Blog

Humidity Adjustments

It appears our humidity sensor’s maximum value has lowered over time. This is, unfortunately, common with our particular weather station’s sensor. The process is accelerated by 24 hour fan aspiration. We’ve seen this behavior in 3 other units we’ve deployed in the past.

When we first installed this sensor last July, the top possible humidity reading was 98%. The ceiling has since lowered to 95%.

After a round of tests using an independently calibrated device yesterday & today, and in an effort to stay within +/- 3% of true ambient humidity, we’ve programmed adjustments to the following humidity readings:

  1. 93% +1% = 94%
  2. 94% +3% = 97%
  3. 95% or 96% = 100%

Because we use a 5 minute rolling average for humidity at times you will see values between 94%, 97%, and 100%.

The good news is, based on experience with this sensor model, the biases should now be fairly solidified and predictable. As always, though, we will test to verify its performance every so often.

****

NOTE: We send corrected humidities to CWOP & Weather Underground.

New Rain Gauge

Today I installed a new tipping spoon automatic rain gauge that will report real-time rainfall to the website. This is described by the manufacturer, Davis Instruments, as an “incremental improvement” over the previous tipping bucket gauge.

For the most part, our automatic gauge has run very close to our 4″ reference gauge which is 6 ft away & 2 ft lower. However, sometimes — particularly during large rain events — we encounter under reports of approximately 25% to 50%. Discrepancies can appear at any time regardless of wind speed or direction. Many others have reported similar, inexplicable anomalies with the old gauge.

Astute, long-time readers may notice the new “Aerocone” funnel in the picture. Supposedly, this cuts down on rain-induced measurement errors by reducing turbulence where rain is collected.

We’ll see how the new gauge performs. Early side-by-side comparisons from other station owners with reference equipment are encouraging. I’m definitely in the “trust but verify” camp nonetheless.

[UPDATE 5/20/19: The new gauge is performing marvelously so far. This weekend the tipping spoon reported 2.70″ of rain. Our reference gauge caught 2.67″. That’s about a 1% departure. Given the distance & design differences between the two gauges, small variances are normal.]

Our Wind Cups Froze

Like every other object in its path, Monday’s ice storm did a number on our anemometer (wind gauge). The pole to which it’s mounted cannot be retracted because… you guessed it. So nothing can be done but wait.

It’s going to take either a big gust of wind (> 25mph), some sunshine and/or warmer temperatures to free the spinning cups. Until then, wind speeds will read 0. With temps barely cracking 20 in the next 5 days, it might be awhile before it’s fixed.

[UPDATE 2/5/19 6PM]: High winds forecast for Thursday may provide sufficient energy to loosen the ice. Fingers crossed.

The directional indicator is currently turning as it should.  So if you see “N”, that means winds are coming from the north.

Just FYI, the National Weather Service says this is only the 2nd time in the 2000s that an Ice Storm Warning has been issued. It’s just not something we are accustomed to up here. That’s doubly true for February. We got about an inch of freezing rain out of that storm!

The Deluge Overwhelmed Our Rain Gauge

Manual rain gauge with 11″ capacity

We’ve received 3.11″ of rain at our location just north of the Vet’s Home since midnight. That’s an all-time record for our station.

Unfortunately, our automated rain gauge had some issues overnight and missed 1 full inch of rain. Yikes! Normally, it is pretty accurate. For the last two big rainfalls (on 9/1 and 8/27) it read within a few percent of our reference gauge. I’ve now inspected the tipping bucket, cleaned it out (the cups were pretty dirty), and returned it to service. I’ll keep an eye on it.

For today (9/5), I’ve overridden the automatic gauge with the manual measurement. You’ll see the wrong value in the stats graph at the top of the stats page. The precipitation tables further down the page should be correct, though.

How big was that rain event? It looks like Marquette set a new daily rainfall record for September 5th for the 24 hours ending at 7AM this morning. The official COOP station at the Water Treatment Plant on Lakeshore Dr. reported 3.41″ during that time. The old record was 1.3″ set in 2007. By comparison, for the 24 hours ending at 8AM this morning, we measured 3.29″.

Marquette has seen even greater rainfall in its history. On September 29, 1881, 4.44″ fell!

Altogether since August 27th, we’ve recorded 7.51″ of rain! I don’t know if that’s a record stretch for the city, but it sure knocks my socks off!

We took a small amount of water into our basement. Nothing serious, though. Apparently, we aren’t alone as there are reports around town of basement flooding. Let us know in the comments if you’ve been affected by all this rain we’ve had.

Adjusted Humidity Readings

We are now adding 5% when humidity readings reach 95% since that is the maximum our sensor will read. For many years Davis Instruments’ sensors have struggled to achieve 100% humidity. Unfortunately, the problem seems to worsen with time. When we placed our sensor in service last November 28th, it would reach 98%.

Also, we are adding 1% to 92% (to make 93%) , 2% to 93% (to make 95%), and 4% to 94% to reduce the jump between 91% and 100%. Tests demonstrate these adjustments closely approximate reality.

You may occasionally see other readings besides 93%, 95%, 98%, and 100% due to 5 minute averaging of humidity values.

Raw values are available on the stats page.

Here’s some more info on this problem if you’re interested:

We have a backup weather station for emergencies. It also contains a Davis VP2 station with an SHT31 sensor that is less than 2 years old. It maxes out at 96% humidity, but it also has an average +8% bias below 80% during the warm season (May – Sept). So it has a wet bias in the middle range in addition to a dry bias in the top range. Keep in mind that the maximum error for this sensor is supposed to be 3.5% according to the manufacturer.

We are not the only ones to experience these issues. Many other Davis VP2 weather station owners have reported them (in a thread approaching 1200 posts). Davis says they are “looking into it”, although that was almost 2 months ago now. Some particularly brave souls have attempted to informally wire up older (SHT11/SHT15) and/or unsupported sensors (SHT75) with varying degrees of luck. We’re not keen on performing ad hoc experiments with our primary temperature/humidity sensor. Some combination of Murphy’s Law and common sense would argue that such slapdash “solutions” will inevitably break at the worst time. Indeed our various attempts with other sensors on the test bench have not gone well. Therefore, we await a viable, proven fix preferably from Davis Instruments.

[UPDATE 9/21/18: Most of those enterprising individuals referenced above are now indicating an array of problems with the SHT75 sensor ranging from sudden failure to gross errors. It’s uncertain whether the fault lies with the sensor or the implementation. Either way, at present there appears to be no good workaround to the issues mentioned above with the SHT31.]

[UPDATE 10/23/18]: We recently swapped out our SHT31 sensor suite as part of a preventative maintenance routine. The new humidity sensor arrived with a +6% average (wet) bias below 55F, an average +4% bias above 55F and a maximum reading of 98% (after several hours in saturated air). We have corrected these deficiencies with custom programming.

When Weather Apps Fail at Weather

I wrote about this yesterday on Twitter. Given the limitations of that platform, however, I couldn’t really go deep. Let’s dive in.

Here are two screenshots taken within 60 seconds yesterday afternoon:

 

The 1st image is of a popular weather app recently redesigned for the iPhone. The 2nd image is our universal web app loaded in Safari on an iPhone.*

Continue reading “When Weather Apps Fail at Weather”

Temperature Readings Just Improved!

Custom Davis radiation shield

There was a Problem?

So perhaps you’ve noticed lately on sunny days forecasted highs have been lower than the actual highs by several degrees. Well… it turns out that’s not entirely true.

During the deepest part of winter when the polar vortex was regularly occurring (late December into January), we were having equipment issues. Normally, we measure temperatures inside an enclosure that has a fan running on solar power during the day and batteries at night. Unfortunately, the weather destroyed the electronics that controlled the battery charging, among other casualties.

As a quick fix, we installed a passive (fan-less) shield for our sensor. Passive shelters are fine during our normally very cloudy, windy winters. Wouldn’t you know it, the sun decided to show up and, along with it, gentle breezes! That makes for beautiful winter days in Marquette. It is also a recipe for solar-induced sensor errors.

The snow acts like a mirror to the sun and the reflected radiation gets into the sensor and heats it up. How much? A rough estimate, based on before and after tests of running with and without a fan, is 3-5 degrees F. Studies have shown that, in fact, on completely calm, sunny days over a fresh blanket of snow, errors can reach up to 18 deg F! Continue reading “Temperature Readings Just Improved!”