[time-nuts] Weather/units question for European members

Steve Byan stevebyan at verizon.net
Sun May 25 16:22:30 EDT 2014

On May 23, 2014, at 11:12 PM, Mark Sims <holrum at hotmail.com> wrote:

> The nice thing about measuring temperature via sonic measurements is that the measurements are unaffected by solar heating of the apparatus... it does not need to be in the shade.

I stumbled on this paper a while back when I was investigating a similar idea:

Wen-Yuan Tsai et al, "An ultrasonic air temperature measurement system with self-correction function for humidity", 2005 Meas. Sci. Technol. 16 548

It uses both time of flight and phase measurements.

Since this is for rocketry, note that RockSim (a commercial package for simulating the trajectory of a model rocket flight) lets you choose a variety of units for windspeed and barometric pressure. I've used meters per second and hectoPascals with the kids I mentor on rocketry.

One funny thing about weather measurements is that the data that NOAA reports is not what it would seem. The standard ASOS data (which is what NOAA reports in its local current conditions) includes barometric pressure in inches of mercury and in hectoPascals. It turns out that neither is the actual barometric pressure. 

First, both are compensated to sea level, so they are not reporting the station pressure. 

Next, the in.Hg measurement is actually "altimeter setting", which is the value which, if set in the Kollsman window of a standard aviation mechanical altimeter located at the ASOS site, will cause the altimeter to indicate the height above sea level of the ASOS site. So it's really not related to sea-level barometric pressure in any direct way; it's not compensated for temperature nor for humidity, etc. It's just based on the standard atmospheric model as used by the standard aviation altimeter. There is a straightforward way to derive station pressure from the altimeter setting, so it's not entirely useless if you are not an aviator.

Finally, if you try to compare the reported in.Hg barometric pressure versus the reported hPa barometric pressure, you will often find that the two values are not related by the standard conversion factor from in.Hg to hPa. That is because the ASOS hPa value is actually the average of the current station pressure, corrected to sea level (I don't know what factors are included in that correction), and the sea level corrected station pressure from 12 hours previous. This averaging is to correct for the diurnal variation in station pressure resulting from solar heating. Without this correction, the weather fronts would oscillate back and forth on the weather map with a 24 hour period. So unless you are drawing weather maps, the ASOS hPa value is useless.

So, when RockSim asks the user to input "barometric pressure", exactly which one does it mean? Note that it also asks for the height above sea level of the launch site. Does it take altimeter setting and assume that it is measured at the height above sea level of the launch site, derive the station pressure from that, and then apply a temperature and humidity compensated version of the standard atmospheric model to calculate the air density profile for the simulated rocket flight? What if the station height above sea level isn't the same as the launch site above sea level? Does it even take any of these complications into account, and just assume that the number you enter for "barometric pressure" is the station pressure at the launch site? If so, note that most folks just enter the barometric pressure number reported by the local weather forecast.

This is one of the dangers of relying on closed-source programs for science and engineering; you can't tell what it's really calculating.

It seems like "what is the barometric pressure" should be a simple question, but it turns out to be quite subtle.

Best regards,

Steve Byan <stevebyan at me.com>
Littleton, MA 01460

Steve Byan <stevebyan at me.com>
Littleton, MA 01460

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