Many tools needed in forecasting

With the amount of technology available today in the field of meteorology, people often wonder how we can “miss” anything with the all-seeing “eyes” of weather satellites. What you “see” isn’t always what you get with these weather snapshots, though, and the limitations of these tools sometimes pose a challenge for meteorologists.

I liken the job of a meteorologist to that of a doctor. In order to make a proper forecast or prognosis, it is very important first to make an accurate diagnosis. Satellite and radar imagery, like MRIs and X-rays, can provide valuable information. But they do not paint a complete picture. Direct observations with human eyes are always preferred to get the most accurate information. In the case of weather data, however, direct observations are not always possible everywhere. That is where satellite imagery comes into play.

Satellite technology has been a wonderful advance in the science of meteorology. Two of the products often used by meteorologists are the visible satellite image and the infrared image. These are the pictures and loops that you regularly see on a television weathercast. Though we can gain a mountain of information about current weather and infer future trends from this view of the clouds, there are limitations to what can be seen.

“Visible” satellite imagery is a powerful tool. Like a camera, the satellite takes a visible picture of the Earth from above. With a visible image, it is possible to see the wispy form of cirrus clouds contrasting with the bubbly appearance of cumulus or storm clouds.

This “camera” also can capture the image of smoke from wildfires or ash plumes from volcanoes. Mountaintops in the Northwest can be picked out from the white specks of highly reflective snow. In some areas in the wintertime, what often looks like widespread cloudiness across a large area is actually a blanket of snow. We can differentiate the snow from the clouds by looking at a time lapse or “loop.” The clouds move; the blanket of snow does not.

As valuable as visible satellite imagery is to a meteorologist, it has an important limitation. It is only available during daylight hours because pictures can’t be taken without sunlight.

When we lose daylight, we have to turn to infrared satellite imagery. In this technology, the satellite measures the heat radiated from clouds to paint a picture. High clouds are colder than low clouds, and a computer can enhance the image to shade or “color” the temperature differences. It is more common to see infrared satellite images on television because they are available day and night and can be used to show a 24-hour time lapse.

If you pay much attention to this type of satellite image, you likely will notice one of its weaknesses. When the ground is extremely cold, say on a cold clear winter night, the satellite will “think” that it’s seeing a cold cloud. Though weather reporting stations are reporting clear skies across the region, the satellite loop will show a swath of clouds. Likewise, dense fog on a mild moist morning may not show up at all because the fog is too warm to register as a cloud. The satellite loop might not be showing anything, but surface observations will report fog with reduced visibilities.

Surface weather observations of sky conditions, taken by human eyes or by an instrument called a ceilometer, are crucial for providing what’s called “ground truth” to satellite data. As a meteorologist, we have to always be on our toes to make sure that what we “see” on the satellite image is what you’re really getting