In addition to these forms of clouds, we may often notice, particularly during high winds, fragments of clouds hurrying across the sky. These are known as “scud”; they are generally pieces carried off by the winds from the main bodies of clouds. Occasionally two forms of cloud are present at the same time. This is ordinarily taken as a case of Cumulus and Stratus, and has become known as Cumulo-stratus; but, if observed in the zenith, it may readily be noted that the two forms of cloud are distinct, and they had better be dealt with separately. The appearance of Cumulo-stratus is an effect of perspective. Clouds float at varying altitudes, according to the latitude and elevation of the ground; the vertical temperature and adiabatic gradients determining the level at which the vapor becomes visible as cloud. It is desirable in all cloud observations, that note should be made of the approximate relative altitudes of clouds and of their velocity of motion. This is particularly desirable when dealing with the stratiform clouds, whether as ordinary Cirro-cumulus or as very high Cirro-macula. The beautiful coloring of clouds results from the breaking up of light beams in passing through them or along their edges. This phenomenon is caused by diffraction, and to it is due our lovely sunrises and sunsets. When the sun is high in the heavens, the light is white, but as the orb nears the horizon, and its rays pass through thicker layers of atmosphere, the smaller light waves get gradually cut off, until the sun sinks as a red ball below the horizon. The largest waves of light produce the red rays and the after glow which are so beautiful. Sunrise and sunset effects are matters of much interest, but are of too complicated a nature to be fully gone into here; we must, however, notice them briefly, because of their importance in weather forecasts. Soft sunset colors indicate fine settled weather; fiery brilliant hues denote change to stormy or wet weather. Other color effects in clouds are due to phenomena, known as halos and coronas. Halos appear as rings round the sun and moon; they are caused by the shining of the orb through very high Stratus or Cirrus clouds, and have a diameter of 42°. Sometimes shades of color, resembling those of a rainbow, are visible--red appears on the inside and blue on the outside. These rings of color are due to the reflection and refraction of light passing through the fine ice crystals of which high Stratus or Cirrus clouds are composed. Occasionally a complicated series of beautifully colored rings is noticeable. Generally speaking, these rings are due to the thinness of the high cloud through which the light is passing. Still more curious arrangements of halos sometimes occur.