July 19, 2023

Thermometer Discrepancies

I have two digital thermometers that measure daily outdoor and indoor temperatures. The oldest is my Acu-Rite.

It was cheap, has no bells and whistles, but records daily maximum and minimum temps, which is what I want for my daily record keeping.

The second is newer, a Logia that Dan and I bought ourselves for Christmas last year. 

Although cheap for it's class, it has a lot of fancy features that are useful. The biggest problem I have with it, is that the time is always an hour off. I've reset it, tried it on daylight savings mode or not, even changed time zone, but when it does it's radio check to update itself, it reverts back to this hour-off flaw. I need to mention that because even though the time on these thermometers appears to be an hour different, these photos were actually take less than a minute apart. Which brings me to what I want to blog about, i.e. they are reporting significantly different outdoor air temperatures.

I've been puzzling over this, and the first thing that comes to mind is sensor location. The Acu-Rite sensor is attached to a post on our back porch. 

It's in the shade, but  there are two factors that influence temperature:

  1. The porch is concrete. Concrete absorbs and retains heat. That's why it's always so much hotter in towns and cities with more concrete than plants and trees. 
  2. I cook and can on the back porch and use a fan in the window to vent the heat. That hot air is vented to where the sensor is.
The Logia is located on a pole above our carport.

It's elevated to about second-story height and is in the full sun. I expected it to be the one to report higher temps, but that hasn't been the case. Could the modest elevation make such a difference? I don't know.

I've kept track of the maximum and minimum temps of these two, and the discrepancy is consistent. What I found curious, though, is that the Acu-Rite's higher temp is close to the Logia's heat index temp. And that led to a discussion between Dan and myself about what temperature is and which is more accurate: what the thermometer says or what if feels like. Dan felt the Acu-Rite was more correct because of how hot and sticky it's been. That led to talking about objective and subjective assessment: dry heat versus humid heat, wind versus no wind, and all that. When the humidity is low, it feels better. When there's a breeze (another factor) a hot day is more tolerable. So, which one should is more correct? Which one should I use for my daily record keeping?

As an experiment, I changed the location of the Acu-Rite sensor. I hung it behind the solar battery bank box. When I later checked the read-out, the temperature reported ten degrees lower than the old spot. But it's darker and damper there, plus it picks up the cooler crawlspace temp, which is vented by Dan's exhaust fan in the crawlspace door. 

I use the Logia's max/min button to notate the daily highs and lows. I don't record humidity, because the high is always in the early morning before it heats up. But maybe I should start recording a high and low for humidity too. And perhaps I should start jotting down the high for the heat index. Because while I believe objective data to be the most significant, it's the conditions we have to work outdoors in that impact our experience. 

I have to say, that this exercise has made me look at weather reports differently. Now, I wonder where they got their highs and lows from. Where is the thermometer located? On the roof of a concrete building? Atop a pole in a blacktopped parking lot? In the shade over a grassy lawn? They never say, but it makes a big difference. 

In the end, I moved the Acu-Rite to a completely different location. It's now being used to assess pantry and crawl space temps. And I'll start recording noting humidity and heat index in my monthly garden reports. I'm not sure any of that will be useful in the long-run, but it sort of suits my personality.

Does anybody else record things like rainfall and temperatures? What are your thoughts?


Will said...

As an aside on unsuitable thermometer locations, the claimed hottest temperature reading in England came from a thermometer situated beside an airport runway and was recorded at the same time as three Tornado fighter jets were on the runway, with their engine exhaust in the vicinity. Hardly a valid location and reading!!

Leigh said...

Will, ha! That's exactly what I mean. Great fun for people who like to sensationalize things, but not useful for factual data gathering.

Ed said...

I used to record both temperature and rainfall like my father and grandfather before me in their journals. Between the three of us, we had about 80 years of data written down.

But I stopped writing down temperature as an exact number a handful of years back. Like you said, it is highly variable depending on placement, wind, humidity, direct sun vs. shade, etc. Instead, I have gone to a more descriptive way of keeping track by righting what it feels like to me and/or how it is affecting the vegetation. So I will say something like it was hot and humid today with no breeze so everything was withering in the direct sunlight. More words but I know mentally how it was affecting me that day over just righting 89 degrees.

I do still keep track of moisture with a digital gauge though it isn't always accurate. My gauge only tracks the last 24 hours while I generally record rainfall from midnight to midnight. So rainfalls overnight can be problematic in recording. But I don't really use this information other than to look back in the past months from a current date to see how much rain has fallen. Beyond a few months, it becomes largely irrelevant unless I'm reading about the rains proceeding significant floods in our past.

I think somewhere along the way I've read that a meteorologist is supposed to take their official temperature reading from a shaded, sheltered spot with no outside interference like concrete, exhaust fans, etc. Similarly, they have rules for where to place rain gauges and other things. I for the most part, break those rules as I prefer convenience to read the information over getting super accurate information.

Annie in Ocala said...

I kept track of rainfall for about 8 years but stopped in 2020. And I would write down extreme temps, hot/cold/snow flurries/hail etc. But not daily, just when they occurred. I'm not much of a record keeper and just recorded it on my calender and keep them. Hot is hot, but I personally deal with higher humidity better than dry climate. And the difference in temps in different areas is interesting. As well I kept 2 rain gauges a couple hundred feet apart and they measure differently also....

Leigh said...

Ed, describing it as you do, is probably the most useful way to do it. Speaks of a weather journal! My most useful data is rainfall, mostly to keep track of when to water the garden. The rest is mostly for interest's sake. It's how I remember that the summer of 2017 was the hottest we've experienced.

I think you're correct about guidelines for placement of weather recording equipment. But like you, we have to put things where we're able.

Annie, I reckon it's mostly habit. Like you, I just got it down on the kitchen calendar. In looking back over those calendars, I discover I haven't been very consistent over the years. Probably something description like Ed mentions is more useful.

Anonymous said...

I have found with both of mine one is the same as yours, and the other one is a different brand, but they’re all pretty much the same especially the ones that don’t cost an arm and a leg that you can pick up at a home garden store, even a Costco.
And they all seem to pretty much have the same flaws that I’ve seen.

So with that said, I just started playing with them this year. I got the same kind that you have fairly inexpensive. My really fancy one just died it wouldn’t do so much. It didn’t do anything. Let me put it to you that way. when purchased originally at Costco in Norcal, it was about $100. Now I’m using the bits and pieces of the smaller ones that I picked up at Lowe’s on the valley floor one day what I found is they somehow are extremely sensitive to heat sources such as any place I hang any of them if they come into direct sunlight I get extraordinarily high temperature readings and they aren’t accurate for what they’re dealing with probably. If I walk out to where they are it’s not 110° at 7 o’clock in the morning .
Also if they’re placed in areas like front porches, back porches anywhere there’s a covering it catches the heat and tucks it under the eaves as you probably know, and it doesn’t cool off until hours after that, so immediately we altered the nature of the temperature is it as hot as it says it is because it’s hanging out under the eaves of the roofline probably not . I’m pretty much done . It can get really frustrating and even then I don’t think I’m getting an accurate read wherever I put it. It’s a weird thing I’ll be interested to see how this all ends up for you.

Caroline M said...

I don't record anything, I just keep an eye on the outside temperature in the Spring and Autumn because of the risk of frost. As soon as you start hanging the sensor on a convenient support (a fence in my case) you get a distortion. I know my fence gets the afternoon sun on the other side but for what I want it for, it's good enough.

Leigh said...

Goatldi, those high overnight temps on the back porch thermometer were really off this year. The Logia said it was 68 and the back porch thermometer said it was 82! So many factors, like you say. But my Acu-rite was my only thermometer for years. Now I'm wondering how accurate my records really are. Also wondering, so what? LOL!

Caroline, yes, being able to watch out for potential frost is useful. I'm guessing attaching one to a tree would give slightly cooler readings, considering how trees have such a cooling effect on the environment.

Rosalea said...

We keep a record of rainfall throughout the growing season. It is hard to place the thermometer to get an accurate reading. Our sending unit is in a slatted box, about 6ft. up a tree on the N side. When it reads +4C or less, there will be frost on my gardens.

Pioneer Woman at Heart said...

We don't record weather or rainfall, but you are right about the weather forecasters. They are never correct in our area. I can look at the sky and know that it will rain or not rain better than them. It would be useful to have one of those though.

SmartAlex said...

I have struggled to find a suitable location for our outdoor thermometer sensor. I also know that our vegetable garden, surrounded by so much gravel, is typically 9 degrees hotter than ambient. I know that the gravel pad where we intend to build our greenhouse is the hottest spot on the property. I can feel the radiant heat when I walk up on it. I always check our thermometer reading against several weather sites. I know the rain gauge is very subjective because it can be a down pour here, and our neighbors a couple of miles away can get absolutely noting. The National Weather Service has requirements for thermometer placement.

The Coop network has provided climate and weather data for over 100 years. Consistency of the measurements is an attribute of the network, and it has been maintained by rare and/or gradual change, and established standards for exposure, of instruments over the life of the network. In order to preserve the integrity of the network, NWS has established standards for equipment, siting, and exposure.

Temperature sensor siting: The sensor should be mounted 5 feet +/- 1 foot above the ground. The ground over which the shelter [radiation] is located should be typical of the surrounding area. A level, open clearing is desirable so the thermometers are freely ventilated by air flow. Do not install the sensor on a steep slope or in a sheltered hollow unless it is typical of the area or unless data from that type of site are desired. When possible, the shelter should be no closer than four times the height of any obstruction (tree, fence, building, etc.). The sensor should be at least 100 feet from any paved or concrete surface.

Precipitation gauge siting: The exposure of a rain gauge is very important for obtaining accurate measurements. Gauges should not be located close to isolated obstructions such as trees and buildings, which may deflect precipitation due to erratic turbulence. To avoid wind and resulting turbulence problems, do not locate gauges in wide-open spaces or on elevated sites, such as the tops of buildings. The best site for a gauge is one in which it is protected in all directions, such as in an opening in a grove of trees. The height of the protection should not exceed twice its distance from the gauge. As a general rule, the windier the gauge location is, the greater the precipitation error will be.

Leigh said...

Alex, I think you summarize all the problems beautifully. I'm guessing, too, that equipment become less accurate as it ages. I think for many of us, there isn't an ideal location on our properties. So, at best, all we can hope for is the gist of weather conditions in that particular spot.

SmartAlex said...

Just a P.S. on the rain gauge. We collect rainwater in a 500 gallon tank from our downspouts. I know about how much rain I need to top it off at any given time. BUT. A slow, steady rain will fill it so much faster than a the same amount of rainfall that comes in a windy storm that blows moisture back off of the roofs. Which leads me to believe that a horizontal, driving rain probably has a hard time landing in a rain gauge.

Leigh said...

Alex, I would agree with that. I also find that my tube rain gauge, which, even though out in the open is nonetheless "protected" from southwestern rain because of a young oak tree.

In the end, all measurements are approximates. I think that's a good guideline no matter where the data is sourced from.

Leigh said...

I'm not keeping up with my comments!

Rosalea, I have found frost to be a curious thing. I used to think it meant 32F, but have discovered there are variables. That, and microclimates! I wish I knew more about figuring out where they are.

Kristina, I currently look at two, sometimes three, weather sites but still can't decide which one is the most accurate. Because none of them are!

Nina said...

We have an old dial window thermometer outside my mudroom. It came with the house. After several years of watching it and seeing it wasn't horribly accurate in the summer, but extremely accurate in the winter, I stuck a shelf in front of that part of the window. When it gets cold here, I don't need to know it's -25 to know it's cold. I find that just discouraging. Now I don't worry about it, don't watch for it, and more stuff gets done in the winter, other than worrying about keeping the fires going. It gets pretty warm here in the summer, but extreme heat is generally just for a couple of days in a row and humidity is a given, I just work with it as it comes.

Anonymous said...

Good info, thanks for blogging.
I have kept records of temperature and rainfall for 32 years in 3 different states.
I use an old school thermometer and rain gauge, I have seen these same issues so I take the readings at the same time everyday.
Nothing is perfect but I can look back comparatively. Life is short, garden.

Leigh said...

Nina, it's interesting how consistently inaccurate thermometers seem to be. I'm wondering now if I even need to continue with my record keeping! (Except for rainfall so I know when to water)

Anonymous, good advice!!!

daisy g said...

Hmmmmm, what an interesting thing to learn about.
I do record rainfall in my garden journal, but not temps. It's mostly to keep track of when I need to irrigate.

Enjoy your weekend, Leigh!

Leigh said...

Daisy, I think, in the end, the best information to record is that which we can use. Like rainfall. Probably the only time the temperature records come in handy is when Dan and I are discussing past summers or winters. Otherwise, there's not much to use it for.

Toirdhealbheach Beucail said...

Heat index is a fascinating development to me. Once upon a time growing up, I do not remember that we ever knew of or considered it. It was just hot.

It is also slightly less useful (I think) in that I cannot easily replicate it but have to rely on other authorities (or at least, I do). Having to rely on other for data interpretation always makes me a bit uncomfortable.

Leigh said...

TB, good point about having to rely on "experts" for interpretation. They don't always seem to know what they're talking about (weather and otherwise.) There is a formula for calculating heat index, but it requires being able to measure humidity as well as temperature. I think wind is a factor in how it feels too, but I don't know if they take that into consideration.

bornfreev said...

The description of you and Dan in conversation instantly reminded me of my husband and I. We will start with something we have observed that slowly rolls into the science of the subject and/or semantics to the point that we just stop, look at each other and laugh!

"Simple" can be very complex. I love it.

Leigh said...

Bornfreev, that's part of it! But I think discussions like this really help the creative process, and hopefully, the outcome.

I so agree about how complex simple can be. Especially when it comes to simplifying ones life. It's a complicated path to get there!