Rainbows normally occur after rain during the afternoon and evening hours and in the eastern sky. Occasionally we get rainbows in the morning before rain in the western sky. It is quite rare to get a rainbow before a shower during the evening, but that was the case for many this evening. Why? We’ll let’s first look at what causes to get a rainbow.
To get a rainbow you need rain in front of you and the sun behind you. In order to get both rain and sun in the sky you need something like a pop up shower or thunderstorm. These are more common during the afternoon and evening hours when the atmosphere is the most unstable. During the afternoon the sun is starting to lower in the western sky. This combined with the fact that usually precipitation usually moves west to east where we live, the rainbow will be to the east after the shower or thunderstorm.
In the morning, since the sun is rising in the east, the rainbow will occur before the approaching shower from the west.
What happened this evening was a bit special. Because there is an upper level low pressure system to our south, our showers and thunderstorms this evening were moving from east to west.
So when the thunderstorm was approaching from the east, the sun was still shining from the west. This gave us pre-rain rainbows. Some were quite bright.