January 19 2013

What Causes Snow?

Every winter, we watch our once green grass wither and turn brown and all the leaves fall off our shrubs. When we look out our window at our lawn, it does not look very pretty. That is—until the first snowfall.  Along comes some winter snow and blankets our lawn, bushes and trees in white. So what causes snow?

Snow forms within clouds so it won’t snow on a cloudless day. And snow needs a temperature below freezing. Snow clouds are full of moisture— saturated air that has condensed into a liquid much like the condensation you see on the outside of a cold glass of water. The warm air reaches the glass, becomes cold and saturated, and turns to liquid—the water beads on your glass. So within the cloud you have this condensed liquid and when the temperature falls below freezing, it turns into ice. It is interesting to note that the design of a water molecule leads it to develop into six-sided crystals. A snowflake is a bunch of crystals all stuck together. Picture these ice crystals joining until the snowflake is big enough—heavy enough—then it falls down toward the  ground. Gravity takes over and the heavier snowflakes begin to fall. Even in midair the snowflake will join with more crystals that it bumps into as it is falling.  When the temperature is right at freezing, snowflakes will be bigger because there are a little bit melty—a little bit wet and ‘sticky’ so the crystals they encounter will stick to them more easily. When it is extremely cold—far below freezing—snowflakes will be smaller and the snow is kind of dry. If you’ve ever tried to make a snowball or snowman and you can’t get the snow to mass together, this is why.

The snow clouds form when warm air flows up and over cold air.  And water vapor that is in the warm air mass, along with the movement of the air as it rises, makes clouds form. The more water vapor that is in and around the cloud and the stronger the updrafts that make the water vapor condense, the greater the chance of snow forming inside the cloud. This is why, in mountain areas, you’ll have snow and precipitation on one side of the mountain but the other side receives minimal amounts of snow. This is because as the air is forced to rise on the ascension side of the mountain, the water vapor forms and comes down as snow or rain.  By the time the clouds cross the mountain to the other side, there is hardly any moisture left in them.

One last interesting point about snow—it actually helps your lawn. It contains atmospheric nitrogen, which melts down into the ground as the snow melts, and this nitrogen improves your spring grass. Nature is pretty cool, isn’t it?