A Measured Approach – Tracking Precipitation at CLT

You’ve probably heard local news media, reporters and meteorologists refer to “precipitation measured at Charlotte Douglas International Airport” (CLT), but have you ever wondered exactly how and where it’s measured on the airfield? It’s critical to any weather event or forecast that might impact CLT, its airline partners and passengers.

As part of our new “CLTea” blog, we are introducing a feature called “Five on the Fly," where we ask five questions about Airport-related topics and interview people who we think you will find interesting.

Our first guest is National Weather Service (NWS) Meteorologist and Observing Program Leader Christopher Horne:

Why is an official weather measurement done at CLT Airport?

We consider weather elements measured at CLT Airport 'official,' meaning that they originate from a trusted government source. Prior to the Automated Surface Observing System (ASOS) commissioning in the mid-1990s, CLT had a National Weather Service (NWS) office on Morris Field Drive. NWS employees there took manual weather observations every hour.

The ASOS at CLT is owned and maintained by the NWS here at Greenville-Spartanburg Airport (GSP) in Greer, S.C. (The Greenville-Spartanburg Weather and Forecast Area extends into parts of North Carolina, South Carolina and Georgia.)

Charlotte-Mecklenburg Storm Water Services and the United States Geological Survey (USGS) also maintain a dense network of automated rain gauges throughout the county called FINS.

Where exactly is the ASOS measuring precipitation at CLT?

The ASOS Sensor Pad is located between Runways 36L and 36C near the Sierra Taxiway across from CLT Fire Station 41.

The ASOS rain gauge at CLT is a ‘weighing-type’ rain gauge (called an AWPAG or All Weather Precipitation Accumulation Gauge), meaning that rain, melted snow and ice collect in a type of ‘bucket.’ The liquid's weight is converted to inches with a resolution to the nearest one-hundredth of an inch. That data is transmitted via fiber optic cable in different forms to different users. The Federal Aviation Administration (FAA) contracts weather observers, whose office is at the base of the new control tower, to augment and back up the ASOS when necessary. They can do this using a manually read standard rain gauge.

The ASOS has a ‘present weather’ sensor that can automatically detect when rain or snow is falling and even assign an intensity to the precipitation. However, the system is not smart enough to tell if sleet or hail is occurring, but the FAA contractors can augment the present weather sensor output to assign the proper type of precipitation.

Since the ASOS cannot measure the amount of snow that has accumulated on the ground, the weather observers there also keep track of the accumulation.

The rain and snow data from CLT is monitored, collected, and quality-controlled here at the NWS office, and then officially archived in the climate data of the United States by folks at the National Centers for Environmental Information (NCEI) in Asheville.

How often are measurements taken?

Precipitation accumulation is constantly monitored/tabulated, and amounts are transmitted externally every five minutes. Common precipitation accumulation intervals are also transmitted hourly with a one-hour total, every three hours with a three-hour total, every six hours with a six-hour total and daily amounts.

Here at the GSP NWS office, we currently have 16 meteorologists on staff. Eleven are forecasters who provide routine 24/7 coverage, and the additional five program leaders and management who are forecaster-qualified, fill in shifts when needed and supplement the forecast staff during active weather events.

NWS electronic technicians are responsible for routine maintenance and repairs on the All-Weather Precipitation Accumulation Gauge and all the other sensors on the ASOS pad at CLT.

What else is measured?

The ASOS Sensor Pad contains a 10m wind tower, on top of which is a sonic wind sensor that constantly measures wind speed and direction. Also on the pad are temperature and dewpoint sensors, a laser beam ceilometer, a visibility sensor and a freezing rain sensor. The ASOS barometric pressure sensors are in a separate location at CLT Airport.

Why is this data so important to CLT and Charlotte?

Like other official automated gauges across the region, rainfall data from the CLT Airport gauge are monitored for rainfall amounts and, most importantly, rainfall rates.

One of the NWS's paramount responsibilities is issuing flood and flash flood warnings. Data from the CLT Airport gauge is used to monitor potential flooding on Airport property and along the various creeks/drainage basins flowing through adjacent areas of the City of Charlotte leading to the Catawba River.

Weather data from the CLT ASOS is used locally by FAA personnel in the tower, Terminal Radar Approach Control (TRACON) and at the Air Route Traffic Control Center (ARTCC). Commercial and general aviation interests use the data for safe flight operations. Local media outlets also use and display the data.