The sky begins to darken and the summer breeze you have been yearning for all afternoon begins to stir and gather momentum. You glance upwards and notice the leaves on your maple or poplar trees have turned into a mosaic pattern of shimmering silver and green. A storm or a rain event is approaching. You are witnessing one of the oldest forms of storm forecasting; leaves on trees turning upside down.
While it may seem, especially during a dry period, that these leaves are flipping over on their own to beckon relief from their thirst, trees nor leaves have the power to turn over on their own. Atmospheric conditions are responsible, mainly moisture and wind. The moisture and wind that change the atmospheric conditions to bring rain or a rainstorm are bought on by weather systems called fronts.
Under general conditions most weather patterns or fronts move from west to east. When a rainstorm approaches your area, the wind is normally coming from the east in the form of a cold front. The air preceding the storm is moist cool air. As this air enters your area several atmospheric changes begin to occur. The cool air of the front causes an updraft of the warm air in your area to rise from the ground upwards. As the warm air collides with the cool air, a breeze develops. As this wind pattern picks up speed, the direction of the wind changes as the cooler air envelopes the warmer air. Usually rain or a rainstorm will quickly follow. Sometimes the air is so dry, the moisture is quickly evaporated so no rain will touch the ground.
Leaves on most of the deciduous trees in your yard react to this incoming moisture and wind pattern changes by seemingly turning over. The moisture or humidity in the air settles on the leaves making them more pliable and softer. When the wind speed picks up, these leaves will be lifted up and turned in all directions exposing many undersides in the process. As the sky darkens, the undersides of leaves become more visible and the mosaic patterns of lighter green mixed with the darker green becomes more enhanced.
By observing these leaf patterns along with a darkening sky and wind, you can predict rain is on the way only in the immediate future. Tune in to your local weather station for any predictions concerning later hours or days. Do not be surprised if no rain drops fall. Sometimes the system passing through is not strong enough to reach the ground especially if the surrounding air conditions are overly dry.