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The Many Densities of Water

The Many Densities of Water

One of the most amazing phenomena, that almost everyone sees, is the density of water's various forms.

All matter exists in different phases - most commonly solid, liquid and gas. For water, that takes the form of ice, water (liquid) and water vapor (gas). But leaving the discussion there disguises the many amazing forms that each of those phases takes.

This post is dedicated to the magnificence of water, and its various forms and phases. To put that into something we can touch and feel, let's look at one of the more fascinating characteristics of water – its density. Keep reading to learn more and find out how to measure the density of snow; you’re sure to be surprised by what you find!

What is density?

Density has to do with how closely packed molecules are in a given object, relative to their mass. Take Aluminum, for example, as compared to Lead. The same size cube made from Aluminum is more than four times lighter than Lead.

We all know that water is heavier than ice, and of course water vapor is lighter than both. But what about snow? What about mist? And how about very cold ice? Does the density of water change depend on its temperature? It turns out that the answers are highly unexpected.

More about water density

Let’s take a moment to appreciate one of the most critical features of water density. A typical rule of thumb when dealing with heat is that it makes things expand. So hotter liquids should take up more volume, and colder liquids should take up less.

Since liquids are free to move and mix, cold regions will tend to sink and warm regions which are less dense will tend to rise. Thus, temperature of the liquid will be warmest at the surface and coldest at the bottom. This is called the thermocline.

Turns out that is only part of the story. Once the temperature falls below 4°C, the density of water starts to increase, by only about 0.01% (see Fig. 1 below). But that’s enough to force the coldest water to the surface of the lake, flipping the thermocline completely.

This effect allows fish to survive winter by forcing the lake to freeze from the top down, allowing the fish to stay in the warmer 4°C region. This reversed thermocline stops the vertical convection which would otherwise cause the lake to lose its energy at a much faster rate, possibly freezing completely.

Density of ice and water at different temperatures

Fig. 1: Density of water as a function of temperature. The inset gives a close-up of the density around 4°C Figure created by Klaus-Dieter Keller and obtained from https://commons.wikimedia.org/wiki/File:Density_of_ice_and_water_(en).svg.

But one of the coolest effects of water is how the density changes when it snows. There are two effects at play. One is the density of the ice in the snow itself. The other is the shape and structure of the snow crystals, and how closely they can pack when they hit the ground.

If I were to ask you to draw a snowflake, you might draw something with 6-fold symmetry like this:

Obviously, this snowflake won’t pack very tightly. But it turns out there are quite a few types of snowflakes whose creation depends on the temperature and humidity when the snow is created. These types of snowflakes can be seen in the diagram below.

Fig 2. Morphology of snowflakes, obtained from http://www.snowcrystals.com/morphology/morphology.html and based on work by Japanese physicist Ukichiro Nakaya in the 1930s, and many others since then.

Measuring the density of snow

You can see that the diagram above that the density of the snow is not shown. That’s something we can measure! So, here’s the experiment, if you live in a climate which has snow.

  1. Find an inexpensive graduated measuring cup of any size, preferably one with a large mouth.
  2. Place the measuring cup outside during a snowstorm – you may want to tape down the base to protect against wind.
  3. Let the cup fill up half-way or so.
  4. Note the volume of the snow by reading the mL scale, VS. Also note the shape of snowflakes.
  5. If possible, obtain the temperature and humidity from a nearby weather station.
  6. Then take the cup inside and allow the snow to melt.
  7. Then measure the volume of the water, VW.

We can easily express the density of the snow as compared to the density of room temperature water, by taking the fraction and converting to percent.

Make measurements over a few snowstorms, to see if you can determine the density for various areas on the snowflake morphology diagram above.

As you think about the density values you’ve measured, under what conditions do you think you’d like to ski, or to hike? 

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