The NBC Nightly News explained last night that global warming was “spawning extreme events” like the massive early snowstorm that crippled the eastern seaboard. Only a national broadcaster would think people are dense enough to believe that heat causes a snowstorm. And yet this meme is repeated everywhere usually containing some reference to heat drawing more water into the air. Having taken an interest in weather mechanics, the actual mechanism of precipitation is rather different from their intuition.
Precipitation is caused by the uplift of air. The moisture content is not the limiting factor, the lift is what triggers precipitation. An incoming cool air mass is denser and so the lighter, warmer air rides above it forming a wedge. Stratus clouds visually demonstrate this. As an air mass is lifted, it cools beyond its dew point and can no longer hold its moisture and it rains or snows. In other words, precipitation is caused by cold air, not warm. If you think about it, this explains why it doesn’t always rain when the air gets hot. We have many days of 30 degree temperature without rain contrary to the NBC version of weather where more heat makes it rain or snow more. Without cold air lifting the air, it makes no difference how much moisture is in the air. With greater lift, the more the air mass is cooled beyond its dew point. In any case, even a fool knows that for precipitation to fall as snow and not rain, it must be cold!
A fun mental image for me is thinking about the low pressure systems that give us our precipitation events. Low pressure systems spin counter-clockwise and inwards like a cyclone. Since they spin inward, the air must be forced upwards like a giant vacuum cleaner. As the air rises it cools, squeezing out the moisture. It gives me great pleasure picturing a giant Dyson Ball rolling its way across Canada!
“But Dave, wouldn’t warmer air hold more water vapour and when it meets cool air, it would precipitate more? ”
In the abstract, this is true but in real life there are problems with this. Continental air masses do not have much available water to evaporate (demonstrated by how dry our air is even when it is very warm) and therefore warmer air can’t take advantage of this increased capacity. Air masses over the ocean don’t have this problem but are instead limited by the temperature of the water. Raising the air temperature has little to no effect on the vapour pressure at the ocean surface which determines the amount of evaporation. For example, even though the air temperature in the summer in the north Pacific can be 30 degrees, because the water temperature is so cold, the air isn’t saturated with moisture as it is at the equator where water temperatures are much higher. Increased potential capacity is countered by the limiting evaporation rate determined by vapour pressure.
Of course it’s easy enough to test their initial assumption of a warming world. Here’s the latest global land temps from the new Berkley data set: No trend this millennium while we put out record CO2!
At some point, I would have thought a national news reporter would take 30 seconds to check any temperature data set if only to quote how much the globe has supposedly warmed!
Here is a cherry-picked satellite data set (RSS) of the entire global averaged temperature going back 14 years. No significant trend (though mathematically the trend is cooling)
Curiously, 3 of the last 4 largest snow extents have occurred in the last 4 years. I’d love to listen to the explanation of how more of the earth can be covered by snow in a warmer world! Here’s the observed data from Rutgers Snow Lab.
An enquiring mind can quickly compare very warm years such as 1998 and observe below average snowfall extents. The news story equating warmth to snow is proven to be wrong. As I said, a fool knows this but it’s important to compare our ideas to observation since it takes so little time. Comparing snow extent to temperatures, you realise how little relation there is. Hopefully after reading the mechanics of precipitation, you can appreciate the reason for the poor correlation.
Note: warm air in the absense of cold air can produce convective induced local precipitation. Warm rising air expands and cools below its dew point causing rain. So if the world was warmer, convective precipitation could occur faster, though not more often. Observations show that precipitation is not following any trend.
Eventually, the wheels will completely fall off man made global warming hysteria. When they do, media outlets will report about new findings and new breakthroughs and completely absolve themselves of their current incompetence. It will be as if there never were millions of people shouting and pointing to real numbers. No newscast will look back and wonder how so many media people failed to take 5 seconds to do some fact checking.