Professional Pilot, January 2016

of clouds and even the shadows they may cast Because it is dependent on light visible imagery is only available when the surface below is in sunlight Additionally depending on the time of day the cloud patterns may appear to shift with the position of the sun relative to any irregularities in the cloud deck or large obstructions such as mountain ranges When the sun is at a low angle near sunrise or sunset tall clouds or higher cloud cover may cast significant shadow over lower clouds At higher sun angles multiple cloud decks may become indistinguishable from one another if they all reflect the sunlight with similar intensity Visible imagery also tends to be of a much higher resolution than its infrared counterpart This makes it good for identifying features such as the overshooting top of a towering cumulus amidst a solid cirrostratus deck It is also excellent for identifying cloud patterns such as cloud streets or the wispiness in cirrus that indicates strong high altitude wind shear On clear mornings it is even an excellent choice for seeing radiation fog that has settled into the valleys overnight Infrared imagery Infrared imagery is also normally presented in gray scale This is because the information on any given map tends to be gleaned from a single wavelength that is optimal for looking at certain properties Certain infrared wavelengths between 9 13 micrometers 58 PROFESSIONAL PILOT January 2016 are highly responsive to the thermal energy being emitted by an object The more energy an object emits the warmer the object is and this is picked up as a stronger reading by the infrared sensor Post processing transforms this information into a gradation from black to white with black representing the warmest temperature and white representing the coldest Thats how we differentiate higher cloud tops from lower ones Temperatures decrease with increasing altitude and so high clouds tend to be colder than low clouds Thunderstorm tops and cirrus will be presented as brighter white while low stratus will appear as dark gray Some processing systems will colorize the colder temperatures to better delineate these high cloud tops and indicate areas where precipitation is likely The main strength of infrared thermal imagery is the ability to see clouds at night Because the clouds are constantly emitting some heat energy as a result of their kinetic energy as well as the release of heat due to condensation of water vapor they will be visible to infrared sensors However infrared thermal imagery also has some important limitations Because the amount of energy being measured tends to be far lower than that in the visible spectrum the sensors are coarser in order to capture measurable amounts of energy meaning that the resulting image will be coarser as well 4 km pixels vs 1 km for visible Additionally infrared may have difficulty showing low clouds because they may be roughly the same temperature as the underlying Earths surface giving them a similar shading Also clouds are bad at transmitting heat energy and so the sensors only pick up what is being emitted upward by the topmost cloud layer Thus the thickness of clouds is generally impossible to discern from infrared imagery and it can be difficult to distinguish a high overcast deck from tall cumulus tops or anvils as both will appear bright white Even a thin high deck can hide any lower clouds Water vapor Shorter wavelengths of infrared energy 4 to 7 micrometers provide a different benefit These wavelengths are excellent at showing how much water vapor is in the air Information from satellite based sensors operating on this wavelength can be translated into a water vapor map that will show the moisture patterns of the middle troposphere at around 12000 to 18000 ft Such mapping is important for determining where moisture may be flowing into a region where a dry line front might produce supercell storms or as in the example that led this article where drier air might be inhibiting storm formation The 3 basic satellite images over your route of flight can provide a wealth of information that along with a good forecast upper air and surface weather data can help you set an optimal route Like these weather data products looking at a series of satellite images from the past few hours will help you also determine how quickly systems are developing or moving and if there are any corridors clearing up or shutting down Charts showing observations and model estimates of conditions along your route will help you to identify major features and possible conditions you may want to avoid but a picture of the actual conditions can be worth a lot more for helping you see where you may want to alter course And send in your pireps so fellow pilots know if the conditions youre experiencing are matching up with the weather info you received Karsten Shein is a climatologist with NOAA in Asheville NC He formerly served as an assistant professor at Shippensburg University Shein holds a commercial license with instrument rating GOES 8 launched in 1994 provided visible and infrared imagery for a decade over the eastern US before being decommissioned The Geostationary Operational Environmental Satellite GOES program has provided continuous weather observation from space since 1975 In addition to imagery the GOES satellites also serve as a relay for surface observations house radiometers to monitor space weather and have an atmospheric sounder to observe temperature profiles and ozone levels