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Using weather satellite imagery

 

For just over 40 years weather satellites have provided valuable information to weather forecasters and pilots. However, the format of the imagery provided by satellite often required dedicated equipment so that the pictures could be printed out as they became available.

The accessibility to computers being what it is today makes accessing satellite imagery as easy as most other weather products available to the pilot. Still, many pilots are unaware of the existence of satellite imagery and/or the value of using such information in the pre-flight planning process.

This month, we will review the basics of weather satellite imagery and provide some insights into their use.

Although there are a myriad of weather satellites, both U.S. and foreign, in orbit of the earth, the U.S. generally has two satellites operating in geosynchronous orbit over the equator and five sun-synchronous "polar" orbiting satellites.

The geosynchronous satellites are referred to as GOES (geostationary operational environmental satellite) and are designed for short-range analysis of weather situations. The sun-synchronous satellites are referred to as POES (polar orbital environmental satellites) and provide all sorts of meteorological data including support for weather analysis and forecasting.

Together, these two satellite systems provide a complete weather monitoring system for not only North America, but also the entire globe. Both satellite systems are operated by the National Oceanic and Atmospheric Administration’s (NOAA) National Environmental Satellite, Data, and Information Service (NESDIS).

To understand how these satellites collect their data it is helpful to understand a few terms. In the case of GOES, geostationary means that the satellite is in a synchronous orbit that essentially renders it motionless relative to the surface of the earth. That means that these satellites move at the same speed of the earth thereby allowing them to hover over one position on the surface. As such, the GOES system is able to constantly watch the same area for changes that signal significant weather events.

POES satellites, on the other hand, are in sun-synchronous orbits which provide consistent lighting as they scan the Earth below. These satellites pass the equator and each latitude at virtually the same time each day.

The orbital plane of a sun-synchronous orbit also rotates approximately one degree each day, eastward, to keep pace with the Earth's orbit around the sun. Roughly 14 polar orbits are completed daily by the POES system.

The POES are an important compliment to the GOES imagery since the later images becomes distorted above roughly 50 degrees latitude poleward.

Two types of imagery are available from satellite; visible and infrared. The two types combined yield a good deal of information about the types of clouds, extent of coverage, and thickness. Temperatures at the tops of clouds can also be determined and, as a result, the approximate height of cloud tops can be determined.

With visible imagery, the main source of data is from reflected sun light. Specifically, different objects reflect a different amount of light so that the amount of brightness in an image can be easily associated with land, water, clouds, etc.

Brightest regions on visible imagery will be thick clouds or snow on the ground (an important distinction since with snow on the ground, bright areas may actually be devoid of clouds). Darkest regions on the visible image will be oceans and forests due to their lack of reflectivity. Since visible images are essentially photographs within the existing light, they are only available during daylight hours.

Infrared imagery is created by sensing temperature differences emitted by clouds or the surface. With this type of imagery, brightest regions will be the coldest surfaces or clouds.

Clearly, in the wintertime in North America the spread between surface temps and temps of clouds aloft are not nearly as large so the infrared image may have less resolution than desirable.

Darkest regions on the infrared image are the warmest clouds or surfaces. Unlike visible images, infrared images are available for both day and night.

Color enhancement is available for infrared images and helps to differentiate areas with smaller temperature differences. Since there are many organizations who color enhance the basic imagery, it is important to have a colour legend appropriate to the particular image you are using. Also, do not assume that each organization uses the same colour-coding scheme.

While the visible and infrared images together form a powerful tool for analyzing large scale systems and cloud coverage, I find one additional image very useful. The GOES system also provides a water vapour image for 10,000 feet and above.

This imagery is also infrared but limits the image to infrared emitted by water vapour only. Bright regions on this type of image indicate moist air, while dark regions are dry. Signs of moist air aloft are generally indicative of weather potential since moisture is required for the formation of clouds in the first place.

Water vapour images are also available with colour enhancement, so the same caution about colour-coding applies to these as well.

There are several important features that will be evident when reviewing visible satellite images. First, cloud development circulation around low pressure systems will be quite obvious and give a quick birds-eye view of the extent of cloud coverage.

Both the infrared and water vapour images will show thickness of that cloud coverage and extent of moisture. Analysis of cloud coverage using these charts are also useful when planning long flights and since water content is a critical component of cloud formation as well as icing at this time of year, if water vapour is present in sufficient quantities above 10,000 feet, there is also a good chance it exists in large quantities below as well.

On occasion, we see ribbons of clouds float by (called cloud "streets" by some). It turns out that these are important clouds since they indicate regions of instability generally created by wind shear of some sort.

The full extent of these clouds is rarely visible and no forecast adequately covers their existence. Luckily, these cloud formations will be readily visible on the visible satellite image. This is useful for planning a circumnavigation route.

Also, where reporting stations are few and far between, the nature and extent of cloud coverage can be ascertained using the visible imagery. What really helps the pilot is the map overlay on both the visible and infrared images. All of North America is neatly mapped out, making state-by-state or province-by-province analyses easy.

Since many organizations process the official NOAA imagery prior to releasing them to the general public, it is important to check the time on each. Images may be updated anywhere from every 15 minutes to every hour depending on who is issuing them.

If a sequence of images is properly ordered and updated often enough, the formation and dissipation of weather is readily visible and can be easily followed as it moves across the country. Some of these sequences of images can be put in motion using a "loop" option.

Some additional satellite products are available from NESDIS and include fog and low cloud coverage and depth, microburst potential, clear air turbulence, and aircraft icing potential, among others. Since most of these are still considered experimental products, use them with caution and, as always, back up any go, no-go decisions with approved weather products.

Satellite imagery offers the ultimate in large-scale weather analysis. Used in combination with surface analysis, prognostics, and other textual forecasts, they complete the pilot’s weather briefing more completely.

One good place to start for access to weather satellite imagery, is at http://www.nws.noaa.gov/. Click on the satellite tab and you’ll find all three types of imagery.

This month’s Pilot Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor at the University of Illinois, Institute of Aviation. He holds a joint appointment with the Professional Pilot Division and Human Factors Division. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 160 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.