Water in liquid form can be removed from a shelter by draining or pumping. Water vapor can be removed by relatively dry air flowing through the shelter or by mechanical dehumidification. Construction materials may contain large quantities of water which, during the early life of a shelter, could be a severe problem. Concrete may require months of drying before the water content is sufficiently low so that it does not keep the humidity high in the shelter. Before a new shelter is stocked it should be operated for several days in the same manner that it will be during the standby period. The humidity should be checked to see that it does not rise to levels excessive for the materials that will be in the shelter. Many underground structures may be subject to flooding from storms, ground seepage, broken water lines, etc. Such occurences may require early replacement of some or all of the stored foods, depending in part on the type of package protecting the food. Water entering the shelter by leakage or flooding should be confined to as amall an area as possible and then removed promptly. If it is allowed to remain over an appreciable area the humidity will build up to an intolerable level. Entrance of moisture into shelters by diffusion through the shelter walls, ceiling, and floor will occur in most parts of the country. Any shelter that is located in moist earth and in which proper humidity is maintained for food storage will have moisture entering by this method. There may be no visible sign of moisture. In shelters that are not dehumidified the amount entering can be determined by humidity and airflow measurements. In dehumidified shelters it can be determined by measuring the amount of condensate from the dehumidifying equipment. (These determinations assume that moisture is not entering the shelter from other sources.) The quantity of moisture entering by diffusion can be reduced to a negligible level by proper construction. If the problem is found after the shelter is constructed, the quantity of moisture entering may be greatly reduced by coating the inside surfaces of the shelter. Items stored in the shelters may contain water that can evaporate during storage and thus affect the humidity. Cellulosic materials, such as fiberboard used for cartons, wood for boxes, and cotton for bedding, are the most likely carriers of this water. Green wood may contain water up to three times the dry weight of the wood. Any cellulosic materials that have been air-dried or allowed to come to equilibrium with the surronding air may still contain an appreciable quantity of water. Figure 7 shows the equilibrium moisture content of wood at various relative humidities. For example, if wet wood is placed in a shelter held at 70 percent relative humidity the wood will dry down to 12 to 13 percent moisture. While the quantity of moisture that might enter a shelter by this method is small, it is quite significant in some instances. Moisture can be gained or lost by air movement through the shelter; the direction of shift depends on the moisture content of the entering air and the conditions, primarily temperature, within the shelter. Moisture will be gained in the summer when the outside air contains a large quantity of water vapor. Shelter temperatures will be below the outside air temperatures so that air entering the shelters will be cooled. This cooling increases the relative humidity. Any materials capable of absorbing water will take in additional moisture. If the air is cooled sufficiently it reaches 100 percent relative humidity. The temperature at which this occurs is called the dew point. Any cooling below this point causes condensation of moisture. This condensation normally occurs on surfaces that are cooler than thè air, and the moisture may cause rapid deterioration of many materials. Moisture will be lost from the shelter during cool days when the amount of water vapor in the air is small. Shelter temperatures will be higher than the outside air temperature so that air entering the shelter will be warmed. This decreases the relative humidity of the air. Materials with absorbed moisture will dry and the rate of evaporation of any water in liquid form, such as surface condensation, will be increased. The important influence of air circulation on humidity levels in shelters is illustrated by data in Table 1, where average annual air temperatures can be compared with average summer dewpoints (columns 1 and 5) at many locations in the United States. As stated earlier, the average temperature in a shelter is approximately the same as the average annual air temperature. In late summer and early fall, shelter |