Contrails can be quite numerous and cover a considerable area. Contrails were observed from the surface all day from NASA Langley in southeastern Virginia during November 20, 1995. Data taken at 1-km resolution from the NOAA-14 AVHRR instrument near 1430 Local Time revealed a wide-ranging network of contrails both in and out of ambient cirrus clouds over much of the mid-Atlantic coast. Figure 3 is a specially enhanced pseudo-color, multispectral image constructed from the AVHRR data. It shows a variety of cloud features as well as contrails. Langley is located on the large peninsula at the southwestern end of the Chesapeake Bay. Contrails will not be visible on the AVHRR imagery unless they are least 0.5 km wide. Thus, these contrails are persistent and spreading and may have been responsible for the development of the thick cirrus cloud extending southeastward over the northern end of the Chesapeake Bay. This example is typical of the contrail cloud conditions that occur over much of the U.S.
Contrail statistics were developed from a special set of hourly daylight observations taken by U.S. Air Force weather observers at 19 bases scattered over the contiguous U.S. These data, spanning the period from April 1993 through March 1994, were analyzed at NASA Langley to determine the occurrence frequency of persistent contrails. Figure 4 summarizes how often contrails were observed during April and October 1993 over the available sites. These data represent the minimum occurrence frequency because they include only those cases when low-level clouds did not obscure the observer's view. As evident in figure 3, contrails will occur over low-level clouds. Therefore, the total frequency for all cases probably be greater than shown here. The spatial variation of contrail occurrence generally corresponds to the main commercial filght routes. Note that contrails occur relatively frequently over Langley Air Force Base next to NASA Langley Research Center because it is in a heavily travelled air corridor. There is a dramatic seasonal cycle to contrail frequency as shown in figure 5. Contrails are seen most often during the late winter and early spring. They are rare during midsummer but not for long. Some of this seasonal behavior may be affected by cloud obscurations.
The surface-based occurrence frequencies will be correlated with satellite observations to account for the cloud obscuration effects. Contrail occurrence can then be related to contrail coverage and radiative properties as observed by the satellites. By relating these parameters to aircraft fuel usage, it will be possible to estimate the present and future minimum impact of air traffic on clouds and the radiation budget. Other indirect effects on cloud amount and microphysical properties will require much additional study and the results of SUCCESS.
