Seasonal cycle of hiking conditions in Northern Finland

annual cycle of weather conditions. In spring, grass may already be getting green in the south while skiing conditions may still be perfect for a good while in Northern Finland. The times of the seasons may also vary a great deal within Northern Finland. For example, as rivers are flooding and the snow is melting in Oulanka National Park, winter still lingers on for several weeks in Käsivarsi Wilderness Reserve. This is often comes as a surprise for hikers unfamiliar with the seasonal cycle in Northern Finland, whereas more experienced visitors are challenged by unseasonal weather fluctuations caused by climate change, including lack of snow, storm winds or drought.

Visitors often contact our customer service to ask about the conditions at destinations. They are particularly interested in transitions between winter and summer as well as the autumn colours and mass occurrence of mosquitoes. This article describes the seasonal cycle in Northern Finland from the viewpoint of hiking conditions. The different sections also include tips from the customer service for monitoring the conditions from home.

Talvi taittuu kesään Pohjois-Suomessa touko-kesäkuun aikana. Tarkempi ajankohta vaihtelee vuosittain muutaman viikon sisällä sääolosuhteiden May or June mark the transition point between winter and summer in Northern Finland. The exact time varies within a few weeks from year to year, depending on the weather conditions. In early summer the terrain and hiking trails may still be wet in many places, as the last bits of snow melt in shady areas and valleys. This time of the year, it is advisable to choose trails that do not lead close by streams or across mires. This is also a good idea because wet soil is more vulnerable to erosion.

As ice and snow melt in early summer, water levels in streams are higher than usual. A flood in a river can develop very quickly and reach high levels. The volumes of snow and ice in the winter gone by and temperatures in late spring affect the level and development of floods. If a heat wave causes large amounts of snow and ice to melt rapidly all at once, floods usually reach record heights but the water levels also drop quickly. If the spring weather is cool, however, this may delay the floods. The time of the spring flood varies from mid-May to mid-June. Heavy rainfall can also cause stream waters to rise up to flood levels.

As climate change increases the average temperatures, summers may be hot and dry. For example, in summer 2024 all small streams and ponds in Northern Lapland dried up. Finding drinking water was suddenly a challenge. More than 30 wildfires occurred in this region.

Hikers in Northern Finland are often also interested in the timing of the mass occurrence of insects. This phenomenon is strongly linked to the weather, as mosquitoes hatch out of their pupal cases when waters are sufficiently warm. Cold weather slows down and reduces the occurrence of insects, however. The insect season usually starts around Midsummer and continues until early or mid-August. A cold June may sometimes delay its occurrence. In addition to warmth, mosquitoes depend on moisture. More mosquitoes are always found in mires and humid areas, while windy weather on the fells keeps them away.

The autumn colours are another phenomenon of the snow-free period that interests hikers. Autumn colours in the northernmost parts of Finland appear in the first half of September, moving southwards from there. The most important factor affecting the timing of the autumn colours is the latitude, in other words day length. Frosty nights enhance the autumn colours and slow down the falling of leaves.

The autumn turns into winter between October and November in Northern Finland. While it may snow on northern fells even in July, permanent snow usually comes in October or November. The snow goes through many transformation during the winter. From early winter till March it is soft and will not carry a hiker. Progress through the thick layer of snow is slow.

In spring the rising daytime temperatures, occasional rain when the temperature reaches above zero and night-time frosts cause the snow cover to freeze hard. This hard snow allows a skier to cover long distances effortlessly. The snow usually freezes in late March or early April, and these conditions may last from mid-April up till mid-May.

The snow season includes a period during which the conditions are too challenging for both skiing and walking, typically in November and May in Northern Finland.

The terrain in Northern Finland is characterised by large height differences. This means that the snow conditions can vary significantly even within a small area. On the open fells the wind hardens the snow earlier in winter, but in the shelter of the forest it takes longer for the snow to freeze in spring. The fells also channel rain clouds, which means that one side of a fell area typically has a higher rainfall than the other. This is why the snow cover may be thicker on one side.

Heavy snowfalls, winds and temperature fluctuations cause different layers of snow to form, some of which may be very likely to start sliding down steep slopes. For example, fresh snow that has fallen on top of a frozen layer is liable to start moving as a slab if exposed to any pressure. The avalanche risk is especially high in ravines and on steep slopes of fells and hills.

The impacts of climate change can be seen as the shortening of the snowy period in winter. Previously, winter was guaranteed to arrive in late October. Today a permanent snow cover usually forms in late November and sometimes only in December. Above-zero temperatures are also increasingly seen in mid-winter, bringing rain rather than snow. Milder climatic conditions have increased precipitation in general, which means that the snow cover may be thicker than usual.

In mild weather, winds increasingly reach storm force. Storms in late winter have become more frequent, especially in fell areas. Unpredictable winter weather requires better anticipation and preparation for winter excursions of hikers. Sometimes better anticipation may mean staying at home if the conditions are too challenging. Alternatively, you can head for a safer destination.

Permanent ice cover begins to develop in Northern Finland when the temperature remains below zero 24 hours a day. Smaller ponds freeze first, while large open stretches on lakes and great rivers get their ice cover last. If the early winter brings cold weather and low precipitation, thick and sturdy black ice will form, whereas a heavy layer of snow falling on thin ice acts as insulation and slows down ice development. If the winter does not bring long periods of freezing temperatures, the ice cover will remain relatively thin and melt faster in the spring. In rapids and narrow straits in lakes, ice remains dangerously thin throughout the winter.

The impacts of climate change can also be seen in the formation of ice cover. It develops later and melts earlier in winter. A hollow ice cover is increasingly common in early winter. Hollow ice is formed when a water body freezes during a flood. As the water level drops during the winter, the ice is no longer supported by the water. This means that the unsupported hollow ice is much more likely to break under pressure.

Black ice is today seen less frequently, as periods of very cold sub-zero temperatures are getting rarer. Slush ice, or frozen slush, has replaced strong black ice. Slush ice is formed when snow presses down on the ice cover, allowing water to rise on top of the ice. When the temperature drops, the snow slush freezes into slush ice. It can only carry one half of the weight that blue ice can take. It also melts faster in spring.

The changing behaviour of ice makes moving around on waterways considerably less safe for hikers, as you can no longer identify dangerous areas in the old way. Weak ice has also become a challenge for maintaining hiking destinations, as most winter maintenance at hiking destinations in Northern Finland is carried out using snowmobiles and riding along waterways. In Oulanka National Park, for instance, some of the winter maintenance has not been possible at all, as the Oulankajoki River has flowed all winter without a sufficiently strong ice cover.