Indications of foul weather include a change in wind direction and speed due to low- or high-pressure systems, an increase in ocean swells, thunder, lightning, the formation of low or dark clouds and red or pink hues in the sky at sunrise or sunset. These are the traditional methods for detecting an approaching storm.
Stratocumulus clouds are clumpy layers of clouds closer to the surface that indicate a storm is on the horizon. Thicker nimbostratus clouds that appear in layers but block the sunlight have the potential to emit precipitation. Towering cumulus clouds indicate the most dangerous weather. These form vertically and produce precipitation along with thunder and lightning. If the conditions are right, these clouds create updrafts that lead to tornadoes. Wall and shelf clouds are the beginning of an approaching storm that has the potential to produce tornadoes as well. Signs of a tornado include hail, rotating clouds and a sky with a green hue.
Thunderstorms produce wind upwards of 60 miles per hour. Hail often accompanies thunderstorms, especially those with the potential to produce a tornado.
Before the invention of modern weather instruments, such as weather satellites and buoy reports, weather observers had to look to the swells of the ocean, the color of the sky and lights around celestial bodies to predict weather.