Latent Heat

In Colorado, July has more thunderstorms than any other month of the year.  If you have been watching the TV weather or read the forecast discussions that appear at the top of your daily forecast from us, you no doubt have heard the reference to dew point and or relative humidity and the chances for thunderstorm activity.  The higher the dew point and humidity the better the chances for showers and thunderstorms.  Once the dew point goes above 50 along the Front Range, chances increase significantly for heavy to severe thunderstorms.  Other than the fact that higher humidity means there is more moisture to work with, why does increased moisture mean bigger more severe storms??  It all has to do with ‘Latent Heat Energy’.

Latent heat energy is the heat and energy stored in water vapor.  Water vapor is the gaseous form of water.  Now most of us are familiar with the liquid form of water, we see it every day in our rivers, lakes and streams, not to mention at home when we turn on the faucet.  To get liquid water into the gas form all we have to do is heat it.  In other words add energy to it.  It’s as simple as boiling water, we add heat to it, it boils and turns into a gas, if we leave the heat on long enough all the liquid water will be gone.  We have added heat energy to the water vapor.  Outside of the house it is not necessary to boil the water. Water evaporates all the time from lakes, rivers even from your backyard.  It still takes the same amount of energy input to transform liquid water to gas.  And the water vapor stores this energy until it returns to a liquid form and releases this stored heat.

This is where showers and thunderstorms come in.  As the sun heats the surface areas around us the land areas heat up the surrounding air.  Warmer air is lighter and begins to rise.  As this air rises it cools and at a certain point it reaches a point where the water vapor turns back into a liquid.  The air has reached saturation point.  As meteorologists we call this the liquid condensation level.  This can be thousands of feet high or right at the surface (fog) depending on how much moisture is in the air and the actual air temperature.

When water vapor returns to the liquid state it gives up the heat energy that turned it into a gas to the surrounding air.  This in turn allows the developing storm to grow even taller and stronger as this heat energy is added.  So the more water vapor that is converted to liquid in the form of clouds and rain the more energy the developing storm has to work with.

Here along the Front Range we as a general rule are pretty dry.  For example. today, July 6th, DIA has a dew point of 35 and a relative humidity of 13%. So pretty dry!  To the east in Limon the dew point is 55 and the humidity is 35%.  This is typically the case, moisture tends to increase the further out east onto the plains you go.  So that any storms that form along the Front Range will stay pretty weak, with gusty winds, some lightning, but little in the way of rain, until they tap into the higher moisture levels to the east and the higher energy levels stored in the higher levels of water vapor to the east.  Only when the higher moisture levels make it all the way up to our Front Range will we get the severe thunderstorms.

So there you have it, actually pretty simple, the more available moisture the more heat energy a developing storm can tap into and use.  So the next time you hear a reference to a high humidity day you will know we have a better chance for some heavy to severe storms.