An Eclipse Every Night?
Posted March 23, 2009
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!