As mentioned in other articles on these web pages, trees, shrubs and grasses respond rapidly to increases in precipitation associated with the North American Monsoon. In Mexico, large swaths of the land go from desert to subtropical rain forest in a matter of days as the initial monsoon rains begin in May or June. Plants have the ability to take moisture from the soil and place it into the air through a process called transpiration. Transpiration is a means for a plant to breathe, grow, and even cool its leaves and stems. In fact until northern Mexico greens up, it is difficult to maintain monsoon moisture in the Southwest for extended periods of time. To a lesser extent, unusually green (wet) conditions in the west Texas and eastern New Mexico grasslands can be a monsoon moisture source under surface easterly flow conditions. Thus, "greenness" is tracked by satellite, and can be compared to normal conditions to determine if long-term moisture will be available for thunderstorms in the Southwest.