Satellite imagery provides a great deal of information on past, present, and future weather. Each cloud swirl, streak, and puff can be associated with a particular weather system or phenomenon. And when images are viewed by a trained voyager with a good understanding of weather dynamics, routing and course decisions can be made with confidence.
The most important weather element that satellite imagery provides to the voyager is cloud data. Analysis of cloud altitude, shape, and movement goes a long way toward painting what the associated air mass is doing.
Clouds are categorized by both their height and shape, with heights falling in three general categories: low – 6,000 ft., 42° F cloud top temperature; middle – 15,000 ft., 10° F cloud top temperature; and high – 22,000 ft., -30° F cloud top temperature. (Cloud top temps can vary; the preceding figures represent average readings.) Temperature numbers are important because they reflect vertical cloud development, which indicates the strength of surface weather conditions.
Individual cloud shape is either fat (stratus) or billowy (cumulus). Cloud shapes are described using the terms shield, band, line, street, finger, or element. Though clouds might, at first glance, appear to have no easily recognized pattern, there are, in fact, distinct patterns associated with the formation, growth, and dissipation of lows, highs, troughs, and ridges that a mariner can learn to recognize.
Clear areas in satellite imagery are normally associated with high pressure regions, where air masses are of low humidity and therefore relatively cloud free, whereas areas of low pressure have varying degrees of cloud development depending on the amount of moisture present and the degree of convective and advective motion within the atmosphere.
To understand cloud patterns a basic knowledge of jet stream behavior is necessary, since it is the undulating waves of jet stream winds that bring together (converge) and separate (diverge) air masses at the surface and result in weather systems and their associated cloud patterns.
Jet stream winds are plotted on weather charts known as 500 millibar (mb) charts — the name refers to the pressure at which these winds are located, generally at around 18,000 feet above sea level. Jet stream winds, as they undulate from north to south in response to the movement of large masses of cold and warm air, form troughs and ridges. Ridges are found in areas where the jet stream follows a clockwise motion and troughs where there is counterclockwise movement.
When satellite images are examined in conjunction with 500-mb charts, areas of likely low formation can first be identified on the 500-mb chart and then matched with cloud patterns on satellite imagery to reveal the type and extent of low development. The opposite approach can also be taken if a distinctive low pressure cloud formation is seen, but no corresponding trough is seen on the 500-mb chart. This usually means a rapid change is taking place within the atmosphere and the original forecast will need to be modified.
Comparing satellite images with surface and aloft weather charts allows a voyager to compare real time satellite imagery with analysis and predictions done by meteorologists. By comparing charts and imagery, the development and movement of lows and highs, their associated cold and warm fronts, and other weather features can be closely monitored.
For example, rapidly developing and intense low pressure systems — often referred to as “bombs” due to their tight and symmetrical isobar lines — develop in 12 to 24 hours. Weather charts, which are prepared and broadcast every six hours, might not provide much warning, but satellite imagery would show the clouds associated with these systems thickening and lowering, and forming into the tight “comma cloud” associated with low pressure systems.
A perceptive voyager with access to satellite imagery would detect cloud formations associated with an intense low and have some warning of its approach. Not to be forgotten, though, are the other cues of a rapidly developing storm, such as a falling barometer, increasing wind with changes in direction, and an increase in seas and swells.
US weather satellites are operated by NOAA and provide both visible and infrared views of the atmosphere as they sweep around the earth. Visible images resemble black and white photographs and show the amount of light being reflected back from the earth. Large thunderstorms are the best reflectors of the sun’s rays, returning 92% of the sunlight hitting them back to space. Thunderstorm clouds appear brighter white and are easily seen on visible imagery. The known reflectivity of various surfaces can be used when viewing visible imagery to distinguish between high, middle, and low clouds.
Infrared imagery, on the other hand, shows energy being radiated from the earth’s surface and the tops of clouds. This radiated energy is represented as temperature and is displayed using distinct shades of gray. High clouds, which are very cold, appear white in infrared imagery, and land, which is warm, appears dark.
Used together, visual and infrared imagery provides a voyager with cloud location, thickness, height, and movement. At night only infrared images are available, but during daylight hours both visible and infrared imagery provide updates on the Earth’s weather every few hours.
When seas are calm and skies are clear, rays from the sun will often reflect off the water, causing a bright spot in satellite imagery called sun glint. Since sun glint only appears when seas are low, satellite imagery indirectly provides an indicator of sea state.