What is the status of the ozone hole?

Posted on October 6, 2014 by WeatherGuys Editor

Ozone occurs about 18 miles above the 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 ozone hole occurs high over the continent of Antarctica. It is not actually a hole, but rather the appearance of very low values of ozone in the stratosphere. Typically, the Antarctic ozone hole has its largest area in early September and lowest values in late September to early October.

The Antarctic ozone hole varies in size each year, from nearly zero in 1980 to an area larger than North America in 2000. The amount of ozone in the atmosphere is now routinely measured from instruments flying on satellites.

The size of this year’s ozone hole reached a maximum size in September of about 7 million square miles. It is about the same size as the ozone hole in 2011 and 2012.

The ozone hole forms through the destruction of ozone over Antarctica. The winter atmosphere above that continent 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 strong winds prevent warm air from the equator from reaching these polar latitudes. These extremely cold temperatures inside the strong winds help to form unique types of clouds called polar stratospheric clouds, or PSCs.

PSCs begin to form during June, which is winter time 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 which rapidly deplete ozone. The depletion is so rapid that it has been termed a “hole in the ozone layer.”

Thanks to the Montreal Protocol’s phased global ban on chlorofluorocarbon (CFC) use and the natural decay of these chlorine compounds, the stratosphere will be CFC-free near the end of the 21st century. In their absence, the ozone layer will repair itself naturally.

The good news is that the size of this ozone hole is showing signs of shrinking. This recovery is a prime example of the power of employing science research in the shaping of public policy.

We would be wise to learn from this example to inform our collective approach to climate change.

Category: Climate, Meteorology

Comments Off on What is the status of the ozone hole?

What is the meteorological origin of these beautiful fall days?

Posted on September 29, 2014 by WeatherGuys Editor

The beautiful, crystal clear early fall days that we have recently enjoyed are characteristic of September in southern Wisconsin.

Such days are often quite cool to start, with morning temperatures in the 40s, but often warm by late afternoon when temperatures can soar to the mid- to high 70s.

Such conditions are associated with regions of high pressure either migrating past us or developing over us.

These high pressure regions are characterized by gentle, persistent sinking of the air from the middle troposphere to the surface – most times are rates of only about 150 feet per hour.

The sinking forces the air to warm by compression which reduces the relative humidity, accounting for the deep blue skies.

The clear skies, extending into the overnight hours in such episodes, allow for substantial cooling of the surface by radiation so that the temperature can be quite low by the time the sunrise occurs again, more than 12 hours later.

(Don’t forget that by late September, after the equinox, the night is longer than the day).

The development of regions of high pressure is now well understood to be associated with the passage of upper-tropospheric “ridges” in the flow.

These ridges are regions where the upper-level flow of air is constrained to turn clockwise over horizontal distances of hundreds of miles.

On the eastern edge of such clockwise turning flow regions, the laws of physics compel the air to gently sink and a high-pressure region is either newly created or sustained in that location.

The unsettled weather that accompanies a surface low-pressure region compels us to refer to it as a “storm.”

Though it is not common to refer to high-pressure regions as “storms,” they are, just like their low pressure counterparts, spawned by the passage of upper-tropospheric waves.

In fact, the longest-lived storm in the solar system (as far as we know) is a region of high pressure – Jupiter’s Great Red Spot.

Category: Meteorology, Seasons

Comments Off on What is the meteorological origin of these beautiful fall days?

Do September temperatures foretell the nature of the winter?

Posted on September 23, 2014 by WeatherGuys Editor

After the recent abnormally cold period, which has left us 3.2 degrees colder than normal thus far in September, a lot of people have been wondering if September temperatures can be a harbinger of what is to come in the winter.

Everyone recalls last winter as a persistently cold season during which we experienced a four-month period (December 1 – March 31) with an average temperature that was 7.44 degrees below normal. Interestingly, last September was 2.7 degrees above normal and last October was 1.0 degrees above normal.

Records from the preceding four winters (2009-10 through 2012-13) provide an interesting, though inconclusive, answer to this question. Of those four prior winters (December through February only), two were colder than normal — 2009-10 by 0.26 degrees and 2010-11 by 1.50 degrees. In 2010-11, September and October averaged 1.67 degrees warmer than normal, while in 2009-10 those same months were 1.14 degrees colder than normal.

The winters of 2011-12 and 2012-13 were 7.01 and 1.32 degrees warmer than normal. Their corresponding Septembers/Octobers were 0.74 degrees warmer and 0.35 degrees colder than normal.

Thus, based upon a very small but recent sample, one might tentatively conclude that abnormal cold or warmth in September and October has little to do with the nature of the coming winter. The most recent three-month outlook issued by the Climate Prediction Center is equally inconclusive for the Midwest, suggesting equal chances for colder- or warmer-than-normal conditions for our months of September, October and November.

Category: Climate, Meteorology, Seasons

Comments Off on Do September temperatures foretell the nature of the winter?

What is an equinox?

Posted on September 15, 2014 by WeatherGuys Editor

The equinoxes (from “equi,” meaning “equal,” and “nox,” or “night”) occur when the sun’s rays strike the equator at noon at an angle of 90 degrees.

In the Northern Hemisphere, the vernal or spring equinox occurs around March 20, and the autumnal or fall equinox occurs on September 22 or 23.

During the equinoxes, the sun is above the horizon for all locations on Earth for 12 hours. This year the fall equinox occurs on Sept. 22 at 9:29 p.m.

The tilt of the Earth’s axis is responsible for the seasonal variation in the amount of solar energy distributed at the top of the atmosphere and plays a key role in determining the seasonal variation in surface temperature.

The Earth’s axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because the Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of the Earth’s axis to the sun is always changing as the Earth orbits around the sun.

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

On the equinoxes, the axis is not pointed at or away from the sun. This results in all areas experiencing a little more than 12 hours of daylight.

The September equinox is considered by many to be a sign of the beginning of fall in the Northern Hemisphere, and a marker of spring in the Southern Hemisphere.

Category: Meteorology, Seasons

Comments Off on What is an equinox?

How does the Farmer’s Almanac make its forecasts?

Posted on September 8, 2014 by WeatherGuys Editor

The Farmer’s Almanac makes seasonal forecasts and recently came out with its winter forecast.

The Farmer’s Almanac does not share how it makes its forecasts so it cannot be judged scientifically. There is no proven skill of its forecast accuracy.

It also makes a weather forecast for specific time periods in a given season.

Such detailed forecasts are not trustworthy scientifically.

Seasonal weather forecasting is a modern-day science challenge. The National Weather Service’s Climate Prediction Center also makes seasonal forecasts.

They explain the underlying principles of their forecast and provide validation of their forecasts publicly.

These modern day seasonal forecasts rely on the known relationships between climate and some key forcing mechanisms, such as El Niño.

An El Niño is a periodic warming of the equatorial Pacific Ocean between South America and the Date Line.

This warming is a natural variation of the ocean and is used to predict departures from average conditions rather than to make specific weather forecasts.

For example, a year with a strong El Niño leads to less snowfall than average in Wisconsin.

These seasonal forecasts also take into account the climatic impacts of other global oscillations.

These relations are uncovered by research conducted by atmospheric scientists, and while we understand these relationships, we cannot yet predict the occurrence of these key forcing mechanisms, such as the development of an El Niño.

Currently, there are no strong global patterns developing that allow for a confident prediction of our winter weather conditions.

There are equal chances our wintertime weather conditions will result in temperatures above, below or at average.

Category: Climate, Seasons

Comments Off on How does the Farmer’s Almanac make its forecasts?