Unusual Weather Phenomena Photo Gallery Part 2
2008-05-23 15:50:10.000 – Matthew Morin, Space Grant Intern
Halo & Cloud Irredescence
One of the benefits of being on top of a summit 6,300 feet above sea level is that you have a 360° view that stretches as far as the horizon. When the atmosphere is clear of clouds and haze, one can spot the mountains in southern Canada as well as the sun’s reflection off the Atlantic Ocean. Another bonus of potentially having visibilities up to 135 miles is the ability to see a plethora of weather phenomena. I didn’t come to realize this fact until I went through a slideshow of the all the pictures I took while interning at the Mount Washington Observatory. Here is part 2 of my Unusual Weather Phenomena Photo Gallery.
Basically, virga is any form of precipitation that doesn’t reach the ground. The conditions needed for virga to take place are a precipitating cloud with drier air below. As the precipitation falls from the cloud, it enters a different airmass which is warmer and/or drier than where it originated. Eventually the falling precipitation reaches a level where it evaporates and disappearing from our view. Sheets of gray that fade off towards the ground are not so uncommon around here. So why am I considering virga an unusual weather phenomenon? It’s because it is indeed an unusual sight for some people living in valleys and other areas closer to sea level. Virga is more easily seen from a distance which is difficult when trees, buildings, and higher terrain obstruct one’s view.
Rime ice, friend or foe? I say “friend”, though seasoned weather observers here might say “foe”. Rime ice is one of my favorite features of Mount Washington. We are in fact known for extreme riming conditions, especially in the transition seasons of fall and spring where there is ample moisture being transported by high winds in a below freezing airmass. Rime ice is basically freezing fog that forms when supercooled water droplets in the air hit a surface and then freeze instantly on contact. The rapid freezing traps in air which gives rime ice a milky color. Rime ice is very interesting to look at because it usually takes on a feathery or spiky appearance as it grows on exposed objects such as poles, railings, and even our weather instruments! During riming conditions, observers have to climb up to the tower and knock the rime ice off of the wind measuring instruments. During moderate to heavy riming, deicing of the instruments usually has to be done hourly or sometimes even more frequent. Deicing prevents the instruments from seizing up and/or reporting bad data. Luckily, rime ice is easy to knock off due to the light and fluffy nature of its formation.
I still get excited every time I see a halo around the sun, though my friends tend to give me strange looks. A halo is seen around the sun (or moon or any bright source of light) under specific atmospheric conditions. Naturally, the sky has to be mainly clear of clouds which would otherwise obstruct the view of the observer. Additionally, there has to be ice crystals in the upper atmosphere which reflect, refract, and sometimes split incoming sunlight into colors similar to a rainbow. Halos are unusual because the ice crystals have to have a particular shape and orientation in order for the observer to see a bright ring around the light source. Besides being nice to look at, a halo signifies that moisture is moving in high up in the atmosphere. Sometimes this could be the precursor to a warm frontal passage. Therefore, Mount Washington Observers note “HALO AROUND SUN” in the remarks section of the weather report sent out to the National Weather Service.
Thankfully, I have even more interesting weather pictures to show and talk about. So stay tuned for part 3 of my Unusual Weather Phenomena Photo Gallery coming in June!
Matthew Morin, Space Grant Intern