GPS Systems Collect Water Vapor Data from Ivan’s Eyewall

As researchers at NOAA’s Forecast Systems Laboratory continue to study data from the recent hurricanes, unprecedented information is discovered. The first known GPS ground-based observation of upper-air moisture within the eyewall of a land-falling hurricane was recorded as Hurricane Ivan pounded the U.S. central Gulf Coast on September 16, 2004. FSL and its partners use cost-effective GPS instruments and the GPS Surface Observing System (GSOS) to improve weather and water forecasting. Researchers recognize water as the principal mechanism by which moisture and latent heat are transported and cause "weather," and as a greenhouse gas, water also plays a critical role in the global climate system.

Hurricane Ivan, rated a strong Category 3, made landfall around 2 AM CDT (0700 UTC) September 16 with winds exceeding 58 m/s (130 mph), and pressure in the eye measuring 945 hPa. According to the National Hurricane Center, landfall occurred about 22 km (13.6 miles) east of the U.S. Coast Guard Differential GPS site at Mobile Point, AL (MOB1). This is consistent with a GOES-East water vapor image taken at 0645 UTC. Contact with MOB1 was lost at 0758 UTC. The minimum pressure reported by the National Data Buoy Center GSOS unit installed by NOAA at the USCG site was 948.4 hPa between 0645 and 0655 UTC. The maximum quantity of precipitable water vapor retrieved from the GPS signal delays by FSL was 80.4 mm (approximately 3.16 inches) between 0630 and 0700 UTC. Precipitable water vapor is defined as the height of a column of liquid water that would form if all of the vapor (water in its gaseous state) were to completely condense.

The GPS and GSOS equipment at Mobile Point successfully collected data within and almost through the hurricane eyewall, resulting in two notable events: the first known ground-based observation of upper-air moisture from near the center of a land falling hurricane, and the highest recorded level of precipitable water vapor in the atmosphere using GPS meteorological techniques. The previous record measurement of water vapor in the atmosphere occurred 10 days earlier (September 6, when Tropical Storm Frances passed over Tallahassee, FL) with a value of 75.7 mm or about 2.98 inches.

We thank our colleagues at the U.S. Coast Guard, NWS National Data Buoy Center, and NOS National Geodetic Survey for their assistance in making this achievement possible.

More information on GPS meteorology within NOAA is available at

Contact information
Name: Seth Gutman
Tel: 303-497-7031