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Abstract does not appear. First page follows. Introduction In many areas of California, warm-season crops such as cantaloupes, cucumbers, tomatoes, and peppers are planted during winter or early spring and are artificially provided with an environment suitable for growth during a period of adverse temperature conditions. This is accomplished by means of paper or plastic structures placed over the growing plants which provide, in effect, a miniature greenhouse. In the past, paper was used almost exclusively as hot-cap material. Recently, however, plastic has been used as continuous row covers or tunnels and has been shown to be superior in many respects to the conventional paper (Shadbolt and McCoy, 1960).5 As these covers often remain in place for as long as three months, temperature, humidity, and other environmental factors must be maintained in order to provide good growing conditions for the plants. Previous studies (Shadbolt and McCoy, 1960) have indicated that the shape and method of ventilation of these continuous covers have a marked influence on the soil and air temperatures as well as on plant response. It was considered desirable to expand these studies and to determine more critically the effects of these factors on the temperature patterns throughout the enclosed air and soil. Accordingly, experiments were conducted at the University of California Imperial Valley Field Station near Holtville and at the University of California, Riverside, during the winters of 1960 and 1961. Polyethylene mulching has also been used in many vegetable-growing areas of the country to increase earliness and yields. In contrast to raised covers, mulches involve the control of conditions primarily below the soil surface. Both clear and black films have been successfully used. Clear film has been shown to provide a greater increase in soil temperature than black film (Voth

The microclimate of plastic shelters used for vegetable production