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applestar
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Why is sunlight stronger at higher elevation?

Why is sunlight stronger at higher elevation? Both jal_ut and Lorax have mentioned this.

Its obviously not the distance to the sun. Air is "thinner"? But that term refers to availability of oxygen right? Well, less pollution, So I suppose clearer and less particulates to obscure compared to sunlight reaching lower elevations, including natural ones -- water vapors, pollen, spores, dust in general? (except when the volcano is erupting....)

Sun reaches the higher elevations earlier at sunrise and until later at sunset, too, hmmm.

Does less oxygen mean more of the other gasses like co2 and N or are there less in equal proportions?

To sum up, what do all these mean for growing a garden?

Dillbert
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if you're talking serious elevations - like the Rockies -

the clouds go around the mountains - or get pushed up and over the mountains - dumping their water as they chill out at the higher elevation - moisture btw _is_ the cloud....

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lorax
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I can speak for the serious altitudes of the Andes, where I live. Our ozone layer is thicker than over any other high elevation (yay, equator!) but we're pushing into the middle layers of the troposphere, where there is less moisture (since the mountains tend to strip the clouds, as Dillbert mentions) and less oxygen. Our atmospheric pressure is also greatly reduced (I experience swelling when I travel from the coast to Ambato - there's literally less holding me in up here).

What it means for gardeing is threefold: first of all, because of reduced atmosphere (I live about 3 km up) the sunlight hits us faster than it does at sea level, and there's less atmosphere actually there to impede the light and heat. This means that daytime temperatures can be extreme, even mitigated by the fact that it's cooler up this high. I can have 40 C days easily (it's shaping up to be one now!). However, that same effect works against us at night, meaning that it cools off quickly and fairly drastically - 5-10 C is a normal overnight. This means that whatever you grow has to be both heat and cold resistant. The plants that are found naturally in high-altitude environments, cacti included, often have white or silver fur (look up Espeletia, an Ecuadorian endemic, for a neat example) to act as natural sunscreen and insulation. Plants that don't naturally have that, like say tomatoes, need special treatment in order to deal with the sun's effects. Shadecloth and greenhouses are used in my area, which is where some 90% of all tomatoes for the country are grown - they die in full sun. You also get the odd effect I mentioned where water droplets on leaves can cause focussed sunburn, which means that most irrigation up here is by drip, rather than overhead.

The Andes in particular also generate their own unique wind systems, and in my part of the chain at least, that's driven by the volcanoes and the shape of the mountains. When the "seasons" change, we can get extreme winds of over 100 kph, and that means that whatever you're growing also has to be wind resistant, or you have to figure out a way to support it when it does get blowing. Volcanic eruption wreaks havoc at high altitudes, as the quantities of ash produced are enough to give us winter weather (cold, wet) even in the deserts.

Thirdly, at very high altitudes below the snowline, it either rains all the time (wet paramo) or it doesn't hardly rain at all (altitude desert - where I live). Depending on where you are, that means you've got to ensure insane drainage or grow xeric-friendly plants and irrigate like a madwoman. Drip systems are the only really good option in the deserts, and in the wet paramo people generally grow water-hungry and cold-tolerant crops, like squash and beans.

There are some things I can't grow at all up here in the rare air - pineapples are grumpy and refuse to fruit (even in the greenhouse, so it's not a temperature issue) and there are numerous fruit trees that can't stand the cold overnight temperatures. However, in terms of oxygen, there's nothing doing - if it will grow for me up here, it's not because the plant is more tolerant of rarified air (plants produce O2, remember?) but because it's more tolerant of the extreme conditions. And the altitude actually works for me - I'm in the only location on the equator where stone fruit can be grown, because we get cool enough nighttimes for the trees to be comfy. That means I've got plums and peaches in my yard (blooming right now, actually), something that I couldn't really do anywhere else in the country - my province is famous for its stone fruits, and my city is sometimes called The Big Peach.

Ohio Tiller
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I don't know why but I do know that several years ago I was out in Idaho Elk hunting and one day at camp it was very warm and I had just got a mid day solar shower. (They are not a warm shower) The sun felt real good so I sat down in the chair with out a shirt on and laid back to soak up the warm sun. 15 minutes later I was burn pretty darn good!

Bobberman
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Relected light from snow and closer clouds meets the incoming light and intensifies it at the higher elevations!
I enjoy fishing ,gardening and a solar greenhouse! carpet installation repair and sales for over 45 years! I am the inventor of the Bobber With A Brain - Fishing Bobber!

lily51
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You were on the right track... Less atmosphere (not just O2) to disperse the UV radiation, so it is more intense.

Bobberman
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Less atmosphere also means less water vapor in the air to stop the light! Lower elevations have lots of moisture in the air! so the light hits the water and actually heats it some!
I enjoy fishing ,gardening and a solar greenhouse! carpet installation repair and sales for over 45 years! I am the inventor of the Bobber With A Brain - Fishing Bobber!

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tomf
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As there is less atmosphere between you and the sun there will be more of the sun's energy hitting you. The thin air does not hold the heat as well as the thicker air down bellow although. Death Valley is below sea level and there for has a denser atmosphere, this makes it very hot. The dense air holds more heat.
Last edited by tomf on Sat Sep 22, 2012 3:44 am, edited 1 time in total.

lily51
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There's a difference between heat and intensity of UV light. Heat takes into account the mass of material in addition to temperature. You can have very intense UV rays without the warmth, as anyone who has had a sunburn in winter will attest.

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jal_ut
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Several factors. Less air for the sunlight to pass through.

Also at this 5000 foot altitude in Utah, we usually have relative humidity of around 15 - 30%. Water in the air acts as a screen too. Low humidity means brighter light.

Yes, air is thinner at higher altitudes. That means less of all the gasses that make up air. Less molecules of gas in each square foot of air. Compare it to clear windows or tinted ones in your vehicle.

Yes, less pollution will make the sunlight stronger too.

Ohio Tiller has it right. It only takes 15 minutes to get a burn.

We also sometimes have extremes of temperature swing too. Over 50 degrees in a 24 hour period is common.

What does it mean to the plants? Most plants seem to get along fine with the intense bright sunlight. Light is, after all, the power that makes plants grow.

You must be really careful bringing out hothouse transplants though, and carefully harden them off. The bright light can put them down in one afternoon.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-

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jal_ut
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You can have very intense UV rays without the warmth, as anyone who has had a sunburn in winter will attest.
Ya, if you go out in winter after a storm and the skies are clear as a bell and the precipitation has removed the pollution, and the bright sun is reflected from the new snow, you can get a sun burn faster than in summer.

If you are going skiing, wear sunscreen on any exposed skin.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-

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