Why lakes and ponds freeze from the top down

Weather Wednesday

Wintry mix
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FARGO — Watch a hot air balloon and you’ll know that hot air rises: the burner heats up the air inside the balloon, causing the molecules and atoms inside the balloon to spread farther apart than the air outside. This changes the density of the air inside the balloon, making it less dense than the cooler air around it, allowing the balloon to float high in the sky.

The same goes for water, warm water rises and cold air sinks, so by that logic: lakes and rivers should freeze from the bottom up, but we know that’s not the case.

Frozen lake

Water is incredibly unique because when it reaches a particular temperature, 39.2 degrees to be exact, the density of the cooling water will start decreasing. This is similar to a hot air balloon, since the colder, but still liquid water rises back up, pushing the slightly warmer water down deeper below it. As this less dense water continues to cool it eventually reaches the freezing point and expands as it transitions to its solid ice state. And at the top is where the ice will stay and continue to thicken as the water gets colder through the winter.

Frozen lake

Thankfully water works in this quirky way: if water froze from the bottom to the top, there would be dire consequences for aquatic life since shallow lakes and rivers would freeze solid, the plants, animals and anything else under the surface would freeze with it.

DNR Ice.png

According to the DNR , if you want to try to catch anything waiting in the warmer, liquid water below you will need at least 4” inches of solid ice, 5-8” before you bring out the ATVs and snowmobiles and closer to a foot of solid ice before you bring the vehicles on the frozen lakes. You should have 13-17” of solid ice to bring trucks onto the ice and 20” for a truck with a wheelhouse shelter.

Jesse Ritka is a StormTracker meteorologist and holds the AMS Certified Broadcast Meteorologist seal of approval.

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