	7	7	16	25	36	46	52	48	39	30	18	10
Max (°F)	18	18	27	39	52	63	68	61	50	37	27	21

Across the country, precipitation is not very abundant, being everywhere below 700 millimetres (27.5 inches) per year, even though they are quite frequent, and well distributed throughout the year. In Helsinki it amounts to 650 mm (25.5 in) per year. The rainiest periods is summer, especially the month of August, along with autumn. The relatively least rainy (and snowy) period is the one that runs from February to April. In winter, a light snow often falls, which may not even be counted in the statistics (if it does not reach a minimum amount of equivalent rain).
Here is the average precipitation in Helsinki.
Average precipitation - Helsinki

Helsinki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Year
Helsinki	J	F	M	A	M	J	J	A	S	O	N	D	Y
Prec. (mm)	45	35	35	35	35	50	70	80	70	75	70	55	650
Prec.(in)	1.8	1.4	1.4	1.4	1.4	2	2.8	3.1	2.8	3	2.8	2.2	25.6
Days	10	8	8	7	6	8	9	11	11	11	12	12	113

In the north, the amount (but not the frequency) of precipitation is lower, especially in spring and autumn. In Rovaniemi, 575 mm (22.5 in) of rain or snow per year fall.

The sun in Finland is rarely seen from November to January, while from May to August it shines about half the time (compared to the length of the day) in Helsinki, and a little less in the north.
Sunshine - Helsinki

Helsinki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Helsinki	J	F	M	A	M	J	J	A	S	O	N	D
Sun (hours)	1	3	4	7	9	9	9	7	5	3	1	1

The sea in Finland is cold, and in winter it can freeze across the country. In summer, it reaches 13 °C (55.5 °F) in the far north, and 17 °C (63 °F) in Helsinki and the Gulf of Finland, so it's not advisable to swim in the sea, if not for the brave.
Sea temperature - Helsinki

Helsinki	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Helsinki	J	F	M	A	M	J	J	A	S	O	N	D
Sea (°C)	2	1	1	2	5	9	15	17	14	10	7	3
Sea (°F)	36	34	34	36	41	48	59	63	57	50	45	37

When to go

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Summer, from June to August, is the best season to visit Finland.
Temperatures are generally good for outdoor activities, hovering around 18/22 °C (64/72 °F), at least when it doesn't rain (in the evening or during rainy days it can be cooler), while it rarely gets hot, even though sometimes the mercury reaches 28/32 °C (82/90 °F).
The days are long, especially in June, when you can enjoy the "white nights" in the south, and the midnight sun in the north (see Rovaniemi and Lapland). June is less rainy than July, but it's a bit cooler.
It is worth while to recall that in the Nordic countries, generally from mid-August the summer begins to decline: the days get shorter (even though they remain long), and there is a slight decrease in temperature and an increase in the number of rainy days.
The sun in Finland does not shine very often, even in summer, when there are several rainy or cloudy days, but at least in this season the sky is not always overcast, and the sun often peeks through the clouds.

If you want to visit Finland in its winter look, the best month is March, when the country is still covered with snow, but the temperature is milder, and the days are longer than in the previous winter months.
Easter can be a good period, even when it is advanced, for cross-country skiing in the north, to take advantage of the longer days, while hoping for the sun to come out.

The period from December to February is not ideal, because in addition to very short days (in the Christmas period the sun doesn't even rise in the north), there sometimes can be a bitter cold, even in the south, especially in January and February. In February, at least the days become longer.

What to pack

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In winter: cold weather clothing, synthetic thermal long underwear, fleece, parka, wind jacket, gloves, warm boots.
In summer: clothes for spring and autumn, being ready to add or remove the outer layer, t-shirt, but also long pants, jacket, sweatshirt or sweater; raincoat or umbrella.