Weather Conditions Associated with Outbreaks of Facial Eczema

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Abstract
To obtain more definite information than that available to date on the weather conditions associated with outbreaks of facial eczema, the records from climatological stations in districts in which outbreaks of facial eczema frequently occur were examined. The data indicate that a season can be considered potentially dangerous either if the soil temperature at 8 inches, measured at 9 a.m., has reached an average of 62.5°F or over during November, or if the main body of moisture in the soil, as distinct from surface layers wetted by smaller rains, has reached a deficit of 1 ½ in. by the end of November. A more definite judgment can be made by the end of December. It appears there is a high probability of subsequent outbreaks if the 8 in. soil temperatures average over 65° F during December, and there is no “soaking” rain during December. By soaking rain is meant quantities of precipitation greater than that required to bring the soil up to field capacity. The data do not suggest that such rains automatically render a season safe, but that if soil temperatures are marginal, such rains, particularly if spread over several days, will reduce the probability of widespread outbreaks. The commencement of the first toxic period in a season, as judged by liver damage in lambs grazing in the field, appeared to be preceded either by two or more periods when falls of rain greater than just a few points are accompanied by or immediately followed by two or more successive nights with grass-minimum temperatures of 54°F or over, or alternatively by a single prolonged group of high grass-minimum temperatures with rain. Also, the rain that was judged to initiate the first toxic period was itself accompanied by or immediately followed by two or more successive nights when grass-minimum temperatures were 54°F or higher. It appeared that where a period of high grass-minimum temperatures was accompanied by particularly heavy or continued rain, its effectiveness in either inducing toxicity or preparing pasture for toxicity was reduced. Periods of high grass-minimum temperatures with rain arise from weather disturbances accompanied by inflows of warm moist air. Meteorological conditions which result in this situation are described. They can generally be foreseen a day or two ahead.