Why it snows

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)


Even more accurate metrics can be found by measuring ocean heat content.  This too is dropping.  Here is the latest from NOAA:


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.



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2 Responses to Why it snows

  1. rainswept says:

    Based on my reading, it appears that the most important factor in what is reported in mass media, who is given a pulpit, & who is vilified, is… the official state position. If such a position is lacking pundits are thrown into consternation & generally default to doom.

    Nothing: not science, not logic, not even a community’s moral compass, trumps government. The smart (though despairing) money is on AGW prevailing in every public arena until the political wind shifts.

  2. geof says:

    How it could snow more in a warmer world: a thought experiment.
    Take a ball, of constant temp 5 degrees. It is capable of snowing on this ball, but only if the temp is below 0. Clearly, there will be no snow on this ball.
    Now, change the conditions, such that one half of the ball warms by 10, the other half cools by 8. This ball is now warmer, but one half of it is colder than 0, and it can snow on it.

    The answer to your riddle is that the heat is not uniform on Earth, but can concentrate. If the Arctic were to warm by quite a lot, it would still be bloody cold up there — but the extent of the portion of the Earth that can experience cold/snow conditions can increase whilst the whole still warms. This can happen when the AMO goes strongly positive (? do I have this backwards), warm air can circulate and stay up in the Arctic, and cold air blast deep down into N America and Eurasia. This has been happening the last few years, and from what I’ve been reading meteorologists are wondering why it’s been increasing, and if we’re starting to experience a new norm.
    I’m not claiming this is the correct answer, but it is a possible answer. Another, one I know isn’t right but could be for some other planet: Winter is getting colder, hence more snow, but all other seasons are getting warmer, hence a warmer world.

    Now, my question for you: how could a temp series that displays no significant trend tell pundits anything? You make it sound as though 14 years were a long time, but then you accurately report that the time series does not display any statistical significance. You note the trend is down, but whether it’s flat (your BEST graph) or down is much the same to you it seems. It doesn’t matter, though, because without significance testing the data could be consistent with either warming or cooling trends, as there is a large amount of year to year variability.

    Now, I don’t yet know enough to calculate when data meets the rigors of 2 sigma trend test. But I do understand that the standard deviation for the variability of this series is high, somewhere around .2 C. A warming (or cooling) trend of about .15C/decade could be easily swamped by this amount of noise over 10 years. However, the more years you have, the more you constrain the possible range of answers. You couldn’t hide that warming trend over 30 years of data with noise – if the trend line was still flat, you’d have to abandon the warming hypothesis.

    So then, how far back would you have to take the RSS data before you got a trend that passed a 2 sigma significance test? And what is that trend equal to?
    The pundits are told by the boffins that you need 20 – 30 years of weather data before you can start talking meaningfully about climate and changes to it. And a glance at any of those last-30-yr graphs will tell the pundits that it is warming, and by quite a bit.

    As an aside, I’d love to know why you disagree with the conclusions of BEST – though I see you will still use their temp record 🙂
    And one more: if you want to make a strong claim about global precipitation trends, you should probably use a chart with data from more than 2% of the earth’s surface.

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