The current method that the CoCoRaHS project uses to monitor hail activity, the Hail Pad is a simple but effective way to record the size, number, and directionality of hail stones. Apparently the materials that are being used to construct them however are becoming cost prohibitive.

I conducted 2 experiments attempting to identify methods to reduce the cost of making 1 square foot Hail Pads, my findings are documented in the following steps.

Step 1: $.60 Each


Buying individual precut pieces of styrofoam is not cost effective in a mass production scenario. A variety of polystyrene bulk sheet products are readily available either from local building supply stores, or online. Cutting polystyrene is easy and can be done using a variety of methods.

Individual Precut 12" X 12" X 1" = $3 to $6 each | Bulk Sheet 32 square feet X 3/4" = $8 = $.25 each

Cutting Methods:

  1. Utility Knife & Straight Edge (rough edges, messy)
  2. Circular Saw, Table Saw, or Band Saw (flat edges, fast, but messy)
  3. Hot Wire Cutter (flat edges, fast, no mess)

Aluminum Foil

When cost per unit is a priority consideration, comparative shopping and a willingness to choose a generic instead of brand name is helpful.

Reynolds rey632 750 square feet = $135 = $.45 each | Generic Extra Heavy Duty Foil 750 square feet = $46 = $.14 (prices don't include S&H)

Packaging Tape

You pay more for convenience, buying small rolls of tape on a disposable dispencer with cutter is not cost effective in a mass production scenario. Bulk rolls can be dispenced by hand, and cut with scissors or a knife.

Disposable Dispencer 66' = $3 = $.05/foot | Bulk Roll  327' = $4 = $.01/foot


The styrofoam that I used for this experiment is bead-type expanded polystyrene which is denser and more rigid than the floral foam that is currently being used. It appears that the floral foam crushes more easily and retains the shape of the impacting object with more resolution than the bead-type polystyrene. The main sensing element however is the malleable aluminum foil surface layer which records the impacts by becoming embossed with a negative of the impacting object.

Step 2: $2.00 Each


Expanding polyurethane foam seemed like a good possible alternative substrate material. It's readily available from local building supply stores, and reasonably priced compared to the price for individual precut styrofoam. The foam is intended to be used as insulating sealant to fill gaps and cracks in buildings, to accomplish this it has to expand to fill in voids which makes it an ideal material to form into a substrate. When cured the material has properties similar to polystyrene.

Great Stuff 12oz = $3.60 (makes 2 pads) = $1.80 each + Foil + Tape


The polyurethane foam dispences rapidly and it is difficult to regulate the flow making inconsistent results likely. At an approximate 1" finished thickness the foam has to stay compressed in a form for several hours to ensure it cures enough to hold it's shape. Once cured the general purpose formula foam that I used hardens very rigid making it strong but this creates a very low impact recording resolution. There are other formulas of this foam available that claim to be more flexible and may be more suitable for this application.

Step 3: Untested Ideas

I considered that shredded paper, or cellulose fiber insulation material could be combined with a binder, some sort of adhesive, and compressed in a form to make a potentially suitable substrate for the pads but I'm not in position to test this idea.

I also have an idea for an electronic sensor and data recording system, again I'm not in a position to experiment with this idea.
I don't think they really specify how dense the styrofoam must be in the CoCoRaHS project, so I think you have done a great job finding low cost styrofoam and foil. I wonder how dense the foam must be to get accurate hail measurements. Presumably, they are measuring how far into the foam the hail goes. I guess this would vary based upon the density of the foam.
Thank you. I din't find any substrate density requirement, or current materials cost specified. Based on what I was able to deduce from their "how to make a hail pad" instructions I concluded that they are using a more open cell foam, and that the total minimum cost per pad is about $3. Again based on deduction not specification it appears that they primarily measure the diameter of the hail impacts not the impression depth. Ultimately they should be able to calculate a conversion formula that would account for the lower resolution and maintain reasonable accuracy.

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