I’m working the Visitor Center in Tuolumne Meadows on a Saturday morning in mid-August; the overcast skies have been spewing drizzles for the last two days. Abruptly, the minute droplets of water turn to wet snow and the winds are perceptibly colder. Presently a visitor comes in and wants to know, “How long will this snow keep up?” or “When will this stuff quit and we get some decent weather?” I feel the heavy load of responsibility placed on me by the faithfully believing visitor. I wonder what has transformed the normal skepticism for weather predictors. Nevertheless, I give my questioners the very best and latest information on the weather.

We get our weather reports from the Fresno office of the National Weather Service. We then use our own knowledge of local weather conditions and mix in a little information gained through experience in these high mountains to interpret the official weather report for our visitors. If the official report states that a possibility exists for thunder showers in the high country, we interpret that to mean it is likely to rain in Tuolumne Meadows and the general region. If, as in the case above, the official weather report indicates that thunder showers will continue through Monday, we must interpret that to mean it will continue snowing till Monday. Our high altitude and generally lower temperatures can turn the lowlands rain into snow at any time of the year.

The job of predicting the weather is complicated by many variables, both on the world and Yosemite scales. We all know that weather is caused by the uneven heating of the earth’s surface by the sun. Weather flows from west to east because of the earth’s rotation. So Yosemite weather generally comes from the Pacific Ocean. I say generally because all weather is affected more by the migration of warm tropical air from the Equator than that at higher latitudes; air rising from those areas has a greater eastward momentum than the surface of the planet, and these are called the Westerlies (the southwesterlies in the Northern Hemisphere and the northwesterlies in the Southern Hemisphere). The great difference between the temperatures of the tropics and the polar regions causes a much faster flow of warm air toward the poles. The difference is greatest in winter and causes the high-altitude air streams, called jet streams, to move faster in winter. This is why the airline flights from San Francisco to New York take less time in the winter than in the summer. The faster moving high altitude winds also set up faster moving winds at lower altitudes. Although the difference between the tropical and polar temperatures is even greater in the summer, that difference is modified by the mild temperatures of the temperate areas of the continent (the U.S. and Southern Canada) caused by the tilt of the earth in its swing about the sun. Still, the high altitude winds have a great effect on our weather in the summer.

One such effect is the way the winds come over the mountains. The main crest of the Sierra Nevada runs from north to south, but there are lateral “fingers”, or ranges, running east and west, such as the Cathedral and Clark Ranges. When the strong winds that carry our weather blow over the high crests and ridges of these mountains they are “squeezed” and pour downward into the mountain meadows. This rapid influx of winds, often moisture laden, causes the air mass in the lower regions to be compressed and sends it flowing in any direction possible. So, though we say our weather comes from the southwest, it is not unusual to have ground winds blowing directly opposite to the major storm front moving into the area.

As mentioned earlier, weather is generated by the sun’s heat striking the earth. This seems simple enough, especially when you consider that there are relatively few of its elements to consider. But the land masses, the oceans, ice and snow, plants and animals, and the atmosphere are all so intricately interacting that without long experience, information from all over the world, and the use of modern computers, the modern meteorlogist would have to give up in despair. In all cases, weather is modified as the energy of the sun, heat, is absorbed or reflected and as the heat is transferred from one part of the earth to another. This reflection or absorption of the sun’s heat has tremendous implications for local predictions in Yosemite.

About seventy percent of the sun’s radiation that falls on the earth is absorbed by the earth, and more than half of that is absorbed by the oceans. Some is reflected back into space by clouds, ice and snow, various land surfaces, dusts and other particles in the atmosphere. The earth maintains its balance of heat by radiating the rest back into space as infrafred radiation. Water dominates the weather system of the world, not only by absorbing more than half of the sun’s heat, but also by serving as a heat reservoir. It gives up this heat as vapor which condenses to form rain or snow. The clouds generated by the heating of the oceans capture almost two-thirds of the solar radiation being reflected from the earth. In yet another function, clouds may also affect or modify the weather.

This modification may occur when an ocean current transfers heat to the atmosphere by producing clouds. The cloud in turn shades the ocean current and cools it locally, thus reducing the amount of heat transmitted to the atmosphere. The shading and cooling effect also acts to reduce the amount of heat the current absorbs from the sun. If such systems were not countered by other factors such as differential heating, opposing ocean currents, and the depth of the ocean, the systems would run away with themselves.

In Yosemite, we can see how water dominates the weather to some degree. The fall of snow will increase the reflection of sunlight from a relatively small area. The heat of the sun being bounced back into the sky will lift the moisture laden winds higher into the atmosphere. This cools the moisture, causing it to condense and then to fall as more snow. Besides the snow on the ground to cause rising air currents, Yosemite has immense areas of light colored rocks, the granite, to promote this reflection. These granites act in the same manner as the snow cover in winter.

In summer, the green of the forests modifies the granite reflection, but a great deal of reflective material still causes rising currents. Those currents act on the winds which carry moisture. The tremendously unstable air rushing down on a variety of rising and falling currents is wafted higher into the atmosphere and is again cooled, condensed, and caused to fall from the skies.

In summer, the moisture often falls as rain in Tuolumne Meadows. Due to the generally cooler temperatures, the rain can fall as snow at any time of the year. So I often don’t know what to tell the visitor who wants a really specific prediction.

One of our old time rangers has a special way with such requests. He points to whatever clouds are in the sky at the time. He looks at them in a meaningful way and says, “You see those clouds?” When the visitor nods yes, he says, “You just watch those clouds. Those clouds are very significant.” Then ominously he continues, “You just watch those clouds and you’ll see.” And then he’ll leave them to wonder at the significant clouds. And without ever making an actual prediction he is gaining quite a reputation as a weatherman. For whether it be rain, snow or sunshine the next day, the visitor concludes, “That ranger was right, those clouds were really significant!”

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