University of Washington
Seattle, WA 98195
Surface weather observations from stations on land and ships in the ocean are used to obtain the global distribution, at 5° x5° latitude-longitude resolution, of total cloud cover and the average amounts of the different cloud types: cumulus, cumulonimbus, stratus, stratocumulus, nimbostratus, altostratus, altocumulus, cirrus, cirrostratus, cirrocumulus, and fog. Diurnal and seasonal variations are derived, as well as interannual variations and multi-year trends.
Two climatic atlases have been published (a) using 50 million ship observations for the 30-year period 1952-1981, and (b) using 130 million land station observations for the 11-year period 1971-1981. Two additional atlases have been published on the probabilities of co-occurrence of the different cloud types. Recently an additional ten years (1982-1991) of land and ocean data have been analyzed with greater accuracy at night by use of a moonlight criterion.
Visual observations of cloud cover are hindered at night due to inadequate illumination of the clouds. This usually leads to an underestimation of the average cloud cover at night, especially for the amounts of middle and high clouds, in climatologies based on surface observations. The diurnal cycles of cloud amounts, if based on all the surface observations, are therefore in error, but can be obtained more accurately if nighttime observations are screened to select those made under sufficient moonlight.
Ten years of nighttime weather observations from the northern hemisphere in December were classified according to the illuminance of moonlight or twilight on the cloud tops, and a threshold level of illuminance was determined, above which the clouds are apparently detected adequately. This threshold corresponds to light from a full moon at an elevation angle of 6° or from a partial moon at higher elevation, or twilight from the sun less than 9° below the horizon. It permits the use of about 38% of the observations made with the sun below the horizon.
The computed diurnal cycles of total cloud cover are altered considerably when this moonlight criterion is imposed. Maximum cloud cover over much of the ocean is now found to be at night or in the morning, whereas in the published atlases without the moonlight criterion the computed time of maximum was obtained as noon or early afternoon in many regions (Figure 25.1). The diurnal cycles of total cloud cover obtained are compared with those of ISCCP for a few regions; they are generally in better agreement if the moonlight criterion is imposed on the surface observations. The average cloud cover is found to be greater during the day than at night by 3.3% over land but by only 0.3% over the ocean. Cloud cover is greater at night than during the day over the open oceans far from the continents, particularly in summer (Figure 25.2).
Using the moonlight criterion, ten years (1982-1991) of surface weather observations over land and ocean, worldwide, have been analyzed for total cloud cover and for the frequency of occurrence of clear sky, fog, and precipitation. The global average cloud cover (average of day and night) is about 2% higher if the moonlight criterion is imposed than if we use all observations (see Figure 25.3). The difference is greater in winter than in summer, because of the fewer hours of darkness in summer. The amplitude of the annual cycle of total cloud cover in the Arctic Ocean and at the South Pole is diminished by a few percent when the moonlight criterion is imposed.
The average cloud cover for 1982-1991 is found to be 54% for northern hemisphere land, 53% for southern hemisphere land, 66% for northern hemisphere ocean, and 70% for southern hemisphere ocean, giving a global average of 64%. The global average for daytime is 64.6% and for nighttime, 63.3%.
The 1982-1991 data have not yet been analyzed for cloud types. However, an archive of edited individual cloud reports has been prepared, so that a user can develop a climatology for any particular type for any geographical region and any spatial and temporal resolution desired. The information in the weather reports relating to clouds, including the present weather information, was extracted and put through a series of quality control checks. Reports not meeting certain quality control standards were rejected. Minor correctable inconsistencies within reports were edited for consistency. Cases of "sky obscured" were interpreted by reference to the present weather code as to whether they indicated fog, rain, snow, or thunderstorm. Special coding is added to indicate probable nimbostratus clouds which are not specifically coded for in the synoptic code. This "edited cloud report" also includes the amounts, either inferred or directly reported, of low, middle and high clouds, both overlapped and non-overlapped. The relative lunar illuminance and solar zenith angle are also given.
References
Hahn, C.J., S.G. Warren, and J. London, 1995: The effect of moonlight on observation of cloud cover at night, and application to cloud climatology. J. Climate, in press.
Hahn, C.J., S.G. Warren and J. London, 1994: Climatological Data for Clouds Over the Globe from Surface Observations, 1982-1991: The Total Cloud Edition. NDP-026A, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee.
Hahn, C.J., S.G. Warren and J. London, 1994: Edited Synoptic Cloud Reports from Ships and Land Stations Over the Globe, 1982-1991. NDP-026B, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, Oak Ridge, Tennessee.
