As autumn officially kicks off, look for fall weather to come in like a lamb but out like a lion

As we say farewell to summer, we usher in the colourful, crisper days of a new season. Autumn officially arrives in the Northern Hemisphere Monday at 10:30 pm EDT. But what actually happens astronomically and weather-wise?

At the autumnal equinox, the length of time that the sun is above the horizon is roughly equal to the length of time that the sun is below the horizon – hence the term equinox, which is latin for equal night.

The vernal or spring equinox in March and the autumnal equinox in September are the two times of the year when the length of day and night are nearly the same around the world.

Astronomically speaking, both equinoxes mark the time when the Earth is aligned in its orbit with the Sun such that it appears stationed above our planet’s equator.

Earth’s axis is tilted with respect to the plane of its orbit around the sun, however on these days of the year, it’s tilted neither away nor towards our home star.

For about a month of either side of the equinox, we see the quickest reduction in the length of daylight hours as we head towards the shortest day of the year on the December solstice.

“At this time of year, for instance, the folks up in the Yukon and Northwest Territories are losing an average of 6 to 7 minutes of daylight each day, while down in southern Ontario it is closer to 3 minutes of lost daylight per day,” explained Brett Anderson, senior meteorologist with accuweather.com in an interview with Yahoo Canada News.

"This means less of the sun’s energy is absorbed by the ground, resulting in the downward temperature trend toward winter."

It turns out that it really is all about that ever-shallower angle that the sun shines across Canada this time of the year that plays a big part in the roller-coaster-like weather we usually start experiencing now. Those shorter periods of daylight are elemental to the change from summer to winter.

“Since autumn is a season of transition, we do expect more frequent and rapid transitions back and forth between late summer weather and eventually early winter weather,” added Doug Gillham, meteorologist with The Weather Network.

“While a pattern of above-seasonal or below-seasonal temperatures can set up for a week or two, we are much less likely get locked into a persistently warm or cold pattern that dominates for the majority of the season.”

And this yo-yo weather pattern has been well underway since the start of September right across the country. Parts of southern Alberta and northern Ontario have been hit by snowfalls over a week before the fall season officially gets underway. Meanwhile, the first week of autumn looks like will it will bring summer-like temperatures from the Rockies to the Maritimes, including the places that have already seen snow, Gillham says.

And if we look towards the rest of fall, the forecast for most of the Canada is for below-average temperatures to set in.

“From the Prairies to the Maritimes, we do expect that the end of autumn and early winter will bring below-seasonal temperatures, and an early start to winter in these areas,” he added.

“But the good news is that we still expect a few periods of above-seasonal temperatures between now and the end of October in these regions.”

Anderson’s forecast models seem to agree and point to a possible more temperate weather pattern as we head towards Thanksgiving.

“Based on the data that I am looking at, I don’t see any intrusions of Arctic air into southern Canada through at least early October as the polar jet stream will remain farther north than usual, which traps the cold air up across northern Canada, while milder Pacific air covers the rest of Canada and the northern U.S,” he added.

“This is looking like a classic ‘Indian summer’ pattern across central and eastern Canada for the end of September and into early October.”