Reading the Weather

by Thomas Morris Longstreth

Soon after the close observation of cloud colors has commenced the amazing discovery is made that the same color at sunrise means exactly the reverse of its meaning at sunset.

“Sky red in the morning
A sailor’s sure warning,
Sky red at night
A sailor’s delight.”

Christ seized upon this phenomenon to throw confusion into the Pharisees and Sadducees when they asked that He would show them a sign from Heaven. As Matthew reports it:—“He answered and said unto them, When it is evening ye say, It will be fair weather for the sky is red. And in the morning It will be foul weather to-day for the sky is red and lowering. O ye hypocrites, ye can discern the face of the sky; but ye cannot discern the signs of the times.”

The reasons for this contradictory evidence of color are not nearly so obvious as the fact itself. Taking the scientist’s word for it need not stretch one’s credulity overmuch if he can be followed step by step. He says that sunlight is white light, and white is the sublime combination of every color. If no atmosphere existed about us the light would all come through, leaving the sky black. The atmosphere, however, which is full of dust and water particles, breaks up these rays, these white sheaves of light, into their various colors. The longest vibrations, which are the red, and the shortest, which are the violet, get by and the rest are turned back, mixing up into the color which we call our blue sky.

If the dust and water particles grow so large and numerous as to divert more of the short rays than usual we get a redder glow than usual. This is most noticeable when the sun and clouds are near the horizon for the air through which they appear is nearer the earth and consequently dirtier. If these water globules mass together so as to reflect all the rays alike the result is a whitish appearance. That is why a fog bank, composed of tiny droplets, each reflecting with all its might, can make the sky a dull and uniform gray.

As evening approaches the temperature of the normal day lowers. As the temperature lowers it is the tendency of the moisture in the air to condense about the little dust particles in the air. And as these particles increase in size their tendency is to reflect more and more of the waning rays of light. Therefore if the sky is gray in the evening it means that the atmosphere already contains a good deal of condensed moisture. If the cooling should go on through the night, as it normally would, condensation would continue with rain as the likely result.

If, on the other hand, after the evening’s cooling has progressed and yet the colors near the horizon are prevailingly red it means that there is so little moisture in the atmosphere that the further increase due to the night’s condensation will not be sufficient to cause rain. Hence the natural delight of the sailor.

A gray morning sky implies an atmosphere full of water precisely as an evening gray does. The difference lies in the ensuing process. By morning the temperature has reached its lowest point and if this has not been sufficient to cause cooling to the rainpoint the chance for rain will be continually lessened by the growing heat of the rising sun. The gray, therefore, is the normal indication of a clear cool night which has permitted radiation and therefore condensation to this degree. It is for this reason that we have the heavy fogs of August and September followed by cloudless days.

A red morning sky shows, like the red evening sky, that condensation has not taken place to any extent. But this is abnormal for a clear night causes condensation. The red therefore means that a layer of heavy moist air above the surface levels has prevented the normal radiation. Hence when the day’s evaporation adds more moisture to that already at the higher levels the total humidity is likely to increase beyond the dewpoint with the resultant rain.

These two color auguries are among the most reliable of all the weather signs. Unfortunately the sunrises are scarcely ever on hand to be examined except by milkmen. But a careful scrutiny of the sunset will make one proficient in shades. In summer when the sun burns round and clear-cut and red on the rim of the horizon the air contains much dust and smoke, the accompaniment of dry weather. And as dry weather has a way of perpetuating itself such a sun makes dry and continued weather a safe prophecy. In winter the same red and flaming sun setting brilliant as new minted gold is a sure indication of clear and cold weather. In all seasons the light tints of the evening sky mean the atmosphere at its best. A golden sunset, a light breeze from the west, a glowing horizon as the sun goes down, slow fading colors all constitute a hundred to one bet for continued fair weather. The sunset colors that are surely followed by storm will be discussed in the next chapter.