The Weather Guys

Ozone occurs about 18 miles above the Earth’s surface.

Ozone is both caused by and provides protection from damaging ultraviolet energy emitted by the sun. The development of an atmospheric “ozone layer” allowed life to move out of the oceans and onto land.

The amount of ozone in the atmosphere is routinely measured from satellites. Typically, the Antarctic ozone hole has its largest area in early September and lowest values in late September to early October. This year it was measured to be one of the largest and deepest in recent years, covering just over 9 million square miles.

The ozone hole occurs high over the continent of Antarctica. It is the appearance of very low values of ozone in the stratosphere. The winter atmosphere above Antarctica is very cold. The cold temperatures result in a temperature gradient between the South Pole and the Southern Hemisphere middle latitudes, which results in strong westerly stratospheric winds that encircle the South Pole region.

These extremely cold temperatures inside the strong winds help to form unique types of clouds called Polar Stratospheric Clouds, or PSCs. They begin to form during June, which is wintertime at the South Pole. Chemicals on the surface of the particles composing PSCs result in chemical reactions that remove the chlorine from the atmospheric compounds. When the sun returns to the Antarctic stratosphere in the spring (our fall), sunlight splits the chlorine molecules into highly reactive chlorine atoms that rapidly deplete ozone. The depletion is so rapid that it has been termed a “hole in the ozone layer.”

The Montreal Protocol bans emissions of ozone-depleting chemicals, and that is having a good impact. Observations show the area of the ozone hole is decreasing.

Astronomical fall began on Sept. 22.

Climatologists consider fall as the months of September, October and November. The fall season is also associated with pumpkin patches, fresh apples and beautiful foliage.

Leaves are green in the summer because they contain chlorophyll, which reflects green light more than other colors. Other colors are absorbed by chlorophyll for photosynthesis.

During autumn, the green chlorophyll disappears from the leaves as the leaves stop their food-making process, and yellow and orange colors appear. These colors have been in the leaves all along but we can’t see them in the summer because of the chlorophyll.

The color orange comes from carotene, and the yellows from xanthophyll. The bright red and purple colors come from anthocyanin pigments, which are made from leftover glucose trapped within the leaves of some trees, such as maples.

Brilliant fall colors with bright red and purple colors require conditions in which leaves can make a lot of anthocyanin pigments. The brilliance of a fall color season is a function of the weather.

Sunny days and cool nights are the key weather ingredient for brilliant fall colors. The best weather conditions are bright sunny days and cool, but not frosty, nights.

A drab autumn has lots of cloudy days and warm nights. Leaves begin to turn before we have any frosts. An early frost speeds up the fall of the leaves and brings a quick end to the fall color. A few hard frosts can cause the leaves to wither and fall from the tree without changing color.

Many southern Wisconsin regions have not experienced frost. Of course, too much winds, or heavy precipitation would bring the leaves down.

The pandemic has reduced the travel plans of many. But this weekend might be a good time to go for a ride to see the colors.