Icicles and Watersheds: Part 1

Why are the icicles so long on our house?

On a recent walk just a day or two after our first snow, my husband and I noticed that we had the longest icicles in the neighborhood. Some houses built the same time as our house had icicles, but they were shorter. One new house had almost no icicles.

But what was the most fun, was our own house. The picture below shows our “champion” icicles.

Sketch of icicles on the east side of our house

Figure 1. Sketch of icicles on the east side of our house. The windows to the right of the icicles are about 1 meter high. The part to the right is the front part of the house; the part to the left is the back part of the house.

Notice that the icicles only cover the middle third of the side of the house. To the right and to the left, there are no icicles. Were we to walk on the roof, we would probably find the snow melted in the middle third of the roof, but not on the sides.

Why? Our house was built in stages. The front two-thirds were built were built in 1950. There was little insulation in the roof. A few months before I made this sketch, we tore out the old ceiling in the room in the front of the house and found that the insulation from 1950 was in poor condition, just like the insulation in the middle of house. The new insulation was much better. The picture confirms that the new insulation was working. No icicles implies no water from melting snow. This means that little heat was escaping through the roof, so there was little or no snowmelt on the roof.

Similarly, the back part of the house was built in 1979. When that part of the house was built, we made sure we had good thick insulation in the roof. There are no icicles on the new part of the house. Again – the insulation must be working.

Using the data from our house, can we explain why our house had the longest icicles? I’m guessing that the new house in our neighborhood that had almost no icicles had good insulation – just like the newer parts of our house and the room we just insulated. We could that the snow on the roof of the new house was fairly deep – there was little melting.

What about the older houses with shorter icicles? Let’s imagine an older house with about the same insulation as the old parts of our house (Figure 2). If this is true, the snow would melt at about the same rate (I am assuming that the roof was exposed to the same amount of sunlight per unit area). Why then would the icicles be shorter on the other (imaginary) house?

If you believe my assumptions, the answer is that the area of the roof “draining” toward the eaves (where the icicles grow) was smaller. Say the distance from the top to the icicles on our imaginary house is 5 meters, and the distance on our house is 10 meters. As the melted snow moves down from the top of the roof to the eaves, twice as much water reaches a given length along the eaves for the 10-meter roof (ours) compared to the five-meter roof. It follows that the icicles on our house would contain twice as much water and be longer than on the other house. The icicles may be not twice as long, because the icicles we had might be wider as well as longer.

View of a slice of our house (top)

Figure 2. View of a one-meter slice of our house (top) and an imaginary neighborhood house (bottom). More water is available to flow over the eaves for our house. We are looking at the two houses from the north.

So the amount of water in the icicles is determined by the amount of snow upstream of (or straight up the roof from) the eaves.

This entry was posted in Backyard Science, Hydrology, Watersheds. Bookmark the permalink.