melting

we have another bout of “santa ana wind” this weekend. And as i’m sitting in my own pool of sweat drinking cold water and eating fruits to cool down, I’ve decided it’s time to sort out what the santa ana wind is…

screen grab from wunderground.com

So, here’s a rough explanation for beginners (me). the only formula needed here is pV=nRT.

Short rough explanation: Santa Ana wind is a flow of dry and hot air which a) originates from the cold Mojave desert, b) loses its already-low moisture as it rises up  past the northeastern side of the San Gabriel Mtns, and c) heats up as it speeds down the southwestern side of the San Gabriels toward Los Angeles. It is the main driving force for fanning wild fires in Los Angeles and surrounding areas in the late summer and fall seasons.

I made the black cross because it has been dimissed as a misconception (see FAQ link)

Long rough explanation (there’s a much better explanation here):

The wind: for various reasons, regions of high and low pressures develop in the atmosphere (read the FAQ link above).  When that happens, air wants to flow from high to low pressure so as to equalize, except, because the earth is rotating, air will end up being bent to the right in the northern hemisphere:

Take an air package along this path which is flowing southwestward toward Los Angeles. First it hits the San Gabriel Mountains. Since it can’t flow across the mountains, it’ll do a little climb to the top (the deserts are already at high elevations).

Adiabatic cooling:

As the air package rises, since there’s less and less air above it, air pressure on the package is decreasing, and so is temperature, following the relationship pV=nRT.  But what happens to the moisture inside the air package?

Relative humidity:

It just happens that hot air can hold more water vapor than cold air.  This is because when temperature is rising, water is evaporating more into the air.  But the amount of water vapor that the air package can hold is limited to the saturation level.  In any case, the package is climbing up the mountains and cooling, so at some point, it hits this saturation limit and the excess water vapor will be forced out as condensation (cloud)

Adiabatic warming:

Now the air package made it to the top, but lost most or all of its water vapor content.  The high-low pressure system still pushes the air package down the southwestern side of the San Gabriels.  As it descends, more air above puts pressure on it, so pressure increases, and hence temperature (again following pV=nRT).  So this package is now hot and dry and happily speeding down and spreading.  Just the right condition for me to do one of the rarest things: drinking cold water!

Lastly, just a quick note on “relative humidity”: As the word suggests, it’s relative.  To be more precise, it’s relative to the saturated water vapor the air package can contain at a given temperature.  So here’re the last 2 plots to illustrate this point.

Here you can read “water vapor pressure” as “water vapor content”, which is to say the higher the water vapor content is in an air package, the higher its pressure will be to the surroundings.  What this plot is saying is that you can predict this pressure (hence saturation content of moist) by knowing the air temperature.  So, at any temperature, the air package can hold maximum an X amount of water vapor.  But in reality, only a smaller amount Y ≤ X is available.  So the ratio of Y/X is considered “relative humidity”.  X increases with temperature (see plot above).   During the day, the air is warm (X is high), so humidity is low.  At night, temperature drops, X is now low, so the ratio of Y/X increases, hence humidity increases in the evening and early morning.

Or, you can skip reading the whole post and look here for an animation of air package traveling up and down the mountains to illustrate the Santa Ana Wind.

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About thả diều
writing-challenged opera-addict

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