For years, I have been measuring the rain in our back yard using a standard rain gauge similar to the ones used by the U.S. National Weather Service (Figure 1). Like the gauge used by GLOBE students, rain goes through a funnel into a tube whose horizontal cross-sectional area is one-tenth that of the outer gauge, so that the measured rain is ten times the actual amount of rainfall. This year, I took a GLOBE-approved plastic gauge home. We put this one on a fence along the east side of our yard (Figure 2).

Figure 1. Rain gauge used for observations in my backyard. Normally, there is a funnel and small tube inside, but it doesn’t fit very well, so we pour the rain into the small tube after each rain event. This gauge is similar to those used by the U.S. National Weather Service. This gauge is about 25 cm in diameter.

Figure 2. Plastic raingauge matching GLOBE specs. This gauge is about 12 cm in diameter. Note the tall tree in the background.

Neither gauge is in an ideal location. In both cases, there are nearby trees (Fig. 2, map) which might impact the measuring of the rain. This is a problem a lot of schools have: there is just no ideal place to put a rain gauge. We were particularly worried about the plastic gauge, which was closer to trees than the metal gauge.

Why do we have two gauges? The metal gauge was hard to use: its funnel didn’t fit easily into the gauge, so we had to pour the rain from the large gauge into the small tube after every rainfall event. We got the plastic gauge to replace the metal one. We put the gauge on the fence because it was well-secured. But the first six months we used the new gauge, the rainfall seemed too low compared to totals in other parts of Boulder. So, I put the metal gauge back outside and started comparing rainfall data.

Figure 3. Map of our backyard. Left to right (west to east), the yard is about 22 meters across. The brown rectangular shape is our house; the circles represent trees and bushes. The numbers denote the height of the trees and bushes. The 10-m tree is an evergreen; the remaining trees and bushes are deciduous. The southeast corner of the house is about 3 m high.

How did the gauges compare?

Starting this summer, I started taking data from both gauges. Unfortunately, it didn’t rain much. And sometimes, we were away from home: so this is not a complete record. But I don’t need a complete record to compare the rain gauges.

Table: Rain measurements from the two rain gauges

The results (in the table, also plotted in Figure 4) look pretty good. With the exception of the one “wild” point on 6 October 2008, the measurements are close to one another. We think that the plastic gauge was filled when the garden or lawn next door was watered. This would not be surprising: we have found rain in the plastic gauge when there was no rain at all.

I learned after writing this blog that Nolan Doeskin of CoCoRaHS (www.cocorahs.org) has compared these two types of gauges for 12 years, finding that the plastic gauge measures slightly more rain (1 cm out of 38 cm per year, or about 2.6%).

Figure 4. Comparison of rainfall from the two rain gauges in our back yard. Points fall on the diagonal line for perfect agreement.

I learned two things from this exercise.

First, I probably should have used the two gauges before I stopped using the metal one. That way, my rainfall record wouldn’t be interrupted if the new gauge was totally wrong. (I was worried that the trees were keeping some rain from falling into the gauge. This would have led to the plastic gauge having less rainfall than the metal gauge. And, since the blockage by the trees would depend on wind direction and time of year, I wouldn’t have been able to simply add a correction to the readings.) Fortunately, the new and old gauges agreed.

In the same way, if you want to replace an old thermometer with a new one, it’s good to take measurements with both for awhile, preferably in the same shelter. Suppose the new thermometer gives higher temperatures than the old one. If you want to know the temperature trend, you can correct the temperatures for one of the two so that the readings are consistent.

The second thing I learned is that it is o.k. to reject data if there is a good reason (such as people watering their lawns). It’s also important to note things going wrong – like my spilling a little bit of water on 15 August. If you keep track of things going slightly wrong (or neighbors watering the lawn), you can often figure out why numbers don’t fit the pattern.

I will continue to compare records for awhile, to see whether the readings are close to one another on windy days. If they continue to be similar, I will be able to try a method to keep birds away from the rain gauge that was developed by a GLOBE teacher – Sister Shirley Boucher in Alabama. Keep posted!

(If you are interested in the Pole to Pole videoconference, just scroll down – it’s just below this one. I’m finishing up the puddles blog so that I can write a blog or two on inquiry, using the puddles as my example).

As I was proofreading the puddles blog upon returning to Colorado, I started wondering if the puddle simply had been left behind from the previous week’s rains, and that salt may have kept the puddle from freezing.

I had the opportunity to check this last week, on a second trip to Missouri. Again, there had been rain a few days before I arrived. And again, there was a puddle in the same place. But this time I could see clearly that water was flowing into the puddle (and other places along the road) from gaps in the curb as well as some in the street. You can see this in Figures 11 and 12.

Figure 11. A new puddle (photographed 19 March) at the same location of the one photographed in February, in Columbia, Missouri. Note that water is leaking through a gap in the curb as well as part of the crack.

I also discovered that the puddle was not in a dip in the road, as I had suspected earlier, but it was located in a place the road was nearly horizontal (okay, maybe a very shallow drip): There was actually some flow downhill toward the lowest spot, where water drained into a sewer. Finally, I discovered that the puddle is only about 2 meters (6.6 feet) above the lake.

Figure 12. Closeup of the puddle.

There were other puddles along the road, formed from drainage through gaps in the curb and sometimes gaps in the pavement of the road (most of the cracks in the roadbed are sealed with tar).

After a few days with temperatures rising to around 15 degrees Celsius (59 degrees Fahrenheit, the puddle finally disappeared. Where the water was, a white stain on the road revealed that salt had collected there; and there was drier soil carried along with the water feeding the puddle.

Another day with no puddles convinced me that the pipe connecting the fire hydrants (see earlier parts of this blog) was not leaking.

So, with a little extra data I was able to confirm the hypothesis that the puddle was being fed by subsurface water flowing at least through a gap in the curb (which is ~15 centimeters or 6 inches high) and possibly the crack in the road. Salt clearly also played a role in keeping the water from freezing.

I also found out something else. My brother and sister-in-law’s house was heated and cooled by pumping groundwater up to the house. Remember, the temperature 30 meters (100 feet) down – or even 10 meters (30 feet) down – is close to the average temperature for the whole year (in Columbia, about 13 degrees Celsius or 55 degrees Fahrenheit). So the water pumped up to the surface in the summer will be much cooler than the air temperature, and thus can be used to cool the house. In the winter, the ground water is almost always warmer than the house, so it can be pumped up to warm the house.

But remember – the temperature of the ground water – and the average temperature – is about 13 degrees Celsius (55 degrees Fahrenheit). That’s not warm enough to heat the house in winter, so another method is needed to bring the temperature up from 13 degrees to a more comfortable 20 degrees Celsius (68 degrees Fahrenheit) or so.

Next time: how the investigation of this puddle illustrates the inquiry process – or the “scientific method.”

Based on the last two blogs, the evidence seems strong that the puddle was being fed by liquid water coming from underground springs. A colleague of mine, Kristina Katsaros, pointed out that even a spring-fed puddle might have frozen under such conditions. Maybe the water would freeze on the top, for example.

Kristina has studied Arctic ice and the effects of salt in the sea water on freezing. So she suggested that another factor be considered: salt on the roads. This would be a modification of the spring-water hypothesis, to allow for the effect of salt.

In the United States, road crews often apply sodium chloride or magnesium chloride to roads because these two compounds can melt snow or ice on the roads, making them safer to drive on. This happens because these two compounds dissolve in water. Salty water has a lower freezing temperature than pure water.

On my trip out from my brother and sister-in-law’s house to take a picture of the puddle, the road was solid white. But coming back, I noticed something interesting: The ice and snow was starting to melt in little spots.

Figure 6. Ice and snow melting in spots on road.

When I looked more closely, I discovered a salt crystal at the center of each spot, surrounded by melting snow and ice. You can see a few of the crystals and their impact on the surrounding ice in Figure 7, below.

Figure 7. Salt crystal at the center of widening circles of melting ice and snow. The salt crystal is dissolving in the water, lowering its melting point.

If they spread salt crystals on the road the morning I took most of the puddle pictures (the fire hydrant picture was taken later in the day), I am guessing that salt was used on the road earlier in the winter as well.

So the puddle that I think was spring-fed may have been able to remain liquid thanks to salt on the road. And, since the puddle was in a low part of the road, salt may have washed down to this part of the road during the rains of the previous week.

So – it is likely that the puddle formed from underground water seeping into the road (built with cracks to allow the concrete to expand and contract), and the puddle stayed liquid because of a supply of warmer water, and a little salt. There are of course many more things I could do to confirm this hypothesis, but it seems reasonable given the facts I have available.

Suppose the puddle had been there from the previous week’s rains, and I simply missed it. Then the salt in the road might be sufficient to keep the puddle from freezing entirely.

This is the second in a series about an unusual winter puddle in Columbia, Missouri.

Recall from last time that I mentioned that the water feeding the puddle would be coming to the surface from under the ground – either a broken pipe or water flowing horizontally through the soil

In my recent 27 February blog about the air temperature and the surface temperature , I wrote about the “energy budget” of air about 1.5 meters above the surface (to explain why the maximum temperature was in the late afternoon), and I also wrote about the surface temperature, which reaches a maximum in the early afternoon.

To understand why the water feeding the puddle (and the surrounding soil) wasn’t frozen, we need to learn something about how temperature varies with depth beneath the surface.

The heating and cooling below ground is mostly by conduction. During the winter, the surface vegetation protects the soil from cooling, and upper soil layers protect the soil layers farther down.

For example, Figure 3 compares the air temperature and the temperature just 2 cm below the surface on a corn/soybean farm near Champaign, Illinois, USA (near Chicago). While the air temperature fluctuates quite a bit, the temperature at 2 centimeters below the surface changes much less. Particularly interesting is the cold weather between about 25 February and 5 March, when the soil temperature stayed warm in spite of the cold night-time temperatures. Figure 4 focuses more closely on that time period.

Figures 3 For February and March 2002, air temperature and soil temperature (Ts) at 2 cm below the surface, a corn/soybean farm near Champaign, Illinois (Latitude 40.00621, Longitude -88.29041). Day 30 = 30 January; Day 60 – 1 March, Day 90 – 31 March). Data available on the Web at http://cdiac.ornl.gov/ftp/ameriflux/data/Level1/Sites_ByName/Bondville/FLUX-2002/.

Figure 4. For the 25 Feburary-6 March time period on Figure 5, Air temperature, surface temperature, and soil temperatures down to 64 cm below the ground.

Notice that the soil temperature gets warmer as you go down, illustrating the “insulation” effect of the higher layers of soil. From Figure 5, we see a similar pattern at Smileyberg, Kansas. On average, the soil temperature at Smileyberg was 1.9 degrees Celsius warmer than the air temperature in January, and 1.1 degrees warmer than the air temperature in February. Notice how the ground stayed warm between Days 24 and 32 (24 January and 1 February), in spite of the cold temperatures. This is just like the behavior we saw at the Bondville site.

Figure 5. For grassland site near Smileyberg in Southeast Kansas, the Air temperature (about 2 m) and soil temperature (average 0-5 cm). Data from Argonne National Laboratory, courtesy R.E. Coulter, Argonne National Laboratory.

Thus it is quite believable that there could be liquid water close to the surface, particularly since the air was much warmer the week before I got to Missouri. (Since I saw no frozen water on the surface uphill of the road, the water could have come up through cracks in the road.)

Mostly written 21 February 2008, from Columbia, Missouri, USA

The temperature for the last few days has been below -5°C (about 20°F). The wind on my daily walks is cold but invigorating.

So, I was surprised yesterday when we drove over a puddle and water splashed on our windshield. It froze instantly. Given the air temperature, this is not surprising. The car thermometer read 17°F (-8°C).

How could there be water in a puddle after three days of subfreezing temperatures?

I decided to investigate on this morning’s walk, and found out a few interesting facts. Figures 1 and 2 show the puddle up close and from a distance. My footprints and the tire tracks in Figure 1 indicate that the puddle was “slushy.” It was easy to make footprints in it. So it is not surprising that passing cars were getting splashed when they drove over the puddle. The second picture shows the puddle with a fire hydrant on the north side of the road (and to the east of the puddle). A line of fire hydrants lies to the north of this road; so I conclude there is a pipe connecting them.

Figure 1. Slushy puddle with footprints (left) and tire track (right). At the time of the picture, the air temperature at this location had been below freezing for several days.

Figure 2. Picture of puddle taken later in the day, after more frozen precipitation (ice pellets) covered the rest of the road with white. Note the red fire hydrant. The puddle is at a low point in the road, and the ground slopes downward toward the puddle from the north.

So, I came away with two hypotheses.

First hypothesis. The puddle is being fed from extremely wet soil. There has been a lot of precipitation around here recently. I knew this because I had been monitoring the weather the week before I got here. I am guessing that the soil is saturated and the water table is quite high.

So, the water could just be flowing downhill, perhaps atop the bedrock, which comes quite close to the surface. Or, the puddle could be fed by an underground spring. There are many springs in this part of the country. The bedrock, close to the surface, is Burlington limestone, which has multiple cracks and caves – paths for the water to follow.

Second hypothesis. The pipe connecting the fire hydrants just happened to have been broken here.

These two hypotheses are based on my impression that the puddle wasn’t there before 20 February. In either case, as we shall see, salt added on the road could have kept the water from freezing once it reached the surface.

I talked to my nephew, who was around when the road was built, and he supported the first hypothesis, because they found a lot of springs when they built the road. The springs are fairly active: my sister-in-law said that swimmers in a lake to the south of the road frequently noticed cool spots, where the cool spring water was feeding the lake. The springs also suggest that the water table is normally high. So, after an unusually wet period, it would be plausible that the water is running down the hillside beneath the surface.

Next time: Why wouldn’t the water running down the hillside beneath the surface be frozen?