Blog Entry: November 16, 2010
Sunday's mix of rain and sunshine at Whitman Mission made for some great photography conditions. Even more exciting was the appearance of a rare and spectacular atmospheric phenomenon over the park, iridescent clouds!
While the appearance of iridescent clouds may immediately make you think of rainbows, the science behind them is actually completely different. Unlike rainbows which form as a result of refraction (sunlight bending as it passes through large water droplets), iridescent clouds form as a result of diffraction. Diffraction occurs when a wave of light encounters an obstacle similar in size to the wavelength of the light and bends around it. This is akin to seeing a rainbow of colors reflected off of a puddle of oil or soap. Since the wavelength of light is small (less than 1/1,000,000th of a meter), we therefore need clouds that consist of very small water droplets in order to form iridescent clouds. The clouds also need to be extremely thin otherwise the diffracted light is trapped inside the cloud and won't reach our eyes. Different colors of light are bent at different angles which is why we see a series of alternating colors in the clouds. If the Sun is in just the right position, the iridescence can sometimes take the form of concentric circles around the Sun, when this happens we call it a "solar corona."
Cloud iridescence is faint and often requires placing the sun behind some sort of obstacle (in this case, the Whitman Memorial Shaft) in order to keep the bright sunlight fro washing it out. However, these iridescent clouds were actually bright enough that I could see them easily with my naked eye without having to position the sun behind an obstacle. A final word of caution: be extremely careful when taking pictures towards the Sun. Looking directly at the Sun through a camera viewfinder for even short amounts of time can cause eye damage due the invisible UV radiation that is constantly slamming into your eyeball. Never look at the Sun through a telephoto lens and you'll also want to keep to short exposures so as not to damage the sensor in your camera.