Initial Analysis of GLOBE Contrail Observations

The temperature and humidity in the atmosphere govern the formation of persistent contrails by jet aircraft exhaust. If the temperature is cold enough, and the humidity is high enough, then aircraft can produce contrails that can last for several minutes or even hours. Scientists gather information on temperature and humidity at aircraft cruise altitudes (between 27,000 and 39,000 feet) using weather balloons. However, the amount of moisture at these altitudes is difficult to measure, and the balloon observations are too sparse and inaccurate to allow an exact estimate of the amount of contrail formation across the US. Atmospheric scientists at NASA Langley are using temperature and humidity information from numerical weather prediction models to supplement the weather balloon measurements and to improve forecasts of contrail formation across the country.

Student observations of contrails and clouds through the GLOBE program (and the Earth Day 2004 Contrail Count-a-Thon) are helping NASA Langley scientists to determine the quality and consistency of the humidity data used in numerical weather prediction models. Nearly 1500 GLOBE observations of clouds and contrails across the continental US were compared to relative humidity calculations from Rapid Update Cycle (RUC) and Advanced Regional Prediction System (ARPS) model analyses during April 2004. Figures 1 and 2 show how the relative humidity with respect to ice (RHI) at aircraft cruise levels compares for each model based on contrail observation criteria. Figure 1 shows that the RHI over schools that observed only short-lived contrails averaged 54 and 62 percent for the RUC and ARPS models respectively, while the average RHI measured when spreading persistent contrails were observed was 67 percent for RUC and 77 percent for ARPS. Comparisons like these with GLOBE data will allow atmospheric scientists to improve estimates of the current and future cloud coverage caused by persistent contrails.