There are a number of ways to tell when rain is approaching. Many people fall back on the old "turning leaves" trick to gauge an impending rainstorm. There’s solid evidence to support the theory that this implies rain, since some trees – specifically oak and maple – actually curl or appear to turn over when there’s a high level of humidity in the air, or when the wind is gusting. Since these two conditions tend to precede rain, then watching the leaves on the trees definitely has some merit, as stated in this bit of lore:
"When leaves turn their back, ‘tis a sign it’s going to rain."
You may not realize it, but rainbows are also indications of a brewing storm. Most commonly associated with a rain shower that occurs during a brief burst of sunshine, this is only true if the rainbow is in the eastern portion of the sky. If it appears in the west, then it signifies a looming storm. This is due to the fact that nearly all storms come out of the west, and when the rays of the sun’s light refracts off of those storm clouds, then a rainbow is formed before the rain begins to fall – therefore, a storm is on the way.
Since rainbows only occur in the west during the morning, and in the east during the evening, one of the common pieces of lore involving the significance of rainbows and their signal of rain is:
"Rainbow in the morning, shepherds take warning; rainbow at night, shepherd’s delight."
The halo that one often sees surrounding the moon is seen as quite beautiful by most, and has inspired poets to pen a number of devotions to this lantern of the night. In reality, this halo – or glow around the moon which extends out from its rim – is caused by refracted light as it passes through ice crystals that are intrinsic to certain types of high-level clouds. While the clouds, themselves, don’t offer any precipitation, they often foretell an advancing low pressure system, which tends to bring undesirable weather conditions. Out of this came the lore which states: