Can subtle changes in sky color impact summer high temperatures?

There are spring and summer afternoons when very thin clouds appear overhead and turn the sky a bit white. These types of skies are physically interesting in at least two ways that are worthy of note.

First, the whiteness is a function of the fact that the cloud cover is a thin cirrus cloud. Cirrus clouds are composed of tiny ice crystals that scatter visible light without preference for any of the colors of the visible spectrum (the colors of the rainbow). This particular property is shared by snowflakes as well as by haze droplets. Individual snowflakes look clear but even a small collection of them is white since all of the light that hits the collection of snowflakes is scattered in all directions equally. The same is true of haze droplets which are most common in the summertime in southern Wisconsin.

Second, even the presence of such a thin deck of high clouds (or a thin haze layer) can substantially lower the high temperature of that afternoon. Often, record high temperatures in the summer occur on days with very little cirrus cloud and very little haze in the sky.

Category: Meteorology, Phenomena

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Do volcanic eruptions impact climate and weather?

Plumes of smoke and ash billow from the Calbuco volcano, seen from Puerto Varas, Chile, on Friday. It is unclear if the plume will spread globally, and thus too early to tell what impact the eruption might have on global temperatures.

Plumes of smoke and ash billow from the Calbuco volcano, seen from Puerto Varas, Chile, on Friday. It is unclear if the plume will spread globally, and thus too early to tell what impact the eruption might have on global temperatures.

As an example, the Mount St. Helens eruption in Washington on May 18, 1980, enshrouded Spokane with its ash cloud for days. The result was lower daytime high temperatures by 5 degrees when compared to nearby cities unaffected by the ash. If the plume stays in the troposphere, say below 35,000 feet, it can stay airborne for no more than a week due to precipitation, wind and gravity.

To have a global impact on temperatures, the volcano must eject debris into the stratosphere, above about 40,000 feet. There ashes and gases in the plume can last for a couple of years and be spread over the entire globe.

The eruption can impact global temperature if it spreads globally by the winds and it contains sufficient amounts of sulfur dioxide. The sulfur dioxide can react chemically in the stratosphere to form tiny sulfuric acid droplets. These droplets, particularly if formed in the stratosphere, can reside for a few years and cause a cooling of the global surface temperatures because they reflect solar energy back to space. This was the case with Mount Pinatubo in 1991, which resulted in reducing the global mean temperature of 1992 by about 0.7 degrees. Of course, eventually the plume does disperse.

The Calbuco eruption spewed material into the stratosphere, and satellite observations indicate that the plume contains sulfur dioxide. It is unclear if the plume will spread globally, and thus a little too early to tell what the impact on global temperatures will be from this eruption. But scientists will be monitoring it.

Category: Climate, Phenomena, Weather Dangers

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Why does the wind adversely affect over-the-air television transmission?


MIKE DEVRIES — The Capital Times archives

If you do not have cable, your digital television reception can be affected by storms and high winds.

Antennas intercept the TV signal, which travels as a low-energy electromagnetic wave. The TV waves that are intercepted by the metal antenna cause electrons to move and that generates an electric current — which gets converted to the TV picture. If the TV carrier wave gets disturbed, your TV picture can be affected.

For the best digital TV reception, your antenna should have a clear line of sight to the TV broadcast tower. But many homes do not have that direct line. Any large structures can interfere with the signal. Trees are a common obstacle. On calm days, trees are not much of a problem but on windy days, strong winds cause the trees to sway.

Signals from the TV broadcast tower can overlap chaotically and generate a problem referred to as multi-path interference. This causes the signal to get distorted. If you are located far from the tower so the signal is weak to begin with, multi-path interference can result in a loss of your signal, particularly on UHF channels.

An outdoor antenna, especially if it is on a tall mast, can be moved by the wind which also causes a problem for reception. If you have an indoor antenna, on windy days you can try moving the antenna to a different location, such as an attic. You might make observations to see if there is a correlation between wind direction, wind strength and signal loss.

Category: Phenomena, Severe Weather

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Why are there April showers?

As we head into the second half of April, recent weather has reminded us all of the old saying, “April showers bring May flowers.”

The question, of course, is why does April bring the showers?

First of all, is it actually true that April has an unusually large number of days on which it precipitates? Records for number of rainy days per month are hard to come by, but the showery reputation of April is likely the result of two, interrelated physical processes that characterize the arrival of spring.

First, as the hemisphere warms up at the end of winter and beginning of spring, the large pool of cold air that surrounds the pole in an irregular shape begins to shrink. A predominant way that this shrinking occurs is through the cutting off of local pools of cold air into isolated masses that become orphaned well south of the main reservoir. These features are known as “cut off lows.”

Simultaneously, the sun angle increases dramatically after the equinox (near March 21) and so the heating that can occur during the daytime increases as well. The combination of increased heating of the now snow-free surface and the presence of isolated cold air masses above the surface in the cut off lows, produces the perfect atmospheric conditions for isolated, convective rain showers on days that might begin with sunshine.

Throw in the regular passage of mid-latitude cyclones with their more organized precipitation shields and you have the chance to get a lot of rainy days in the transition month of April.

By May, the cyclone part of the rain threat is often reduced substantially as the jet stream migrates northward with the continued warming of the hemisphere.

— Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

Category: Meteorology, Seasons

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What is permafrost?

Permafrost is ground that is frozen for at least two years. It remains frozen all year and contains plant material that has not yet completely decomposed.

Permafrost is mostly located in polar regions, though it also occurs in some high mountain tops where it is called alpine permafrost. There is no permafrost in Madison, though our ground does freeze in most winters.

When plants grow they pull carbon dioxide out of the atmosphere during photosynthesis. When they die, or when they drop their leaves in fall, the plant material decomposes and returns the carbon dioxide into the atmosphere. It is microbial activity that is active during the decomposition.

In the Arctic, plants grow slowly and they also decompose slowly as plant material gets frozen and is part of the permafrost. Like food in your freezer, the biomaterial does not rot and decompose. So, the carbon in the plant is stored in the permafrost and after many centuries the result is that there is a lot of carbon stored in the permafrost.

Much of the permafrost in Alaska is tens of thousands of years old. Estimates state that the amount of carbon frozen in the permafrost is more than two times the amount of carbon currently in our atmosphere.

The permafrost is starting to thaw and that is a concern for enhancing global warming. As the ground thaws, the microbial activity increases and the plant material currently frozen in the permafrost will decompose, adding carbon into the atmosphere.

It is unlikely that all the carbon in the permafrost will find its way into the atmosphere, but the thawing that is being observed will worsen global warming.

Category: Climate, Seasons

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