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

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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

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How does the wind make waves on water?

Posted on September 2, 2014 by WeatherGuys Editor

Waves form as the wind’s energy is transferred to the surface of water.

A weak gusty wind can make ripples appear on smooth water. These ripples will dissipate quickly once the wind stops.

The size of a wind-generated wave depends on the following:

• The wind speed. The stronger the winds, the larger the force and, thus, the bigger the wave. The wind must also be constant, not just a wind gust here or there.

• The duration of the winds. The longer the wind blows over the open water, the larger the waves.

• The fetch. This is the distance of open water over which the wind blows. The longer the fetch, the larger the waves.

• The depth of the water also plays a role, as it is difficult to generate large waves in shallow water.

Waves in a deep lake or sea can be taller and last much longer.

Waves in the open ocean have been measured to be larger than 100 feet in height.

Waves in the Great Lakes have reached heights of 35 feet. The storm Sandy generated waves on Lake Michigan of 21.7 feet.

High sustained winds from one direction can push water up at one end of the lake, resulting in a storm surge.

Weather can also cause a seiche on the Great Lakes. In French, the word “seiche” means “to sway back and forth.”

An atmospheric disturbance causes waves to slosh back and forth between shores of the lake basin resulting in huge fluctuations of water levels in a short period of time.

Category: Phenomena

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What is a funnel cloud?

Posted on August 18, 2014 by WeatherGuys Editor

A funnel cloud gets its name from its shape — it is a funnel-shaped protuberance from the base of a thunderstorm.

It is composed of water droplets and is often associated with a supercell storm. The funnel cloud often has rotation, and when it does, it’s a harbinger of possible severe weather.

A supercell thunderstorm is a large storm, sometimes 20 miles or more across, that almost always produces dangerous weather.

Supercell storms produce one or more of the following weather conditions: strong wind gusts, large hail, dangerous lightning and tornadoes. The severity of these storms is primarily a result of the structure of the environment in which the storms form. Severe weather requires warm, moist air near the ground and a change in wind speed and direction, or wind shear, with height above the surface.

Funnel clouds are not dangerous unless they reach the ground. We are interested in reported funnel clouds since it is possible that a funnel cloud can become a tornado. If the rotating funnel cloud stretches down and touches the ground, it is called a tornado.

Many tornadoes are at one time funnel clouds, but not all funnel clouds become tornadoes. When a trained weather spotter observes a funnel cloud, he reports it to the National Weather Service, who may then warn the public.

Less threatening is a cold-air funnel. These are generally observed in partly cloudy skies after the passage of a cold front. While they look threatening, they don’t pose a hazard.

Category: Meteorology, Phenomena, Severe Weather

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Has this summer been mild?

Posted on August 11, 2014 by WeatherGuys Editor

This summer has been relatively mild, compared to some we have recently experienced.

In 2012 we recorded 39 days with a high temperature of 90 degrees or greater. This summer we have only had one such day so far — July 22, when the temperature was 93.

Tuesday is the 81st anniversary of the 127-degree high in 1933 at Greenland Ranch in Death Valley, California, that set the national record for the month of August. Such heat is of a completely different character than anything that has ever happened in and around Madison.

The all-time high for Madison was set on July 14, 1936, when the temperature soared to 107 degrees.

It is important to note that 107 here might be more uncomfortable than 127 in the desert, given that such a hot day in Madison is nearly always accompanied by very high humidity, while the desert is always very dry.

In fact, to get as hot as 127 degrees, even at Death Valley, special circumstances have to be met. Most importantly, there has to be a strong flow of air off of the surrounding higher terrain. This sinking air is compressed as it moves downward to higher pressures, and the compression leads to substantial warming. At the same time, this compressional warming lowers the relative humidity of the air, rendering it very dry as well.

Many of us experienced the extreme heat and humidity of July 13, 1995, when the temperature in Madison was 101 with a dew point of over 80 degrees – truly miserable. Still, the question of whether super hot and super dry is more uncomfortable than really hot and super humid is perhaps dependent on personal tolerance.

I think we can all be glad that we have not (yet) been put to that test this summer.

Category: Seasons

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