Regions of low pressure are associated with bad weather...

Now you have the science behind the adage. Picture yourself on a ship in the middle of a mid-latitude ocean. Sailboat There the wind (and thus storm paths) is from west to east. It is morning and you are watching the sunrise. It is red. Since it is morning you are looking east, and the red sky indicates that there is high pressure there. Because you are in the mid-latitudes, the high is moving eastward--away from you. That could only mean that a low, and very likely an associated storm, is moving toward you from the west.  Sailor take warning! Now picture yourself watching the sunset from the ship, and the western sky is red.  That means that an area of high pressure is to your west, the westerlies are moving it toward you, and good weather is on the way--sailor's delight!

Note that this only works in the belt of westerlies, from about 30 degrees to 60 degrees latitude in both the northern and southern hemispheres. The region 30 degrees on either side of the equator is characterized by easterlies (the trade winds). This adage would be opposite in that region.
 
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