Much of Southern and Central Finland will experience 'tropical nights' toward the end of the week, with the southeastern regions most likely to feel their effects.
The Finnish Meteorological Institute (FMI) defines a tropical night as one during which the temperature does not drop below 20 degrees Celsius between 9 pm and 9 am.
"When the weather pattern is quite stable, these will occur every day at least for this week," said Jani Parviainen, on-duty meteorologist at the FMI.
Last week, isolated tropical nights were already seen in places like Lahti in Päijät-Häme and Niinisalo in Satakunta.
Temperatures at sites along the Baltic Sea coast and near lakes have also occasionally remained above 20 degrees throughout the night.
The likelihood of tropical nights increases as waters warm up.
"Water bodies act as heat reservoirs, slowing down drops in temperature at night. Water vapour also slows the cooling of hot air masses," Parviainen said.
Inland surface water temperatures are currently between 23–25 degrees Celsius, slightly lower in Lapland around 20 degrees.
Tropical nights are not unusual in Finland. They occur almost every summer.
11 days of heat in Finland
July has been quite warm in Finland. Today marked the 11th consecutive day with temperatures exceeding 30°C somewhere in the country — though no major records have been broken yet.
The hot streak is expected to continue, and nights in the east may still be tropical next week.
"There is some uncertainty at the end of the week. The warm air mass will gradually begin to move eastward. The longest period of intense heat will remain in the eastern parts of the country. It may cool down somewhat in Western Finland on Sunday," Parviainen said.
