The Weather Above Thunderstorms
In exploring the science of planet Earth, there are no shortages of strange phenomena to captivate your imagination. Many of the most amazing effects our atmosphere has to offer us occur outside of the realm of our daily weather. The atmosphere has many layers, and all the weather that affects us here on the ground is confined to the lowest layer. This is called the troposphere. The layer above that is the stratosphere; where the ozone layer is located. Above that there are several more layers that primarily interact with outer space and help to create phenomena like the aurora borealis, more commonly known as the northern lights. Daily weather doesn’t occur in these layers because the air is too thin. It’s like when you add sugar to iced tea without stirring it. All the heavy sugar clumps sink to the bottom and you have a thicker consistency of tea as you move to the bottom of the glass. The point is, weather in the upper atmosphere looks very different from what we are used to. In these upper layers, you’re mostly limited to chemical and electrical effects, but these effects can be spectacular. Here are a couple that are especially fascinating and don’t need expensive imaging equipment in order to see them.
We’ll start off with clouds that form far above the troposphere called noctilucent clouds or NLCs.
“Although NLCs look like they’re in space,” continues Thomas, “they’re really inside Earth’s atmosphere, in a layer called the mesosphere ranging from 50 to 85 km high.” (31-52 miles) The mesosphere is not only very cold (-125 C -193 F), but also very dry–”one hundred million times dryer than air from the Sahara desert.” Nevertheless, NLCs are made of water. The clouds consist of tiny ice crystals about the size of particles in cigarette smoke.
How ice crystals form in the arid mesosphere is the essential mystery of noctilucent clouds.
Ice crystals in clouds need two things to grow: water molecules and something for those molecules to stick to–dust, for example. Water gathering on dust to form droplets or ice crystals is a process called nucleation. It happens all the time in ordinary clouds.
While there are many sources of atmospheric “dust” available in the troposphere, such as sandstorms, pollutants, volcanic ash, and even bacteria, these sources rarely are able to reach the height of the mesosphere. Theories on what kick-starts the formation of these clouds ranges from space dust to the clouds being an effect of long term climate change. Like most high-atmosphere phenomena, we just don’t have enough information to know for sure.
Next we will move on to something a little bit more exciting. The power unleashed by thunderstorms is often underestimated or taken for granted. Lightning after all is many times hotter than the sun. But there is something else shooting out into the upper atmosphere from powerful thunderstorms. Pilots have seen them for years, but only in the 1990s were they finally confirmed by science. Considered somewhat a fantasy, they were given the name “sprites”.
Red sprites are difficult to observe because they last for just a few milliseconds and occur above thunderstorms—meaning they are usually blocked from view on the ground by the very clouds that produce them. They send pulses of electrical energy up toward the edge of space—the electrically charged layer known as the ionosphere—instead of down to Earth’s surface. They are rich with radio noise, and can sometimes occur in bunches.(http://visibleearth.nasa.gov/view.php?id=78487)
In addition to red sprites, there is also something called a blue jet, which looks like a geyser of blue light shooting out from the top of a storm.
You can try to observe sprites and blue jets for yourself by following these tips from researchers at the University of Alaska:
How to Look for Sprites and Jets
Via the University of Alaska
A clear view above a thunderstorm is required. This generally means the thunderstorm activity must be on the horizon. Additionally, there must be very little intervening cloud cover.
Best viewing distance from storm is 100-200 miles (200-300 km). At these distances sprites will subtend a vertical angular distance of 10-20 degrees. This is 2-4 times the separation of the pointer stars in the Big Dipper.
For observing sprites, it must be completely dark. (i. e. no longer twilight)
Eyes must be completely dark adapted. Use same criteria for this as for astronomical observing. If you can see the Milky Way, then it is probably dark enough and the eyes have adapted enough to see sprites.
Fix your gaze on the space above an active thunderstorm. Do not be distracted by underlying lightning activity in the storm. Block out the lightning if necessary using a piece of dark paper in such a way as to still being able to view what is going on above the cloud.
Sprites will be very brief flashes just on the edge of perceptability. They occur too quickly to follow with the eyes, but their strange vertically striated structure and dull red color may be perceived.
Patience will be rewarded. If the right kind of storm is present and one’s viewing geometry is favorable, then there is a greater likelihood of seeing a sprite than of seeing a shooting star or comet.
While their impact on our daily lives is nonexistent, noctilucent clouds, sprites, and jets are beautiful and fascinating. Click here for a video from the International Space Station of a sprite over Southeast Asia taken on April 30th of this year. If you want to see more awesome views of our planet from space, visit the NASA Visible Earth page at http://visibleearth.nasa.gov/
-Matt Gray, Weather Intern