In 1900, Galveston, Texas, was struck by a devastating hurricane. There was little if any warning from weather forecasters, and observation reports from Cuba were ignored. As a result, some 8,000 people died.

Today, satellites are positioned in different orbits and perform many functions to provide a variety of weather data. A hurricane such as the one that hit Galveston (the event was popularized by the book "Isaac's Storm," by Erik Larson) would be detected and tracked, and appropriate warnings would be sounded.

With satellite weather data and computer modeling now in widespread use – and satellite -based weather products increasingly available – boaters can understand current weather conditions and avoid dangerous situations like never before – if they know how to interpret satellite images.

SAME WEATHER, DIFFERENT IMAGES

For the United States and surrounding waters, the weather community heavily relies on two satellites known as Geostationary Earth Observing Satellites (GOES), which remain fixed over specific locations (hence the name geostationary) by maintaining an orbit that exactly matches the Earth's rotation. GOES-East covers the eastern half of the country, while GOES-West covers the western half.

Satellite observations range from different types of imagery to more specialized products, such as sea surface temperatures. The three major types are visible, infrared and water vapor (see photos). Each type of imagery has advantages and disadvantages; looking at all three together can provide a more complete picture of the atmosphere.

NOAAAbove, a satellite image from GOES-West on December 6, 2007 shows a storm system hitting the Oregon and California coast.

NOAAAbove, a colored infrared satellite image taken at similar time as the previous image. Notice the Pacific coast storm is easier to distinguish in infrared.

NOAAAbove, a water vapor image of the Pacific coast storm. The image shows the water vapor content in the middle and upper parts of the troposphere.

NOAAAbove, a Quikscat satellite product shows surface winds. The winds are blowing in a counterclockwise flow around the low pressure center in the Gulf of Alaska. The strongest winds (up to 45 knots) are found just off the Alaskan coastline.

NOAAAbove, this image shows sea surface temperatures in degrees Celsius. The gray areas are clouds.

Visible imagery, like taking a photo with a camera, is the measurement of the sunlight reflected by the clouds and Earth's surface. Water and land reflect a lot less sunlight than do clouds (which is why clouds appear bright white when sun strikes them from an angle), and therefore clouds in visible imagery appear with differing brightness based on their thickness and structure.

Meteorologists are trained to identify subtle features found in satellite imagery; however, amateurs can get useful information as well. For instance, thin clouds, which will rarely produce precipitation, can often be semitransparent in visible imagery. More troublesome large thunderstorm clouds can be identified from their puffy structure (shadows often form when the sun angle is low in the late afternoon).

Visible satellite images have the highest resolution of the three types, so more fine details can be seen. However, there are a few major drawbacks. First, visible imagery, of course, is confined to daylight hours. Another disadvantage, during the winter months, is that it is difficult to distinguish between snow cover and clouds. Both reflect similar amounts of sunlight, and so they show the same brightness. A trick to tell them apart is to look at multiple satellite images to identify movement. The clouds will move, however slowly. Snow cover will not.

INFRARED IMAGES

Infrared imagery measures the amount of thermal heat, or the temperature, radiating from the upper layer of an object. In this case, infrared satellite imagery measures the upper layer of clouds, or, where clouds are not present, the land and water. The coldest areas show up as the brightest, while the warmest areas show up darkest. Clouds, with the exception of polar regions in winter, are colder than the oceans and land surfaces. The image generated by this distinction in temperatures is a lesser quality than what is provided by visible satellite images. But the big advantage of infrared imagery is that it measures heat, not sunlight, which makes it useful at night. Since temperature is measured, images can be color-coded so the data can be more easily interpreted.

Water vapor imagery is a special kind of infrared measurement of the temperature of clouds and water vapor in a layer of the atmosphere six to 10 kilometers above the Earth's surface. At this altitude, steering currents such as the jet stream can easily be identified. Dry air in this upper region of the atmosphere shows up dark, while moist air and clouds show up bright. Meteorologists often use water vapor imagery loops to study where storm systems are heading – information that any mariner would like to know.

OTHER SATELLITE PRODUCTS

Two satellite observations, both derived from specialized instrumentation on weather satellites, can be of particular importance to mariners: One measures oceanic surface winds, and the other measures the temperature of the sea surface. The SeaWinds scatterometer is a microwave-based radar specifically designed to measure ocean near-surface wind speed and direction.

Sea surface temperatures can be determined from satellite infrared imagery. Recent techniques have been developed to reduce cloud interference.

Weather satellite images are especially valuable over the oceans where other observations are scarce. Mariners can use these images to make well-informed navigation decisions.

Satellite products are available via the Internet from several government Web sites, including those run by NASA and the National Oceanic and Atmospheric Administration (NOAA). They range from user-friendly images that can be used to learn, to extremely specialized data that is aimed at professionals.

Spend some time looking at images, comparing them against published forecasts and reports and they can quickly become a useful tool.