Wednesday, July 3, 2013

A Window on the Upper Atmosphere

The current large scale weather pattern over the U.S. is interesting in a number of ways - its persistence, intensity, and extent. In my last post I discussed how this year's pattern is similar in many ways to what was occurring last year.  I often use the 500 millibar map (this is about 18,00-20,00 feet in altitude) to show the upper level wind and weather pattern. Looking at lines on a map is one thing, but seeing a "real life" visualization of it is something else. Yesterday's water vapor satellite imagery was striking to me as you could easily see where the major weather features were across the U.S.

You are probably familiar with the visible and/or infrared satellite images of clouds or storms. Water vapor imagery is unique in that it can detect water vapor (gaseous state) as well as clouds (liquid state). The water vapor sensors on the satellite see only about the top third of the troposphere (approximately 20,000 to 30,000 feet), so dry and moist areas at upper levels can be clearly seen. On unenhanced images, white areas show where moisture is present, and grays and blacks show where it is dry. Sensors on the satellite detect the radiative energy from the water vapor and clouds. The output voltage of the sensors is proportional to the energy striking the sensor. This in turn is converted to a "brightness temperature", which is the scale use see on the bottom of the image.

Here is the water vapor image from Tuesday, July 2 at 1200UTC (7:00 a.m. CDT). This image is from the National Center for Atmospheric Research (NCAR) Real-Time Weather Data web site.  This image is color enhanced. The driest areas are in reds, and the areas of highest moisture are in violets to greens.

Water vapor satellite image for July 2, 2013 6:45 a.m. CDT

Here is the 500 millibar map for the same time yesterday.

500 Millibar map for 7:00 a.m. July 2, 2013.
Again, the water vapor image with some map features identified.

Water vapor image 6:45 a.m. July 2, 2013 with features marked.
Notice how easy it is to pick out the upper level features depicted on the 500 millibar map on the water vapor image. You can clearly see the upper ridge in the west and the trough in the central U.S. with its center in southern Illinois. Additionally, a small disturbance in southeastern Minnesota (yellow circle) clearly visible on the water vapor image is just barely evident on the map (yellow shading). This is an area of vorticity, or rotation, in the atmosphere. Very often these are the features that help fire thunderstorms. In fact, yesterday afternoon thunderstorms developed over Iowa as this feature moved south southwest across the state. Moisture associated with the developing seasonal monsoon is evident over Mexico and southern Arizona. An area of thunderstorms (green/blue) associated with a weak tropical disturbance is evident over Cuba.

Here is today's water vapor image, 24 hours after the previous image.  The center of the trough in the central U.S. has moved back west into western Missouri. The area of thunderstorms near Cuba yesterday have moved into the Gulf of Mexico.  Dry air off the east coast is associated with the upper level ridge over the western Atlantic.

Water vapor image for 6:45 a.m. CDT July 3, 2013


  1. Much easier to see what is happening. Thanks for posting this

  2. Glad you found it interesting! I look at the water vapor image often. After writing the post on the heat it struck me how the July 2nd image drew a picture of what was going on upstairs.

  3. Thanks for this Steve! Because I live in southeastern BC, 150 mi. north of Spokane, I also check out (for Pac NW weather blog)Cliff Mass weather blog.
    Joe Woodward
    B.S. meteorology 1992, Metro State College (now University), downtown Denver, CO