tue, 01-mar-2011, 18:47

Sunset, Miller Hill

Sunset, Miller Hill

People always ask if we’re the coldest spot in town. I can’t really answer that, but I can find out if we’re the coldest reporting weather station in the region.

Once again, we’ll use PostgreSQL window functions to investigate. The following query finds the station in zone 222 (the National Weather Service region that includes Fairbanks) reporting the coldest temperature every hour during the winter, counts up all the stations that “won,” and then ranks them. The outermost query gets the total number of hourly winners and uses this to calculate the percentage of hours that each station was the coldest reporting station.

Check it out:

SELECT station, count,
    round(count / sum(count) OVER (
        ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
    ) * 100, 1) AS percent
FROM (
    SELECT station, count(*) AS count
    FROM (
        SELECT station, dt_local, temp_f,
            rank() OVER (
                PARTITION BY dt_local ORDER BY temp_f
            )
        FROM (
            SELECT location || ' (' || station_id || ')' AS station,
                date_trunc('HOUR', dt_local) AS dt_local, temp_f
            FROM observations
                INNER JOIN stations USING (station_id)
            WHERE zone_id = 222
                AND dt_local between '2010-10-01' and '2011-03-31'
        ) AS foo
    ) AS bar WHERE bar.rank = 1 GROUP BY station ORDER BY count desc
) AS foobar;

And the results:

                station                 | count | percent
----------------------------------------+-------+---------
 Goldstream Creek (DW1454)              |  2156 |    51.0
 Chena Hot Springs (CNRA2)              |   484 |    11.5
 Eielson Air Force Base (PAEI)          |   463 |    11.0
 Parks Highway, MP 325.4 (NHPA2)        |   282 |     6.7
 Small Arms Range (SRGA2)               |   173 |     4.1
 Ballaine Road (AS115)                  |   153 |     3.6
 Fairbanks Airport (PAFA)               |   125 |     3.0
 Fort Wainwright (PAFB)                 |   107 |     2.5
 Ester Dome (FBSA2)                     |   103 |     2.4
 Eagle Ridge Road (C6333)               |    81 |     1.9
 Keystone Ridge (C5281)                 |    33 |     0.8
 Skyflight Ave (D6992)                  |    21 |     0.5
 14 Mile Chena Hot Springs Road (AP823) |    21 |     0.5
 College Observatory (FAOA2)            |    11 |     0.3
 Geophysical Institute (CRC)            |    10 |     0.2
 DGGS College Road (C6400)              |     1 |     0.0

Answer: Yep. We’re the coldest.

Update: Thinking about this a little bit more, the above analysis is biased against stations that don't report every hour. Another way to look at this is to calculate the hourly average temperature, subtract this from the data for each station during that hour, and then average those results for the whole winter. The query is made more complex because several stations report temperatures more than once an hour. If we simply averaged all these observations together with the stations that only reported once, these stations would bias the resulting hourly average. So we average each station's hourly data, then use that to calculate the zone average for the hour. Here's the query, and the results:

SELECT station, 
    round(avg(diff), 1) AS avg_diff 
FROM (
    SELECT station,
        dt_local, 
        temp_f - avg(temp_f) 
            OVER (
                PARTITION BY dt_local 
                ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
            ) AS diff 
    FROM (
        SELECT location || ' (' || station_id || ')' AS station, 
            date_trunc('HOUR', dt_local) AS dt_local, 
            avg(temp_f) AS temp_f 
        FROM observations 
            INNER JOIN stations USING (station_id) 
        WHERE zone_id = 222 AND 
            dt_local between '2010-10-01' and '2011-03-31' 
        GROUP BY station, date_trunc('HOUR', dt_local)
    ) AS foo
) AS bar 
GROUP BY station 
ORDER BY avg_diff;
                station                 | avg_diff
----------------------------------------+----------
 Goldstream Creek (DW1454)              |     -6.8
 Eielson Air Force Base (PAEI)          |     -3.8
 Fort Wainwright (PAFB)                 |     -3.1
 Fairbanks Airport (PAFA)               |     -2.9
 Small Arms Range (SRGA2)               |     -2.8
 Chena Hot Springs (CNRA2)              |     -2.3
 DGGS College Road (C6400)              |     -0.7
 Ballaine Road (AS115)                  |     -0.6
 College Observatory (FAOA2)            |      1.0
 North Bias Drive (RSQA2)               |      1.3
 14 Mile Chena Hot Springs Road (AP823) |      3.1
 Skyflight Ave (D6992)                  |      3.3
 Geophysical Institute (CRC)            |      3.5
 Eagle Ridge Road (C6333)               |      3.8
 Parks Highway, MP 325.4 (NHPA2)        |      4.5
 Keystone Ridge (C5281)                 |      5.1
 Ester Dome (FBSA2)                     |      5.1
 Birch Hill Recreation Area (BHS)       |      6.8
sat, 15-aug-2009, 08:58

Inside of station enclosure

Inside of station enclosure

In my previous post on my weather station I included a graph of the relationship between the three temperature sensors I’m collecting data from. It turns out that the plot is incorrect because I had messed up the program that converts the raw data from one of my Arduino stations. What became clear after looking at the (fixed!) data is that the enclosure I built isn’t adequate at keeping the sensors cool when in direct sunlight. I could probably improve the design of the enclosure somewhat, but I decided to aspirate the sensors instead.

The photo on the right shows the inside of the enclosure. The sensors are sitting on a platform inside the pipe, and there’s a small muffin fan (the kind you’d use to vent a computer case) on the top. It’s a 12V fan, and at the moment it’s being driven by a 9V AC/DC converter. The plan is to replace the converter with a 12V solar cell that is sufficient to drive the fan. This way it doesn’t consume any electricity, and the fan is only spinning when it’s necessary (when the sun is out). Thus far, I haven’t found a suitable solar panel. The small ones designed to charge a cell phone battery don’t operate at the correct voltage (and probably don’t produce enough current anyway), and the big ones designed to keep a car battery charged are expensive and overpowered for my needs. With winter rapidly approaching, I’ve got plenty of time to figure something out.

The pipe is a piece of 4” sewer pipe that’s been spray painted white and has a series of holes drilled into the bottom. The fan pulls air up through these holes and over the sensor array in the middle. If I had it to do over again, I’d cut the pipe a bit shorter so it’s not so difficult to get into the enclosure. But for keeping the sensors bathed in atmospheric air, it works quite well.

Temperature summary, effect of aspiration

The plot shows the observed temperature for each of the three sensors I’ve got. The blue line is the “west” sensor that’s inside the enclosure I built and is the subject of this post. The red line is the reported temperature from the Rainwise station that sits atop a post attached to the dog yard gate. The green line is the sensor that’s behind the house and under the oil tank. I built the enclosure for the west sensors on July 12th and installed it that evening. You can immediately see the effect of the shielding during the high temperature peak on the 13th. But you can still see the two little peaks that are present in the previous plots. These peaks come from direct sun on the station, split by some trees that shade the station for an hour or two.

On the evening of the 13th I installed the pipe and fan. It was smoky on the 14th and cloudy on the 15th, but you can see the effect of the fan on the following dates. The double peaks are now gone, and the temperature from the west sensors at the high point during the day is now a few degrees cooler than the measurement from the Rainwise station. Also notice that all three sensors are virtually identical on the 15th when it was cloudy and raining.

What this demonstrates to me is that the aspirated west sensor is now the best reference sensor for our site. The Rainwise sensor is a close second, but it’s Gill multi-plate radiation shield isn’t as effective as my aspiration system at reducing the effect of solar heating on the station.

sun, 12-jul-2009, 15:22

Stevenson screen

Stevenson screen

For the past ten days I’ve been collecting data from three sets of temperature sensors located in different places around the yard. There’s the sensor in the Rainwise weather station at the top of the dog yard gate, a collection of sensors out behind the house under the oil tank, and a set of sensors under a collection of yogurt containers on top of a foundation post on the west side of the house.

It’s not easy to get accurate temperature readings. You need to site the sensors where they’ll get a good reading (between 4 and 6 feet off the ground, out in the open and away from buildings and trees), keep the sensors from getting heated by the sun, and keep them dry both from rain and snow, as well as from condensation inside an enclosure. I’ve got the last one figured out, but siting and solar radiation are proving to be big challenges.

The plot below shows the hourly average temperature readings for all three sets of sensors over the last ten days since I added the west sensor.

Temperature sensor comparison

The sensor atop the dog yard gate (the red line) is well sited in terms of it’s distance from large objects like buildings and trees, but it’s too high off the ground. It’s enclosed in a Gill multi-plate radiation shield, which is effective at reducing the effect of solar radiation when the wind is blowing. Compared with other temperature sensors in the region, this sensor is commonly several degrees warmer during the middle of the day, and I think this is because the shield isn’t keeping the sensor cool enough. We do seem to get less wind than in other places, and I think this is why the shield isn’t working as well as it should be. The sensor’s location away from everything does allow it to reach accurate minimum temperatures at night.

The sensor cluster behind the house is effectively shielded from the sun because it’s very close to the north side of the house, and even when the sun is in the north (in Fairbanks, the sun comes pretty close to circling the sky in summer) there are trees behind the house that keep it shaded. But it’s much too close to the house, and the location is far more sheltered than is appropriate. The moderating effect of being so close to the house reduces the diurnal temperature range, clipping the highs and lows compared to the data from the dog yard sensor.

On the west side of the house, I’ve got three sensors sitting on top of a foundation post (a telephone pole driven into the ground). There are several layers of yogurt container on top of the sensors, both to protect them from rain, but also in an attempt to reduce solar heating under the containers. The radiation shielding appears to be almost as effective as the commercial Gill shield over the dog yard sensors (the high temperature peak on the graph is very similar between the two), but something is keeping the temperature from dropping at night. The low temperatures from the west sensors are more than 5 degrees warmer than the dog yard sensor. I suspect the sensors aren’t high enough off the ground, and that the foundation post may be absorbing a lot of heat during the day and keeping the sensors artificially warm at night.

My plan is to place the west sensors inside the Stevenson shield pictured at the top of this post. I’ll raise it to between 4 and 6 feet in the air, and see how the temperatures compare with the dog yard sensors. I’m also working on a solar powered aspiration system in case the Stevenson screen doesn’t have enough of an effect on the high temperatures on sunny days. I haven’t quite worked it out yet, but the idea is to put a small computer fan on the top of a short piece of 4” plastic pipe that contain the sensors. When the sun is shining, the solar panel drives the fan, which pulls air up through the pipe and over the sensors. We’ll see if it’s needed in the next few days.

mon, 28-nov-2005, 18:40
Watershed Opening

I've discussed the problems with our water supply on other pages (going into the tank to clean it, new water tank, watershed) but the most recent pages end just after I finished installing the new tank.

Last winter the outlet pipe that transitions from the base of the tank inside the watershed to the warm garage froze several times. I eventually installed heat tape, but that proved ineffective. Each time it froze I had to disassemble the piping and heat the area with a heat gun until the slug of ice slid out and flow was restored. Not a fun activity at 5 AM when all I want to do is wash my face and make coffee.

This year I tried a new technique -- creating an insulated box around the opening to reduce the cold air flow and a small computer fan to gently blow warm air into the insulated area. I installed an indoor / outdoor thermometer in the insulated space with the probe extending into the shed. As winter approached, the temperature in the shed started declining, but the insulated area stayed right around 50 F.

Yesterday I put my beer thermometer into the insulated area on a whim. The indoor / outdoor thermometer read 48 F in the insulated area and 36 in the shed. But my beer thermometer was reading 30 F! Turns out the battery was close to dead in the thermometer I'd been trusting; the temperature in the shed was actually 26 F, and the temperature in my insulated area was down to 30 F.

I quickly upgraded the fan, and as you can see from the image, it's still 26 F in the shed, but it's a comfortable 52 F inside the insulated area. All the water in the shed has enough thermal mass that it can be below freezing for weeks (by which point it'll be filled again with warmer water) without freezing except around the edges. And the outlet pipe is now a steady 50+ F.

Just goes to show that you shouldn't put too much faith in a single instrument. It hit -34 F in Fairbanks this morning, and if I hadn't noticed it, the outlet pipe would surely have frozen.


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