## Friday, November 2, 2012

### This chart explains why we can expect more extreme weather

This is the Psychrometric chart.  Mechanical engineering students are required to memorize this chart.  OK not really.  But they do become very familiar with the intersecting grid of straight and curved lines in thermo class.

About the chart.  For any combination of air temperature and relative humidity there is a point on the psychrometric chart where you can stick a thumbtack.  Go across the bottom to locate the dry bulb temperature line and then go straight up the line to where it intersects the correct relativity humidity curve.  Once that point is defined so are other physical properties of the air mixture such as the wet bulb temperature, the dew point, the amount of water absorbed, the density of the air and it's overall energy or enthalpy.  Follow the different lines going through that point to read the units represented.

The lines to focus on are those for relative humidity (RH). They curve upward to the right as the dry bulb temperature increases.  The lowest of these lines is for 10% RH, the highest is for 100%.  The maximum amount of water vapor that can be absorbed into air at a certain temperature is represented by the 100% RH line.  The increasing slope of the 100% RH line shows that as air rises in temperature, its propensity to absorb water vapor increases at an increasing rate.  Warm air can absorb far more water vapor than cool air.

Another series of lines that are important here are the wet bulb temperature lines.  If you go back to the tack in the chart and follow the wet bulb line to the left and upward, you can read the wet bulb temperature which is close enough to call the dew point, in other words the temperature below which the air must be cooled in order to induce precipitation.

As the effects of AGW cause average temperatures to rise in the oceans and atmosphere, one effect will be that more mass of water vapor will be absorbed from rivers, lakes and oceans into the atmosphere.  The larger mass of water vapor absorbed into the air will mean the more mass of precipitation will eventually fall out from the air later.

However more water vapor in the air does not equate to more precipitation across all geographical areas.   Humid air must collide with cold air to bring the mixture of the two below it's dew point temperature to create precipitation.  If a mass of humid air is not chilled below it's dew point the water vapor just moves along in the wind and eventually falls to the ground somewhere else.

I think it is reasonable to say on average the amount of precipitation will increase in a warming climate.  Precipitation will tend to decrease in areas where the atmosphere is hottest and increase in areas where the air is cooler.  I think we will see more persistent droughts, wildfires and growing deserts along with an increasing number of massive precipitation events that will dump record-breaking rain/snow where warm humid air masses collide with cold fronts, bringing floods, property damage, economic disruptions. crop losses and infrastructure failures.  This trend doesn't bode well for the agriculture or property insurance industries, or for our economic prospects in general.