What does El Nino mean for southern Wisconsin’s winter this year?

El Nino is an atmosphere/ocean phenomenon in which the waters of the eastern equatorial Pacific Ocean are warmer than normal for an extended period of time.

This unusual warmth forces changes in the distribution of deep convective thunderstorms over the tropical ocean which, in turn, affect the position and strength of the jet stream. Since the jet stream acts a conduit of winter storms and can regulate intrusions of cold arctic air, changes in jet stream characteristics can have a profound influence on our winter weather.

Generally, El Nino is associated with warmer than normal winters in the western Great Lakes states so we can reasonably expect that this coming winter will be milder than the last two. Though there is a tendency for a bit more precipitation in El Nino winters here, since it is usually warmer than normal, there is usually less than the normal amount of snow.

Currently, the sea surface temperature data in the equatorial Pacific suggest that this year’s El Nino may be the strongest on record. If it continues to grow in intensity as predicted, our winter could become memorably mild. One of us recalls throwing rocks into Lake Mendota in late February 1998, near the end of the last really strong El Nino winter, and imagining that might never happen again. Perhaps it will.

Category: Meteorology, Phenomena, Seasons

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When is the autumnal equinox?

On Wednesday morning, we will officially enter fall as the autumnal equinox occurs at 3:20 a.m.

On Wednesday, in common with every location on Earth, we will enjoy exactly 12 hours of daylight and 12 hours of night.

Of course, 12 hours of daylight in Madison (latitude 43N) is substantially different from 12 hours of daylight at the North Pole (latitude 90N) where the sun will barely be visible above the horizon for the 12 hours of “daylight.” On the very next day, the sun will not appear above the horizon at the Pole and will not come back for six months. As the days march on, the same fate will gradually overtake other latitudes in the Northern Hemisphere until by Dec. 21, the sun will not rise even at 66.5N.

The primary means by which air is warmed is by its contact with the surface of the Earth that, when the sun shines, can absorb radiation and heat up.

A leading consequence of the shortening days, especially at high latitudes, is that air masses can begin to get really cold again. Even as we enjoy one the warmest Septembers in quite some time, the areal extent of the cold wintertime air is growing at high latitudes.

As it encroaches southward from the Pole, the polar jet stream, always on the warm edge of the coldest arctic air, begins to take up residence at lower latitudes bringing with it the powerful storms of late autumn and winter.

So, enjoy these beautiful early fall days we are having in Madison because the cosmic deck is stacked against us.

Category: Meteorology, Seasons

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When does the first freeze occur in fall?

The cooldown that we enjoyed over the weekend, after the prolonged warm and humid spell that began September, got us thinking about the inevitable first morning with a temperature below 32 degrees.

We are less than two weeks away from the autumnal equinox, the last day until late March on which the day is at least as long as the night.

In the last 73 years, since the official temperature for Madison has been recorded at the airport, the average date of our first 32-degree night is Oct. 1. However, the earliest such night occurred on Sept. 11, 1955 – after a summer in which a record 40 days with a high temperature of 90 were recorded. This early freeze was nearly equaled by 1962 and 1963, when it occurred on Sept. 12.

More recently, the only fairly early date for the first freeze was Sept. 16, 2007. The latest such night also occurred in the 1950s – Oct. 17, 1959 – with a more recent competitor landing on Oct. 14, 2005.

Interestingly, for a period of 23 years (1940-1963) we have simultaneous records from the official recording site that used to be located in Downtown Madison. The average first freeze at that location is much later, around Oct. 20. The earliest and latest dates at that site are also quite different.

So, though on average the first freeze occurs about 10 days after the equinox at the airport, your own home may record a substantially different first freeze date.

Category: Climate, Seasons

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Is the smoke from western forest fires affecting our weather?

This year there have been many fires on the west coast of the U.S. and Canada.

Most of these fires are in remote regions and were started by lightning strikes. This smoke has drifted over our region. It will not have much of an effect on our temperature or precipitation. However, official weather reports include observations on sky conditions and visibility.

The smoke can cause the sky to appear hazy, even if the smoke is high above the ground. When the smoke is thick it can cause brilliant red sunsets and sunrises. When light beams interact with particles suspended in air, the light can be scattered or absorbed.

The amount of light that is being scattered is a function of the number of particles and the size of the particle relative to the wavelength of the light falling on the particle. Small particles, like those smoke is composed of, scatter blue light. So, as the sun sets and its rays pass through the smoke plume, all the blue lights are scattered out of the path between the setting sun and your eyes, leaving just the red and orange colors.

This results in the sun having a bright red color when it is low on the horizon. Recently, the smoke above us has been thick enough that the red sun disappeared from view before it set below the horizon.

If the winds are right, the smoke can be transported down to the ground. This can cause a reduction in air quality. The small particles that make up the smoke can cause respiratory problems, particularly for children, the elderly and people with asthma.

Category: Meteorology, Phenomena

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How was our summer weather?

The average global temperature of July set highs — it was the warmest month on record since record keeping began in 1880.

The observed ocean surface temperature was the highest for any month in the 1880-2015 record period. The average temperatures over land were also above the average for the 20th century, ranking as the sixth-warmest July since 1880.

The July globally averaged land surface temperature was 1.73 degrees above normal. And so far, 2015 (the months of January through July) is at the all-time record warm temperature for the globe.

Regionally, however, southern Wisconsin’s mean temperature for July 2015 was about 2 degrees below the average for the 1971-2000 record period.

Our August temperature and precipitation is also below the average for the past 30 years.

Preliminary analysis looks like our summer mean temperature for this year will be below normal. We had five days with the temperature at or above 90. The outlook for September is for continued cool temperatures in our region.

The summertime precipitation for southern Wisconsin was below normal. There were four days in Madison when the precipitation was more than 1 inch. Though Madison is not in drought conditions, there is a short-term drought across southwestern Wisconsin and along the shore of Lake Superior in northern Wisconsin.

Category: Climate, Seasons

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