The Weather

I did not find any precise sources concerning the temperatures, but here’s what I gathered anyway.

Basically it almost never really goes around -30°C (-22°F) in Helsinki, but -10°C (14°F) happens. It’s rarely above 0°C (32°F) during winter.
In the North, especially once you’re in Lapland, it’s obviously colder. So -30°C (-22°F) can happen…​ and maybe even less.

About the summer, it’s like a spring in France. You won’t be "too hot" like you sometimes are here, but you shouldn’t be too cold either. The temperatures usually are between 10°C and 25°C (50°F and 77°F).

If you’re going to Finland in winter, I sure hope you like snow!

In Helsinki there’s way less snow than in the inner lands, and I’m not even talking about the North. This is, obviously, mainly due to the Gulf Stream, especially as Helsinki is a harbor. The capital city is still usually covered in snow between December and the end of March. You can sometimes even see snow falls in May or something.

Finland being used to the snow, there shouldn’t be too much problems with public transports because of that.
There will still be black ice though, so stay careful when on foot or in your own car.

The…​ what?
I’m kidding, there’s a lot of that in summer!

Only 1/4th of the Finnish territory is above the arctic circle, and thus knows total night for 6 months, and non-stop daylight for the rest of the year.
But in Helsinki, you still have very short days in winter, and very long ones in summer.

At the end of December, the days last from…​ 11am to 2pm? About so? And even when the sun is "up", it’s actually really low, like a sunrise. And when you think the day is really gonna start, the sunrise fades into sunset. And night. And it’s not even 3pm. The real risks are the lazy mornings: you think it’s very early because it’s night ? Nope, it’s almost noon!

During the summer, it’s the same thing but upside down. At the end of June, night lasts from…​ midnight to 3am? It’s nice when you’re leaving a bar and there’s still some daylight, but window blinds are mandatory if you want to get some sleep!

I personally really like those weird day times, but it can annoy some people.
Also, the very short days in winter tend to encourage depression, so take care of yourself and be careful about that!

If you’re coming to Finland in winter and you have skiing clothes, you’d better take them with!

For the top you should have a good ski jacket, a big sweater, one (or more) t-shirt, a hat, good gloves (maybe even under gloves) and a warm scarf.

For the legs, you can wear leggings, regular pants, and maybe some k-way style pants. Yes, everything on top of the other. For the days with a lot of snow, ski pants are the best.

Concerning your feet, you can have regular shoes with big wool socks, as long as the shoes are not too slippery. Boots can be good when there’s too much snow. And don’t even think about wearing heels.