The Weather Guys

October ended up to be about normal in terms of temperature despite the chilly last couple of weeks. In fact, 14 of the last 17 days of the month were at or below normal. A natural question arises in the face of this cool spell — namely, does this portend a cold start to the winter?

The official November-January forecast from the Climate Prediction Center of the National Centers for Environmental Prediction (NCEP) gives us a slight probability of above-average temperatures for the next three months and even chances for normal amounts of precipitation over that interval. This forecast may arise from the fact that sea-surface temperatures in the central tropical Pacific Ocean are nearly normal heading into the cold season.

Our own harbinger of the early winter focuses on the presence or absence of snow cover in northwestern Canada. The idea is that if there is snow cover there by mid-late October, the chances of producing cold air masses there increase as the daylight hours decrease in late fall. Cold air produced in northwest Canada is often involved in our early season cold air outbreaks.

We are testing this idea in current research. So far this year, there has been unusual warmth (and consequently little snow) in northwest Canada. Thus, our prediction is in line with that of NCEP — the early part of the winter will not be memorably cold.

Our amount of daylight hours depends on our latitude and how Earth orbits the sun. Earth’s axis of rotation is tilted from its orbital plane and always points in the same direction — toward the North Star. As a result, the orientation of Earth’s axis to the sun is always changing throughout the year as we revolve around the sun. Sometimes the axis points toward the sun and other times away from the sun.

As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude.

This tilting leads to a variation of solar energy that changes with latitude. This causes a seasonal variation in the intensity of sunlight reaching the surface and the number of hours of daylight.

The variation in intensity results because the angle at which the sun’s rays hit the Earth changes with time of year.

If you shine a flashlight at the ceiling, the region that is illuminated shrinks or grows depending on whether you point it directly at the ceiling or at an angle. Similarly, the sun’s energy spreads out over differing geographic areas when it reaches Earth’s surface. It is more concentrated during our summer months when the sun is higher in the sky.

This spinning of Earth like a top explains our daily cycle of night and day. The tilt of the Earth’s axis also defines the length of daylight. Daylight hours are shortest in each hemisphere’s winter. Between summer and winter solstice, the number of daylight hours decreases, and the rate of decrease is larger the higher the latitude. The fewer sunlight hours the colder the nights.

How fast Earth spins determines the number of hours in a given day. As Earth orbits the sun it spins about its axis approximately once every 24 hours. But this is slowly changing with time. About 650 million years ago there were only about 22 hours in a day.