Nature Notes

Vol. XII May, 1934 No. 5

More About the Weather

Clouds are formed when the air has been cooled below the point where the moisture, which it contains, condenses. As air rises it cools (as noted in the weather article in the April issue of NATURE NOTES) and as it cools its ability to hold water decreases. Thus we have clouds which is the moisture in the air that has condensed.

Often visitors to Mt. Rainier National Park watch the play of clouds upon the upper slopes of Mt. Rainier. They are curious and very much interested in the way they form, move about and disperse although the atmosphere elsewhere and removed from the mountain may be perfectly clear and cloudless. Warm air from the lowlands, containing considerable moisture, rises into the higher air strata and so doing passes the over frigid, glaciated flanks of Mt. Rainier. Thus it is cooled rapidly by contact with the glaciers and, as it becomes less able to hold its moisture, the clouds that one sees are formed at that place. As these currents continue to rise or move about the air removed from contact with the icy slopes and the clouds disappear, the condensed moisture being absorbed again. We often say to visitors who are concerned about the future state of the weather in the park "Well, there is a cap on the mountain, so we may have rain in a few days". The "cap" referred to is a cloud bank that forms over the summit and obscures it from view. The presence of such a cap is not an absolute indication of rain but it generally indicates the approach of bad weather as the upper air strata in general is cooling the warm lowland air below its power to hold moisture. The cap then is the beginning of generally larger masses of clouds that eventually result in rain or, on the upper reaches of the mountain's flanks, snow. When it snows the moisture in the atmosphere has been chilled below the freezing point and quite often in this park the rain that we experience during the summer months has started as snow high above us. Most everyone has observed storms in which the moisture was falling was really very wet snow that melted before or as it struck the earth.

Cumulus clouds
Cumulus Clouds.

Often when you awaken in the morning you find that dew, in countless drops, covers the vegetation, jewels the strands of spider webs along the trails or in your garden and a walk in the early morning under these conditions will be very moist as you push your way through the vegetation of field or forest. The reason? Generally, as night approaches, the atmosphere becomes less restless and air movements cease. The heavier, colder air settles in the low areas and the earth's warmth is radiated into the upper air strata - the air close to the ground being greatly cooled by this combination. Then if the ground is moist the air immediately above, containing a great deal of moisture, gradually reaches a point when the moisture condenses and is deposited in droplets upon the vegetation. Frost is merely frozen dew brought about by the fact that the temperature during the night dropped below the freezing point.

Quite often in climbing Mt. Rainier we note a sea of fog stretching away toward the horizon upon leaving Camp Muir (10,000') where a few hours rest were taken during the early part of the night. Such a sight is a very beautiful one with the air above clear and sparkling and with only the tops of the higher peaks protruding like islands in a billowy sea. The air before we turned in at Camp Muir was clear in the valleys and all details of the topography visible and this make this fog bank even more remarkable to the climber. The same phenomena may occasionally be seen from Paradise Valley or Yakima Park and is caused by the warmth of the valleys is radiated through the clear air into space as the cold air drains downward from the more frigid mountain slopes above. This generally happens after a period of rain or at least when the air is heavily charged with water vapor and as this heavily charged air cools and the vapor precipitates out and a dense fog bank hugs the earth, leaving only the tops of the peaks exposed to the view of one standing above in the clear air. This fog is dispersed as the day advances again leaving the air in the valleys clear and bright.

Thus the basis of our weather is the energy derived from the sun. Land, water and air react differently to the effects of this energy. The earth, constantly revolving and tipping first from one side to the other as it follows its path about the sun gives us night and day and the various seasons. This widens the effect of the sun's energy upon the globe. The power of air to absorb, transport and deposit moisture accounts for our storms, clouds, sleet, dew, snow, etc. that are both the source and bane of our existence on this planet. But to quote these familiar lines we learn that -

"Whether its cold or whether its hot
We shall have weather, whether or not."

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