Introduction

This page contains Advanced Very High Resolution Radiometer (AVHRR) Satellite images of the Western United States. AVHRR sensors are carried on sun synchronous polar orbiting satellites  (NOAA satellites).. They have an orbital altitude of about 833 km (517 miles) and  a spatial resolution of  1.1 km,  meaning that each cell or pixel covers an area on the ground of approximately 1 km by 1 km.  Each satellite pass covers a large area,  having a swath width of 2400 km as it moves from pole to pole. These satellites have the ability to give global coverage of the Earth twice daily, four times if two satellites are available and six times if 3 satellites are used. AVHRR sensors have 5 bands or channels. The spectral characteristics of these bands are:

The AVHRR satellite bands used for viewing images consist of bands 1, 2, 3 and occasionally  4. Since these bands are located in different parts of the spectrum, they are used for different purposes. A brief description on how they are used is given below:
The AVHRR satellite bands used for creating the simulated natural color images are bands 1,  2 and 3. To create the RGB (red, green, blue) image, band 3 (the thermal infra-red band) is displayed in red, band 2 (the near infrared band) is displayed in green, and band 1 (visible red) is displayed in blue.  (To see full image, go to July 4, 2002 3:22 p.m. )
Band 3 is in the infra red part of the spectrum and is used to pick up the thermal signatures of fires. For display, this band is shown in red tones which enhance the ability to locate fires. Hot objects such as fires will show up as white and cool areas will be very dark. Since this band is a thermal band, it can be viewed during both the day and night.  Band 3 is most effectively viewed during the evening hours, and  throughout the night into the early morning. Due to the heating of the earth by the sun, band 3 tends to get saturated during the day and fires are hard to locate, if they can be located. See the sample image to the right. (To see full image, go to July 5, 2000 8:13 p.m. )
Band 1 is in the visible (red) part of the spectrum and is shown in gray tones. This band is useful in looking at weather patterns and for viewing smoke. Fires cannot be seen using band 1, but sometimes the smoke they produce can show up of the image. To see fires, a thermal band must be used. Since band 1 is a visible band, it can only be used during the daylight hours. Images during the night are dark and nothing can be seen on the image. See the example daytime  image to the right. (To see full image, go to July 5, 2000 8:13 p.m. ) Occasionally band 4 is used if band 1 is unavailable. Band 4 is a thermal band and is not in the visible part of the spectrum. It is sensitive to temperature differences between the land surface and clouds and so by enchancing it the weather patterns can be shown. Images using Band 4 have been enhanced such that they appear nearly the same as band 1.

Navigating to view the images: