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Old 11-20-2003
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One Source for Weather?


Given only one option for assessing weather, the author wouldn't hesitate to select satellite imagery.
Last fall I gave a seminar on new weather products to a group of sailors attending the Annapolis Boat Show. The day was cold and clear with winds from the northwest. A cold front had passed over the day before and so the humidity was low. It was really what I consider a perfect Boat Show day. And, as often occurs at the end of these talks, someone proferred the following question: "If you had to choose just one source of weather information, what would it be?"

Without any hesitation I responded "satellite imagery." Satellite imagery is my succinct answer not because I put less value on pilot charts, text forecasts, or voice forecasts, but because satellite images are more universally available and they provide insight into surface and upper-air features, as well as oceanographic phenomena.

Satellite pictures, both visible (VIS) and infrared (IR) imagery are broadcast to us directly from NOAA's polar orbiting satellites. And there is no charge for accessing this data stream, just as there is no charge for receiving weatherfax or Navtex, or listening to SSB weather broadcasts.


Figure 1: this VIS satellite image (taken 1645 UTC on December 18) shows a broad frontal system passing over southern Florida.
Satellite images can be captured directly using an onboard laptop and helix antenna (look for these distinctive antennas on the stern of almost any ocean racer), or downloaded from NOAA's Satellite Active Archive website (www.saa.noaa.gov). Keep in mind that satellite weather software is needed to decompress these files, so consult the SAA WebPages for suggested programs). Annotated satellite images are also posted on the Marine Prediction Centers (MPC) homepage, www.mpc.ncep.noaa.gov.

This all may seem a little too involved, but another reason why I'd choose satellite images if I had only one source of weather information is because the information derived is both real time and data intensive. I can accurately analyze and forecast weather within hundreds of miles of me and conjure a three-dimensional mental image of large and small-scale weather events.

Just for starters consider that satellite images show low and high-pressure systems, their associated fronts and squall lines, tropical waves, depressions, storms and hurricanes, sea-surface temperatures, and ocean currents. Of course they'll also give you fog, sun glint (areas of calm seas), thunderstorms, wind direction and strength (the latter is derived from observing cloud orientation and grouping).


Figure 2: this infrared satellite image (captured on December 17) has been enhanced using color overlays to illustrate water temperatures in the Atlantic and Pacific.
A directly captured satellite image can be enhanced using color overlays (pallets) to bring out certain features. Add GPS input and you can place a cross hair at your boat's precise position. Satellite images can be geo-referenced enabling their use as plotting and navigational chart. And some programs offer drawing tools that allow the insertion of waypoints and position data, while zooming and profiling features allow specific aspects of weather to be analyzed in detail.

So, show me an example you say? Well, take a look at the VIS and IR images here. I downloaded these images from the Satellite Active Archive (SAA) earlier this week. In Figure 1 above, notice t the cloud-free skies behind the frontal system. This indicates clear, cold air, not unlike the weather on the day I gave my talk. And note the strong cold front moving over the southern tip of Florida. This front is moving to the southeast and is no longer a weather concern for sailors to the north.

In the infrared image (IR) above in Figure 2, the colors indicate temperature with green representing 65 degrees F and blue 55 degrees F. The temperature palette used in this overlay can be adjusted for any temperature range and for intervals as close as a tenth of a degree. To confirm the accuracy of these water temperatures I could consult the real time buoy data made available by NOAA on the Internet (www.ndbc.noaa.gov) or via phone: 288.688.1948 (Dial-a-Buoy).

"By combining satellite data with buoy data and on-scene measurements you can locate features like Gulf Stream, eddies and meanders."

On the day that I gave my talk in Annapolis, I downloaded some similar images and then cross-referenced the temperatures given by checking online with information from buoy 44014 (64 n.m. miles off Virginia Beach, VA) and the Chesapeake Bay Mid Bay buoy located at 38 28.4N and 76 22.8W. Both showed real-time temperatures within three to four degrees of the satellite data. By combining satellite data with buoy data and your own on-scene measurements you can easily determine offsets for calibrating satellite imagery temperatures and then precisely locate oceanographic features such as the Gulf Stream, eddies, meanders, and walls.

You can also use the temperature readings to track cloud top temperatures, which indicate if a weather system is developing and intensifying or weakening and dissipating. As a system grows its vertical height increases, which leads to colder cloud tops. But as a system weakens its cloud base erodes and the cloud height lessens, leading to warmer cloud top temperatures.

Shadows on VIS imagery, seen especially in early morning and late afternoon images, reveal cloud structure and density. Dark and well-defined shadows indicate dense and cohesive clouds, which are most often associated with developing systems, especially thunderstorms.


Figure 3: note how the clouds shown in this image have aligned in rows indicating wind strengths in excess of 30 knots.

When winds blow greater than 30 knots clouds line up in rows. These wind rows (partially visible in Figure 3) show both wind direction and size of the wind field. If winds are greater than 30 knots and traverse a mountainous region, clouds begin to "roll" and align themselves perpendicular to the wind direction. Essentially the clouds are tumbling along and are unable to stay aligned with the wind field.

So on that day in October, using the images I downloaded along with confirmation from the real-time buoy data, I elected not to take my Boston Whaler out on the Chesapeake Bay for the afternoon. Though I dearly wanted a day on the water, I realized that the strong, cold winds were going to produce a hard chop and more spray than my boat can handle. But I also could see the strong high-pressure moving east which, if I had been thinking of heading offshore to Bermuda, would have confirmed that now was an ideal time to depart.

Satellite imagery is a powerful tool for weather analysis and forecasting. It is especially powerful for mariners, who are often out of range for weatherfax and real-time data. Though I would not by choice limit myself to one source of data—I would prefer to have imagery, charts, and text—my first choice would definitely be imagery. So, start watching the Weather Channel, they use satellite imagery for all their forecasts and tap into the Marine Prediction Center and Satellite Active Archives, and you'll be able to keep a better eye on the weather.

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