Spot a rainbow after a rainstorm or by a waterfall and you will usually simply be amazed by its simple beauty.
Scientifically speaking, a rainbow is an optical/meteorological phenomenon, where a spectrum of light appears in the air when the sun shines into droplets of moisture in the atmosphere.
No mystery here. It is a cause and effect event.
But rainbows are one of the greatest earthly signs that God has ever used to convey his promises.
After the Biblical Great Flood, God declared the rainbow to be a sign that he would never flood the Earth again (Genesis 9:11-17).
Now whether the rainbow made its first appearance then, or if God simply said the rainbow, an existing phenomenon, would be his sign, I don't know.
Anyway, the rainbow is one of the simplest and easiest to spot signs of the impending end of the world ("end of the wicked") in the last days.
Joseph Smith taught:
"The inhabitants of the earth are asleep; they know not the day of their visitation. The Lord hath set the bow in the cloud for a sign that while it shall be seen, seed time and harvest, summer and winter shall not fail; but when it shall disappear, woe to that generation, for behold the end cometh quickly." (Teachings of the Prophet Joseph Smith," page 305.)
Also by Joseph Smith:
"I have asked of the Lord concerning His coming; and while asking the Lord, He gave a sign and said, "In the days of Noah I set a bow in the heavens as a sign and token that in any year that the bow should be seen the Lord would not come; but there should be seed time and harvest during that year: but whenever you see the bow withdrawn, it shall be a token that there shall be famine, pestilence, and great distress among the nations, and that the coming of the Messiah is not far distant." (Teachings of the Prophet Joseph Smith," page 340.)
So, the year you do not see the rainbow, the end of the world is not far away.
How could rainbows disappear? God withdrew all light for three days following the crucifixion of Jesus Christ (3 Nephi 8:21) and while that seems scientifically impossible, it happened.
Perhaps, there's some scientific law he will use to prohibit rainbows.
Otherwise, if there was enough fog, smoke, and/or haze around, that might prevent rainbows from being seen.
Still, it is not a matter of how God is going to withhold the rainbows, simply that he will do it.
Note that I have spotted a rainbow in the spring of 2013 and so don't panic yet ....
(The photograph above is Arches National Park, a picture by Elizabeth Arave Hafen.)