The moisture that fuels a storm is not only brought in by advection from a moisture source such as an ocean. Although a great deal of moisture is advected in from a moisture source, there is moisture that is also available from other sources. First, the air already has some moisture even when the air is fairly dry. If the lifting is strong enough, even fairly dry air can cool to the dewpoint to develop precipitation. Second, a major source of moisture is made available by local evaporation. This will be the main emphasis of this writing. The vegetation and soil is able to hold a tremendous amount of moisture. After a precipitation event, plants and the soil will hold on to moisture. Once the precipitation event ends, a supply of moisture from evaporation will constantly be supplied to the air. This moisture supply is temporary. Eventually another precipitation event will be needed to recharge the moisture in plants and the soil. A greater amount of moisture can evaporate into air that is warm. Also, a greater surface area of plants will be able to supply more moisture to the air. Thus, after a precipitation event in a warm location with abundant vegetation it can be expected that significant evaporation will take place after the precipitation event. This can make it easier for the next precipitation event to occur and to occur with a greater volume of precipitation than would have been possible without the supply of moisture from evaporation. Compare a precipitation event setting up when the weather has been previously dry compared to when the weather has been very wet. The very wet weather that previously occurred will allow for a greater supply of moisture to enter the air. This can increase instability and add more moisture to the clouds. Thus, it can be easier for it to rain if the weather has been rainy previously. When the weather pattern turns dry, it can be more difficult to get rain since less moisture is being evaporated into the air and the instability is less. Thus, dry weather can lead to more dry weather until the weather pattern changes so substantially that a significant rain event occurs anyway. Wet weather can lead to more wet weather until also the weather pattern changes so substantially that the wet weather pattern stops. |