The Weather Guys

Why does the moon look red during a lunar eclipse?

In a total lunar eclipse the sun, earth and moon line up and the Earth casts its shadow on the moon.

The moon is always a full moon during a total lunar eclipse and it never goes completely dark. It appears reddish for the same reason that sunsets and sunrises often have a red tint.

The Earth’s shadow has two parts: the umbra and the penumbra. A small amount of the sun’s energy directly shines within the penumbra.

None of the sun’s rays shine directly in the umbra part of the shadow, so it is the darkest part of the shadow. A total lunar eclipse occurs when the moon travels through Earth’s umbra.

The umbra is not pitch black because of the Earth’s atmosphere. Sunlight is scattered, or redirected, in all directions by the Earth’s atmosphere into the penumbra and umbra.

As light passes through the atmosphere, the blue colors are scattered out of the path. Red and orange light passes through the atmosphere and is scattered into the shadow zone. If our planet had no atmosphere, then the moon would be completely dark during a total lunar eclipse.

A total lunar eclipse will occur Monday-Tuesday across the United States and Canada. If it is cloudy or you miss seeing this total lunar eclipse, three more total lunar eclipses will be viewable from our area on Oct. 8 of this year, and next year on April 4 and Sept. 29.

The last total lunar eclipse viewable from our area was in December 2011, so this many over the next year is a bit unusual.

To melt snow, there must be a net gain of energy by the snow. This energy gain can either melt the snow or sublimate the snow, causing it to go from the frozen water phase directly to a gas in the atmosphere.

All objects gain energy from their surroundings, while at the same time losing energy to their surroundings. In this situation, the main energy-exchange mechanism is radiant heat.

An important source of radiant heat is the sun, or solar radiation. Tree trunks are dark and so absorb much of the sun’s energy that falls on them. Trees also lose radiant energy by emitting radiation outward from their tree trunk and into the environment. This radiant energy is much less energetic than solar energy and is often referred to as terrestrial radiation.

Snow is very reflective of solar energy. It is bright white because it reflects about 90% of the sun’s visible light that falls on it. Snow is also an excellent reflector of UV light from the sun.

Snow is also a very good absorber of infrared energy, which is the type of thermal radiation emitted by objects with temperatures observed on Earth.

In an open area, snow does not gain much energy from the sun because of its high reflection, nor does it gain much infrared energy from the atmosphere because the atmosphere emits relatively small amounts of infrared energy.

On the other hand, snow around the base of trees absorbs much of the energy emitted by the tree trunk near the ground. Thus, the energy gains of snow around a tree trunk are greater than in the area away from the tree.

The increased energy gain is often just enough to increase sublimation of the snow around the tree. This leaves holes or depressed layers of snow around the tree trunk.

What else can we say about the past winter?

As our remarkable winter winds down, (though it will remain cold through the end of March, according to most recent forecasts), a few of its additional characteristics are worthy of mention.

First, in Madison we just completed the fifth-longest streak of consecutive days with a snow depth of 1 inch or more — 99 days, from Dec. 9 to March 17. The all-time record streak is 118 days in 1978-79, although we came very close to tying that streak in our snowy winter of 2007-08, which had a 110-day streak.

For some perspective, the average number of days with 1 inch or more of snow cover in a Madison winter (not necessarily consecutive days!) is only 76. So, if you have had a nagging suspicion that we’ve seen snow for an unusually long time this winter, you were right.

This lingering snow is partly a function of this winter’s persistent cold, which we have mentioned a number of times in this column.

Perhaps less high profile, however, is that this winter had 45 days on which at least 0.1 inch of snow fell in Madison — the 13th-highest total of all time. The record for that category is 60 days during our snowy 2007-08 winter. There also was a recent third-place entry with 48 days in 2000-01, when 23 of those days occurred in December.

Finally, we endured 96 consecutive days between high temperatures at or above 50 degrees — from Dec. 4 (when the high was precisely 50) to March 10, when it finally soared to 57. In the past 43 winters, a streak that long has occurred only nine times. It appears that 1971-72 is the record holder with 113 consecutive days from November 18 to March 11.