The Weather Blog

Busy August for Stargazers

It's not often that we get a chance to talk about events going on outside the Earth's atmosphere, but this month presents stargazers with numerous interesting events.  The Perseid meteor shower is still going on, there are a couple of planets that are well positioned, and at the end of the month we'll have a lunar eclipse!

The Perseids peaked over the weekend, although there should still be a couple of days with fine viewing.  The meteor shower results when the Earth passes through a trail of dust in space left behind by Comet Swift-Tuttle.  Our planet reaches the dust trail in August of every year, which is the reason for the annual frequency of the Perseids.  When small dust particles enter the highest parts of the atmosphere at high speed, they burn up and the result we see is a shooting star.

The Perseids are so named because the meteor trails seem to emanate from a point within the constellation of Perseus, although meteors can be seen in any part of the sky.  During the peak of the shower this past weekend, over 60 meteors per hour could be seen.

The Perseids may not be the most prolific of the regular meteor showers, but they are one of the easiest to observe.  All it takes is a comfortable vantage point, preferably with a dark sky away from city lights.  There's no need to focus on any particular area of the sky, and there's no need for a telescope or binoculars - you'll want to be able to watch as wide an area of the sky as possible.  The Perseids are also popular since you can observe them in relative comfort on a warm summer evening, as opposed to the Leonids in November or the Geminids in December.  This year's Perseids have been even better because the moon is new right now.  Last year, the light from the full moon washed out much of the Perseid shower.

Another unique event coming up in a few weeks is the Aurigid meteor shower, which is a very brief show that only occurs at irregular intervals.  The peak is expected on September 1, but the shower only lasts for a couple of hours.  Viewers on the West Coast of the U.S. and in Hawaii will have the best show.

The Aurigids may be masked somewhat by an almost full moon after a lunar eclipse on tap for August 28.  Unlike solar eclipses, eclipses of the moon can be seen anywhere on the Earth's night side.  The best viewing for this month's total lunar eclipse will be on the West Coast and in Hawaii, as unfortunate timing for the rest of the U.S. means that the eclipse will peak in the early morning hours leading up to moonset and sunrise.  The next total lunar eclipse will be February 21, 2008.

Finally, some of our neighboring planets in the solar system are positioned well for viewing at night this month.  Jupiter is in the southern sky in the constellation of Scorpius, where it and the bright red star Antares make a nice pair.   Mars, the Red Planet, rises after midnight in the constellation of Taurus, where it and the nearby orange star Aldebaran make a good show of color.

The weather has been active here in Earth's atmosphere during the past few days.  When we return on Thursday we'll recap the recent heat wave in the U.S., a rare tornado in New York City, and provide a synopsis of the activity in the tropics.

March 3 Total Lunar Eclipse

Residents of eastern North America who have clear skies this weekend will be treated to a celestial spectacle.  On the evening of Saturday, March 3, a total lunar eclipse will occur.  Although the best viewing will be in Europe and Africa, portions of eastern North America will get in on the tail end of the show.

A lunar eclipse occurs when the sun, Earth, and moon are lined up just right that the moon passes through Earth's shadow.  Because the moon's orbit is inclined slightly, these alignments are not common.  If the alignment is particularly favorable, the moon will pass completely into the deepest part of the Earth's shadow, the umbra, and the result will be a total lunar eclipse.  Sometimes the moon will just clip the umbra, giving us a partial lunar eclipse.

Even though the moon will be shadowed by the Earth, it does not go totally dark.  Some sunlight is able to reach the moon indirectly, by way of the Earth's atmosphere.  Sunlight passing through our atmosphere is bent, refracted, or scattered.  The red and orange components of sunlight are often favorably bent so that they can illuminate the moon during a total eclipse, giving it a reddish appearance.  The effect is variable though, so the reddish tint of the moon will be different in each eclipse.

The moon will be hidden in the umbra for a little over an hour during this weekend's eclipse.  Europe and Africa will be able to observe all of this period of totality.  However here in North America, the moon will not have fully risen yet.  Those of us in the eastern U.S. and Canada may get to see a red moon rise during totality but shortly begin to move out of the umbra.  By the time the moon rises in the Central time zone, totality may well be over or very nearly over.  West of there the total phase of the eclipse will be over before the moon even rises.

People in western North America can take heart though.  Even though they will miss most of this weekend's eclipse, they will be treated to totality during the next lunar eclipse, which will be on August 27.

So what will viewing conditions be like here in North America at moonrise?  There is a powerful storm currently centered over Illinois that is forecast to lift toward the Great Lakes region while slowly weakening during the next couple of days.  By Saturday afternoon, it should still be positioned over the Great Lakes.  This means that unfortunately, the view of the total eclipse for the Midwest will likely be obscured by clouds and precipitation.  Portions of the East Coast may be cloud free around moonrise.

Speaking of the powerful storm that's currently clobbering the U.S., in Monday's entry we'll take a look at the damage this storm has wrought, including the ongoing severe weather outbreak in the Southeast and the blizzard in the upper Midwest.

Large Solar Flare

A large solar flare erupted from the sun this past Tuesday evening.  The flare caused disruptions to radio communications here on Earth.  The flare poses little danger to astronauts aboard the Space Shuttle and the International Space Station, as the Earth's magnetic field shields them from the radiation.

The flare was X-class, the most powerful category of solar flares.  It was generated by a cluster of sunspots on the sun's surface that has been designated as sunspot cluster 930, which can be seen in the image at left (from the SOHO [Solar and Heliospheric Observatory] satellite; click to open a full-size version in a new window) as the bright white region near the right side of the image.  Cluster 930 has a history of producing large flares - earlier in December it produced two more X-class flares, and when it was visible from Earth in November it spawned frequent small flares.

Solar flares are often accompanied by coronal mass ejections, or CMEs for short.  These are bursts of high energy particles from the sun's corona that are ejected out into space, where they become part of the solar wind.  Last week's solar flares did have CMEs, but they were not directed at the Earth.  This most recent flare is accompanied by a CME which is pointed toward Earth.  The CME is not dangerous to us down here at the surface, but it will lead to a geomagnetic storm, which will in turn give us a nice auroral display.  Geomagnetic storming is underway as I type, and it could give observers in Canada and the northern U.S. an opportunity to view the northern lights on the evening of December 14 - if they have cloud-free skies.

For more information on solar flares, CMEs, geomagnetic storms, and the aurora, I recommend visiting NOAA's Space Environment Center.  The SEC is the agency that monitors the sun and the Earth for these phenomena, and they have a wealth of real time data, images, alerts, and warnings about current space weather conditions.  In addition, near-real time images of the sun (as well as archives and movies) are available from SOHO. 

Martian Dust Devils

Earth isn't the only place that sees dust devils and landspouts.  The rovers on Mars have observed them there as well.  You can read about them here: http://science.nasa.gov/headlines/y2005/14jul_dustdevils.htm?list99065  be sure to scroll down and see a video clip of a dust devil passing in front of one of the rovers.   A similar scene could be seen on many a summer day in Arizona.  Maybe I can get a research project funded to take a couple mobile doppler radar trucks up to Mars and do some extra-terrestrial tornado chasing!

Earth at farthest point from the sun this week

Many people still have the belief that the earth is closer to the sun in northern hemisphere summer than in winter.  But, in fact, this week of July 4th marks the time each year when the earth is farthest in its orbit from the sun.   Earth is actually 5 million miles closer to the sun in January than it is in July when it is furthest away.  Seasons of course are caused by the tilt of the earth, and not the distance from the earth to sun, but were the earth closest to the sun in July and furthest from the sun in January, summers in the northern hemisphere would be hotter and winters colder.     So when you are sweltering on a hot July day, just be thankful that we are not receiving quite as much heating from the sun as during the rest of the year.

Planetary Conjunction

While not a meteorological happening, an interesting site is nonetheless developing in the evening sky just above the west-northwest horizon.   The planets Venus, Mercury and Saturn are all coming into extremely close alignment, with a very close grouping of all three planets on tap for this weekend.  Mercury and Venus will appear even closer together early next week and will appear almost as one unified bright object on the 28th.