This animation shows cloud changes over the western half of the United States and the northeastern Pacific Ocean over the past five hours. Note that times are Greenwich Mean Time (GMT), because viewers may be located in any time zone.
Colors indicate cloud temperatures and indirectly, cloud heights; because temperatures at the tops of clouds depend primarily on their heights, with the highest clouds having the coldest tops. Cloud colors are ranked progressively in the animation like this:
Red = Coldest/Highest
Yellow = Warmer & Lower than Red
Green = Warmer & Lower than Yellow
Blue = Warmer & Lower than Green
Cyan = Warmer & Lower than Blue
White = Warmest/Lowest
Any red clouds in the display have especially cold, high tops. Clouds like that are strongly associated with severe weather – high winds, thunderstorms, and heavy precipitation; although nearby areas that are not currently under clouds are apt to be unusually clear and sunny during daytime. The tops of yellow clouds are not quite as cold or high as the tops of red ones, but even so, are cold and high enough to be associated with major storms, and likewise progressing further down the list.
White clouds are thin clouds near the surface with relatively warm tops. If they are near the ocean they are called “marine layers.” When marine layers move over coastal land the result is very low clouds or fog, depending on whether they extend all the way down to the surface. Marine layers are strongly associated with poor visibility and mild, stable weather. There may be light drizzle, but generally not heavy precipitation, high winds, lightning, or rapid temperature changes.
Rates of Change
The animation also shows how far individual clouds have moved over the past five hours, which is important to estimating how long current weather conditions in an area will prevail. Total precipitation and wind damage from storms depends not only on their severity, but also on how long they persist over an area. Even though violent storms often have high rates of precipitation and high wind speeds, there may not be much total precipitation or wind damage if they move by very quickly. Conversely, there may be large amounts of total precipitation and significant accumulated wind damage from milder storms that persist over the same area for a long time.