Log in to WRAL.com with one click using your favorite social network:
OR
Log in using your WRAL.com account:



Wrong email/password combination.

Forgot password?

Register with WRAL.com using your favorite social network:
OR
Register for a WRAL.com account using our web form.

Login Options

11:56 p.m. • 2-10-12

Weather Forecast for Raleigh

  • Sat: Mostly Cloudy.
    • Hi: 52° F
  • Sun: Clear.
    • Hi: 43° F
  • Mon: Mostly Cloudy.
    • Hi: 50° F

Other Locations

> 7 Day Forecast

Doppler Image

Marketplace Links

Social Links

Main Menu

WeatherCenter meteorologists

WRAL WeatherCenter Blog

WRAL's WeatherCenter meteorologists take you behind the weather headlines, answer questions and look to the sky to add insight and explanation for conditions in the Carolinas. You also can find us on Facebook and become a fan!

RSS Feed
GOES-13 Full Disk

An Eclipse Every Night?

There is, and it is something you can see in cloud animations in our weathercasts this time of year, but it is not an eclipse of the moon or sun in the usual sense. This one involves the geostationary weather satellites that are in orbits about 22,300 miles above the equator. At this altitude, it works out that the speed required to maintain orbit also happens to match the rotational speed of the earth below, so that the satellite remains more or less stationary above the same place on the ground. That is the reason we can show you the 6-, 12- or 24-hour satellite animations (sometimes with radar imagery added to show where rain is being detected) that give such a good illustration of how clouds and weather systems are moving along, developing or dissipating.

However, twice each year there is a period of several weeks either side of the spring (Vernal) and fall (Autumnal) equinoxes in which the alignment of the sun, earth and satellite causes the satellite to pass through the earth's shadow for a few hours at night. During this time, the satellite's solar panels are unable to generate power and imaging operations are shut down, leading to anywhere from about a 1-3 hour gap in imagery. If you watch our satellite and radar movies during this time period, you may see occasions when the clouds appear to freeze for a bit, then suddenly jump ahead. This is even more noticeable when there is radar data atop the satellite imagery, since the radar data is able to continue smoothly animating while the clouds appear to stop.

In addition to the loss of power due to passing through the earth's shadow, another problem that arises near the equinoxes is that there are periods when the sun is so close to the earth as seen from the satellite that the sensors cannot be operated or they would either produce erroneous data or be damaged. These are called "solar intrusions" and to avoid these the satellite is instructed to refrain from imaging certain "Keep Out Zones" during the parts of the year that the eclipse issue is a factor. All taken together, the eclipse and Keep Out Zone operations result in some gaps in satellite data from about late February until late April, with a similar period  from August into October. The folks who operate the satellites make schedules of these outage periods available online, and if you're interested you can see one for this Spring at the attached link.

A new series of geostationary satellites, beginning with GOES-13 which is already in a "storage" orbit, will greatly reduce the amount of missed imagery, due to new and improved battery technology that will allow the instruments to continue operations despite the loss of solar power for a few hours. I think all of us in the weather biz are looking forward to that!

Read More Posts from this Blog
e-mail print friendly

2 Comments


WRAL.com welcomes your comments on this story. All comments are moderated prior to publication based on our posting guidelines. Please review them prior to posting and if your message is not approved.

This story is closed for comments. Comments on WRAL.com news stories are accepted and moderated between the hours of 8 a.m. and 8 p.m. Monday through Friday.


page 1
sort order: oldest first | newest first

I have been wondering why our television signals have been freezing so often lately.

Thanks for the information!

Very interesting! Thank you!

page 1
sort order: oldest first | newest first

Featured Blogposts

  • scotty and mr wuf

    American Idol and Garner native Scotty McCreery performs at N.C. State's Hoops 4 Hope. The circus is in town. And Olympic-level table tennis stops in Cary. Here's what's happening this weekend.

  • Hoops 4 Hope on Feb. 15, 2009

    The Hillsborough Street Community Service Corporation is sponsoring Play 4Kay events on Hillsborough Street starting Feb. 8 to support Hoops 4 Hope and the Kay Yow Cancer Fund.

  • Heart

    Showering your loved ones with goodies is always fun to do on Valentine's Day, but not if it leaves you drowning in debt! With a little planning and creativity, you can show your loved ones you care and stay within your budget.

Other Recent Blogposts