Astronomical autumn begins in Latvia

Early Wednesday morning, at 3:05 am, the autumn equinox took place, marking the beginning of astronomical autumn in the Northern Hemisphere, according to data from timeanddate.com. At the same moment, the Southern Hemisphere entered spring.

During the equinox, both hemispheres receive roughly equal amounts of sunlight and warmth, and day and night are nearly equal in length worldwide. In Latvia, however, exact balance has not yet arrived — on September 23, daytime is still about ten minutes longer than nighttime. This autumn, nights will first become longer than days only on September 26.

This discrepancy is explained by the refraction of light in the atmosphere: sunrise and sunset are defined by the moment the upper edge of the solar disk appears or disappears at the horizon, rather than its center.

Shrinking daylight

Daylight hours shrink fastest of the entire year during the second half of September, decreasing by about four minutes and 46 seconds per day. This trend will continue until December 21, when the winter solstice arrives and astronomical winter begins.

Latvia also tracks a "solar" seasonal calendar, under which solar autumn runs from August 6 to November 5, with the astronomical autumn's start falling roughly in the middle of that period. The darkest three months of the year, termed solar winter, will run from November 6 to February 4.

Meteorological autumn, determined by average air temperature staying below +15 degrees Celsius for at least five consecutive days, already began in Latvia on September 10 this year.

Near-term forecast

According to the European Centre for Medium-Range Weather Forecasts, Latvia will see significantly above-normal precipitation this week, followed by drier conditions next week, with October overall potentially drier than usual. Air temperatures in October are forecast to be above normal.

Data from the EU's Copernicus programme indicates that, due to El Niño, Europe overall can expect a warm and wet autumn and winter this year, with elevated risks of flooding and storms. In Latvia, winter could bring unusually heavy rain and snow, while the coldest months may turn out to be February and March.