What type of pyranometer does the National Weather Service use?

Instead of the Campbell-Stokes recorder, the NWS uses a device called the Marvin sunshine recorder. A more elaborate device, it consists of one clear and one blackened bulb connected by a thin glass tube partly filled with mercury. As sunlight warms the blackened bulb more than the clear one, the mercury expands and shorts out an electrical contract that then operates a pen that marks a chronograph.

Does the Sun emit a constant amount of energy?

The energy the Sun emits—technically called "solar irradiance"—currently is much more than it did when the solar system was young. Scientists estimate that, billions of years ago, our Sun only emitted about 75 percent of the energy it now does. Now, however, the amount of solar irradiance is fairly stable, though not completely constant. It can vary by as much as 0.1 percent, which may not sound like much, but such fluctuations are big enough to make the difference between moderate and frigid winters, merely warm or searing-hot summers. Solar irradiance can be affected by sunspots, but the internal workings of the Sun are still so complex that scientists do not fully understand what causes these fluctuations. Despite there being no complete consistency in the Sun's energy output, astronomers and meteorologists still refer to a "solar constant," which is the amount of energy of all emissions (measured in watts) per square meter. Readings are taken at the upper extremes of the Earth's atmosphere, which receives, on average, about 1,366 W/m2.

What U.S. cities receive the most sunlight annually?

Yuma, Arizona, is regarded as the sunniest city in the United States, with about 4,000 hours of sunlight every year. Below are some other sunny cities for those who are averse to rainfall. Because what constitutes a "sunny day" is debatable, the figures are estimates of the percentage of hours of annual sunshine, or what meteorologists call the "mean percentage of possible sunshine."

Sunniest U.S. Cities


Percent of Annual Sunshine

Yuma, AZ


Redding, CA


Las Vegas, NV


Tucson, AZ


Phoenix, AZ


El Paso, TX


Fresno, CA


Reno, NV


Bishop, CA


Santa Barbara, CA


Sacramento, CA


Bakersfield, CA


Flagstaff, AZ


Albuquerque, NM


Which state is known as the Sunshine State?

Florida has adopted the name of Sunshine State. Although it does have a warm, often sunny climate, it also has a pronounced rainy season from April through October. Ironically, Florida actually has more thunderstorms than any other U.S. state. Arizona actually deserves the credit of being the "true" sunshine state.

What is a person's circadian rhythm?

Human beings have evolved in a world where there are regular cycles of day and night, and we have thus developed a kind of internal clock that makes us want to sleep at night and be awake during the day. This occurs even when we are, say, underground or in an enclosed factory with no windows where there are no visual cues as to the time of day. The circadian rhythm controls such bodily functions as peristalsis (movement of food through the digestive tract, leading eventually to excretion), blood pressure, melatonin secretion, hormone levels, and alertness versus fatigue. In our modern world, people have tried to adjust to lifestyles that are not exactly commensurate with their circadian rhythms. Electricity and other sources of power allow us to work late at night, drive vehicles, and operate machinery at times when our bodies would rather be asleep. The result is that more accidents involving cars and factory equipment happen at night than during the day. We are also much less efficient at night, even when doing safe activities like balancing the checkbook.

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