The rains in Finland have changed

in Finland summer rains and winter rains are different from each other. In winter, we get steady gray frontal rains, and in summer, deafening rain can surprise you.

Researcher at the Department of Meteorology Mika Rantanen tells what will happen to Finnish rains in the future.

Is Finland rainier than before?

Yes it is. Accurate statistical information has been available since the 1960s. During this period, the amount of precipitation has increased, with the largest increase occurring in winter.

Winter rains have increased by about 20 millimeters over the entire measured period, and in Lapland spring rains have also increased by ten millimeters. The change is statistically significant throughout the country.

However, no statistically significant changes are visible in other seasons.

How big a difference is it from before?

Generally, during the winter months, there is a total of about 200 millimeters of rain, so adding 20 millimeters to that means an increase of approximately ten percent.

However, it must be remembered that the winter climate in Finland is naturally very variable. The years are not brothers, so to speak, and the amount of precipitation varies a lot from year to year.

What can be the reason for the current increase in rainfall?

One could imagine that it is at least partly due to general global warming. Winters have warmed since the 60s, and warm air contains more moisture. The amount of water in the atmosphere increases by about seven percent per degree of warming.

Another reason may be related to changes in storm tracks. Low pressures arriving from the Atlantic cause intermittent storms and winter frontal rains.

The increase in precipitation does not therefore mean that it is unequivocally due to human-caused climate change.

On the other hand, global warming causes a single storm to bring more rain. However, almost no research has been done on the reasons for the increase in winter rainfall in Finland.

How is the warming climate predicted to affect the rains in Finland in the future?

The trend estimated by climate models seems to be that the rains will continue to increase, and specifically in winter.

The most recent study from a few years ago predicts that the amount of precipitation would continue to increase by about ten percent by the middle of the century. Similarly, the winter temperature is predicted to rise by about three and a half degrees in the same period.

This can change Finland’s winter climate dramatically. Due to the higher temperature, the rains come more often as water, so winters without snow are becoming more common.

One could think that by the end of the century, Central Finland will have a climate similar to Minsk in Belarus at the moment. Of course, the comparison only takes rainfall and temperatures into account. The amount of light will not increase.

In Lapland, the situation is a little different, because it is cold enough there, and the temperature may not rise above zero. There, the rains come as snow, and the amount of snow may increase compared to before.

Will it rain more obliquely in the future?

The direction of the rain depends on the direction of the wind. Very slanting rain is usually caused by stormy weather.

Inclined rains are unlikely to increase dramatically at least, because that would require an increase in days with very strong winds, and such changes are not known, at least so far.

What happens to the summer weather?

According to estimates, the amount of rain in summer does not increase as strongly as in winter.

Forecasting and measuring summer rains involves more uncertainty compared to winter rains, because in summer the rains are convective showers. They are created when the sun heats up the earth and the warm air mass rises upwards.

This warm air package cools as it goes up, and the moisture it contains condenses into clouds. The deaf are very local, and they do not necessarily hit the weather observation stations, so there is no information about them.

In the future, the number of rainy days in the summer months will probably not increase, but the rain showers are predicted to be stronger, which will increase the total amount of rain.

Basically, stronger rain showers will be punished more than weak rains, i.e. the hourly amount of rain will increase.

What are the effects of increasing heavy rains?

An ordinary citizen hardly notices if it rains a few millimeters more than before during one rainstorm.

In some situations, even the last few millimeters can be significant, for example if it is about the durability of the city’s stormwater system. In the future, violent heavy rains may lead to stormwater floods in cities.

Published in Science in Nature 7/2024