what causes different amounts of uv radiation to reach the earth's surface at different times?
450979
the condition of the o zone layer and aptmoshpere and the position of the light coming into the aptmoshpere
151400
Well, for one thing, the magnetic poles of the Earth are responsible for how much UV radiation reaches the ground.  So, the poles help regulate the amount and type of sunshine for a plant.  Another thing is that the the magnetic fields surrounding the planet affect the weather in a big way.  Weather affects plants.
127042
1. continental drifting\n\nVery slow but substantial changes.\n\n2. changes in sun's energy\n\nSuspected to be large, but not yet demonstrated conclusively.\n\n3. variations in the positions of the earth relative to the sun\n\nFairly small effect. \n\n4. volcaic eruptions\n\nLarge observable effects in global temperature etc. \n\n5.greenhouse effect\n\nKeeps the earth about 60 degrees F warmer than it would be otherwise.\n\n6. global warming\n\nIn the last hundred years, the earth has warmed measurably. It is not clear if this is due to human activities or not, but many suspect it is.\n\n7. air pollution\n\nHas increased the ozone depletion at the poles in springtime, which introduces more UV lower into the air column. This might be changing the climate, although it is not known for sure.\n\nAlso blocks sunlight locally near sources of soot. It is not clear how big an effect this has globally, but it might be a cooling factor. \n\nAcid rain produced by S02 emissions has been involved in some deforestation, which has probably had an effect on the climate, but again this is not known for sure.
1124409
because at that time it is below the horizon and its rays are reflected to us by different layers of atmosphere.while comoing to us it is diffused
966288
I remember this from college as being quite a long time, but I can't remember it exactly. This is from Wikipedia:\n\n"The high-energy photons (gamma and X-rays) released in fusion reactions take a long time to reach the Sun's surface, slowed down by the indirect path taken, as well as by constant absorption and reemission at lower energies in the solar mantle. Estimates of the "photon travel time" range from as much as 50 million years to as little as 17,000 years."
168153
Describes the Solar Radiation that falls on an area above the atmosphere at a vertical angle: s = 1.37 kW / m².\n\nIn space, solar radiation is practically constant; on earth it varies with the time of day and year as well as with the latitude and weather. The maximum value on earth is between 0.8 and 1.0 kW / m². In Germany, the average annual amount of insolation varies between 950 and 1100 kWh / m², depending on the region.
229495
the sun is not getting hotter out ozone layer is thinner so the suns rays are getting through more
66169
The sun's rays hit the Earth's atmosphere, where the light is scattered by nitrogen and oxygen molecules in the air. The blue wavelength of this light is affected more than the red and green wavelengths, causing the surrounding air to appear blue. At sunset, the sun's light passes farther through the atmosphere, deflecting and decreasing the blue in the air. Scattering by dust particles and pollution in the air causes the sunset to appear red.
475577
You would surmise, that since the sun is the source of the heat for the\nearth and atmosphere, that the atmosphere should be warmer at points closer\nto the sun, like at the TOP of the atmosphere, instead of the reverse.\n\n   But the atmosphere is not warmed directly by the sun...it is warmed by the\nEARTH...!  Most of the energy from the sun arrives at the earth in the form\nof light energy, or short-wave energy. The atmosphere is transparent to\nshort-wave energy, and most of the solar energy reaches the surface of the\nearth, where it is absorbed. This energy warms the surface of the earth,\nwhich, in turn, warms the atmosphere with long-wave energy. The type of\nenergy radiated by a star primarily depends on the temperature. So the\nenergy from the sun arrives at the earth as short-wave (in the visible part\nof the spectrum) energy, and leaves the earth as long-wave (in the infra-red\npart of the spectrum) energy.\n\n   The atmosphere is not transparent to long-wave radiation, and absorbs the\nheat energy from the earth. That part of the atmosphere closest to the earth\ngets the most energy and is the warmest.\n\n   Because the sun is nearly overhead in the latitudes nearest the equator,\nthose locations receive more solar energy per unit area, and become warmer\nthan those locations near the poles. This difference in the amount of energy\nreceived and subsequent heating of the atmosphere and evaporation of water\nfrom the lakes and oceans, is what drives the "weather machine" on the\nearth.
381000
sunspots are associated with solar wind, which is streams of energy being projected outward from the sun's surface. When these energy particles reach the earth they interact with the earth's magnetic field, creating static, and disrupt the normal layering of the atmosphere that radio waves bounce back from. Northern lights are another effect of solar wind hitting the earth.
559456
the Sun,and the earths Bio-mass.\nthe sun makes it hot (you may know that already)\nand the trees(bio-mass) absorb heat and release it .\nso regulating temperatures between day and night.\nthe ozone layer and aquifiris manta(atmosphere ,clouds,mist etc.)\nworks as a filter and determines how much sun(heat)is allowed to enter.
948640
At latitudes between 231/2 degrees north and 231/2 degrees south, the sun is directly overhead at noon twice a year. In these cases, the sun's rays shine directly down toward the surface. The radiation that reaches the atmosphere is therefore at its most intense. \n\nIn all other cases, the rays arrive at an angle to the surface and are therefore less intense. The closer a place is to the poles, the smaller the angle and therefore the less intense the radiation. Due to decreases in the intensity of radiation, average temperatures decline from the equator to the poles. Seasonal changes in solar radiation and the number of hours of sunlight also vary with latitude.
43990
sunspots \n\nDark spots on the surface of the sun caused by magnetic storms. \n\n\nThe number of sunspots goes through a maximum and minimum about every eleven years. During periods of maximum sunspots, the elementary particles associated with the spots cause disturbances in the atmosphere of the Earth and interfere with radio and television communication.
414202
(m)\n\nAs the sun begins to set, the light must travel farther through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes\n\nThe sky around the setting sun may take on many colors. The most spectacular shows occur when the air contains many small particles of dust or water. These particles reflect light in all directions. Then, as some of the light heads towards you, different amounts of the shorter wavelength colors are scattered out. You see the longer wavelengths, and the sky appears red, pink or orange
280135
Both reasons are correct.  The UV rays have less "stuff" to travel through in the air and atmosphere, so they have a stronger effect.\n\nAlso the snow does have a reflective and intensifying quality that it passes to the UV rays.
1137540
Like the other posters have said Australia does reciev greater amounts of UV light reaching our continent, however Australia is located near the equator which is another reason why its easier to get sunburnt than china.\nAs well as China conrtaining laot of photochemical smog and pollution which traps majority of sources of UV and produces more ozone.
814462
The sea, air, and land are all warmed by radiation (light, infrared, etc) from the sun during the day.  The earth also gives off radiation (infrared) all the time.  During the daytime the radiation from the sun is greater than the radiation given off by the earth's surface so the surface warms up.  At night, no radiation from the sun, but the earth is still radiating infrared, so it cools off.