In GLOBE, you use careful measurements to learn things about our environment just as scientists do. Also, some of the data you take for GLOBE is used by scientists. Finally, what you learn from GLOBE about how to take measurements will help if you have the chance to volunteer to help in a local science study run by another organization.

I recently had the chance to help survey a rock layer about 300 kilometers southeast of Boulder, Colorado. We were there to see whether a reddish-brown rock layer was level or tilted (sloped) in some direction. Since I study weather and climate rather than geology, I was there as a citizen scientist — a volunteer, rather than a professional. We took GPS readings of latitude, longitude, and elevation at the top of the reddish-brown layer. The top was fairly easy to see if the layers were visible, since the rock layer above it was white. You can easily see where the layers meet in the cliffs in Figure 1. We call where the layers meet the “contact.”

Figure 1. The rock layers we were following. The contact, or where the layers meet is at the top of the white walking stick (a ski pole) on the left side of the picture. The black line through the picture is where I taped two photographs together.However, most of the time the boundary (or “contact”) between the two layers wasn’t that easy to see. The rock layers were often buried under small pieces of rock. Then, we watched for areas where the small rocks had been washed away (Figure 2).

Figure 2. The same reddish-brown rock layer exposed on the hillside by a stream.We took GPS measurements at the contact between the two layers for about a kilometer along the east side of a canyon on the first day of the field trip. On the second day, we took similar readings along the west side of the canyon. Each time we found the contact, we would lay the GPS unit on the ground there, record the latitude, longitude, and elevation, and then walk until we could see the contact again.

Figure 3 shows the results of our measurements. Combining the measurements for the two days, we found that the top of the reddish-brown layer sloped downward toward the north with about a 3% slope (if you go north 100 meters, the top of the rock layer will be three meters lower). Two other groups were following the rock layer a few kilometers away. Both groups found that the rock layer sloped toward the north, by about the same amount.

Figure 3. GPS elevations along the east side of the canyon (23 September) and the west side (24 September). North is to the right of the graph, so the rock layer is lower to the north.

We were surprised that the GPS elevations worked so well:

• First, the elevations varied rapidly on our GPS units.
• Second, GPS experts tell me the elevation is less accurate than the location.
• Third, we only took one reading at a location (though I would try to average the GPS elevation when it varied).
• Finally, while our GPS elevations agreed well with the elevation of the rock layer from a topographic map on second day, the two elevations differed by slightly more than 20 meters the first day.

In the next few entries, I will describe why we were so successful.

Can you guess how hailstone size can be used to measure the strength of a storm? Here, “strength” refers to how fast the air moves upward in the storm: in other words, how strong the updraft is.

Hailstones grow until they are too heavy for the storm to hold them up. For hail to stay aloft, the air the hailstone falls through has to go upward at least as fast as the hailstone is falling downward relative to the air. As the hailstone gets bigger, its fall speed increases — so it will fall until it hits the ground — or finds air moving up fast enough to stop its fall or even carry it up again.

Careful Doppler radar studies of hailstorms and the appearance of hailstones show that the hail makes an interesting journey though the cloud before it falls. The hailstone in the picture has been sawed in half so that you can see what it looks like inside. Do you see the white and clear layers?

Hailstones start out small — as a single frozen raindrop or ice crystal; and, in one case, a fly! (Yes, I mean an actual insect!). Hail grows by getting coated with layers of ice (which “sticks” to the hail) or water (which freezes onto the hail) as it travels through the cloud. For example, raindrops freezing onto the hail form a clear layer, while ice crystals sticking to the hailstone form a white layer. So the hailstone provides a record of the type of precipitation it went through before falling to the ground.

Similarly, your shoes carry a record of where you were when you walk in the mud. As you clean off your shoes, the layers of mud will remind you of where you were. The light layer of mud came from near the school, and the darker layer of mud from near where you live. The mud on your shoes creates a record of where you walked just as the layers in a hailstone create a record of the conditions the hail has traveled through.

Giant hailstone that fell in Coffeyville, Kansas, showing alternating layers of clear and white ice. The fall speed of this hailstone was estimated to be 47 meters per second. Scientists don’t know what causes the “bumps” on the hailstone, but one hypothesis is that they grow like icicles from water flowing around the hailstone as it falls (remember — the hail is falling faster than the rain!). Picture courtesy of the National Center for Atmospheric Research.

Many books on weather show diagrams of hailstones making several up-and-down trips — perhaps riding the updraft up to where the ice is, then falling to where the rain is, and then getting caught up in an updraft (perhaps the same one), and then falling again. However, in parts of the cloud that are colder than freezing (remember, cumulonimbus clouds are very tall, so their higher parts have temperatures below freezing), there is liquid rain as well as ice and snow. Here, the hail can travel through alternating patches of ice and water without going up and down. Even at temperatures above freezing, the hailstone might travel through regions of melting ice particles as well as rain. Also, the outer layer of the hailstone could start to melt as it falls, and then refreeze as the hail is carried up to where it’s colder. It’s likely that hail travels in many odd ways before finally hitting the ground. Although scientists have some rough ideas of how hail forms and grows, there are many details to be filled in on exactly how.