Skip to content

Posts tagged ‘This Idea Must Die Series’

Should We Replace “Wind Chill Factor”?

In a continuation of our series, based on this book, which discusses scientific ideas that need to be reexamined, Dr.’s Doug Cobos and Colin Campbell make a case for standard operative temperature to replace wind chill factor:

Frost covered plant in early morning

Currently, the forecast is based on air temperature and wind chill. What the forecast leaves out is the effect of radiation.

What are we looking for when we look at a weather forecast?  We want to know how we’re going to feel and what we need to wear when we go outside. Currently, the forecast is based on air temperature and wind chill, which are a major part of the picture, but not all of it.  What the forecast leaves out is the effect of radiation.  If you go out on a cold, sunny day, you’re going to be warmer than you would be at that same temperature and wind speed on a  cloudy day.  It’s not going to feel the same.  So why not replace wind chill with the more accurate measurement of standard operative temperature?

Where wind chill came from:

In 1969, a scientist named Landsberg created a chart showing how people feel at a certain air temperature and wind speed. His chart was based on work by Paul Siple and Charles Passel.  But, Siple and Passel’s work was done in Antarctica using a covered bottle of water under the assumption that you were wearing the thickest coat ever made.  The table was updated in 2001 to improve its accuracy, but since the coat thickness assumption remained unchanged it underestimates the chill that you feel. It also explicitly leaves out radiation, assuming the worst case scenario of a clear night sky. The controversy is detailed in this NY Times article from several years ago.

Ice covered lake with the sun reflecting off the surface, a bench in front of the lake in the snow with a person walking next to it

Siple and Passel’s work was done in Antarctica using a covered bottle of water under the assumption a person was wearing the thickest coat ever made.

During the winter, forecasters use air temperature and wind chill with no radiation component.  In the summertime, they use an index that takes into account the temperature and the humidity called the heat index.  But again, there is no accounting for radiation. Our families deal with this all the time when we take the kids out fishing in early spring. Before we leave, we’ll check the weather report for temperature and wind chill.  But is it going to be sunny or cloudy?  That’s key information. You can see the radiation effect in action when a cloud drifts in front of the sun.  All the kids scramble for their jackets because the perceived temperature has changed.  This is something that none of the indices actually capture.

Understanding the concept:

Standard operative temperature combines the effects of radiation and wind speed to give a more complete understanding of how you will feel outside.  It is a simple energy balance: the amount of energy coming in from the sun and metabolism minus the amount of energy going out through heat and vapor loss. Using this relationship and adding in the heat and vapor conductances, the temperature that we might “feel” can be graphed against the solar zenith angle at a fixed air temperature. For reference, the sun is directly overhead when the zenith angle is 0 degrees and at the horizon at 90 degrees.

Wind Chill and standard Operative temperature chart

Figure: Wind chill and standard operative temperature with respect to sun angle for two wind speeds (1 and 10 m/s) at an air temperature of -5 degrees C.

What’s interesting is that on a clear day when the sun is around 45 degrees (typical for around noon in the winter) and the temperature is -5 degrees C, if the wind is blowing at 1 m/s, you would feel a temperature of 6 degrees C (relatively warm). The wind chill predicts the feel at -6 degrees C, a huge difference in comfort.  This difference decreases with increasing wind speed as you’d expect, but even for the same conditions and wind at 10 m/s, the 45-degree sun angle creates a temperature feel 7 degrees C higher than the wind chill.  Although not huge, this makes a considerable difference in perceived comfort.

What do we do now?

The interesting thing is that all the tools to measure radiation are there. Most weather stations have a pyranometer that measures solar radiation, and some of them even measure longwave radiation, which can also be estimated within reasonable bounds. This means forecasters have all the tools to report the standard operative temperature, which is the actual temperature that you feel.  Why not incorporate standard operative temperature into each forecast? Using standard operative temperature we could have the right number, so we’d know exactly what to wear at any given time.   It’s an easy equation, and forecast websites could use it to report a “comfort index” or comfort operative temperature that will tell us exactly how we’ll feel when we go outside.

Which scientific ideas do you think need to be reexamined?

See weather sensor performance data for the ATMOS 41 weather station.

Download the “Researcher’s complete guide to soil moisture”—>

Get more information on applied environmental research in our

This Idea Must Die: Using Filter Paper as a Primary Method for Water Potential

In a continuation of our popular series inspired by the book, This Idea Must Die:  Scientific Problems that are Blocking Progress,  Dr. Gaylon S. Campbell relates a story to illustrate the filter paper method, a scientific concept he thinks impedes progress:

Folded old paper sitting on a wooden table with a gold antique pocket watch and pen laying in top

There are times when our independent verification turns out to be like the clock and the whistle, and we end up inadvertently chasing our tail.

I remember listening to a story about a jeweler who displayed a big clock in the front window of his store. He noticed that every day a man would stop in front of the store window, pull out a pocket watch, set the watch to the time that was on the large clock, and then continue on.  One day, the jeweler decided to meet the man in order to see why he did that.  He went out to the front of the store, intercepted the man, and said, “I noticed you stop here every day to set your watch.”

The man replied, “Yes, I’m in charge of blowing the whistle at the factory, and I want to make sure that I get the time exactly right.  I check my watch every day so I know I’m blowing the whistle precisely at noon.”

Taken aback, the jeweler replied, “Oh, that’s interesting.  I set my clock by the factory whistle.”

The Wrong Idea:

In science, we like to have independent verification for the measurements we make in order to have confidence that they are made correctly, but there are times when our independent verification turns out to be like the clock and the whistle, and we end up inadvertently chasing our tail. I’ve seen this happen to people measuring water potential (soil suction). They measure using a fundamental method like dew point or thermocouple psychrometry, but then they verify the method using filter paper. Filter paper is a secondary method—it was originally calibrated against the psychometric method. It’s ridiculous to use a secondary method to verify an instrument based on fundamental thermodynamics.

Tunnel looking up from the bottom with square holes in the sides going up to the top

Geotechnical engineers use natural material such as soil and rock in combination with engineered material to design dams, tunnels, and foundations for all kinds of structures.

Where the Filter Paper Method Came From:

Before the development of modern vapor pressure measurements, field scientists needed an inexpensive, easy method to measure water potential. I.S. McQueen in the U.S. Geological Survey and some others worked out relationships between the water content of filter paper and water potential by equilibrating them over salt solutions. Later, other scientists standardized this method using thermocouple psychrometers so that there was a calibration. Filter paper was acceptable as a kind of a poor man’s method for measuring water potential because it was inexpensive, assuming you already had a drying oven and a balance. The thermocouple psychrometer and later the dew point sensor quickly supplanted filter paper in the field of soil physics. However, somewhere along the line, the filter paper technique was written into standards in the geotechnical area and the change to vapor methods never occurred. Consequently, a new generation of geotechnical engineers came to rely on the filter paper method. Humorously, when vapor pressure methods finally took hold, filter paper users became focused on verifying these new fundamental methods with the filter paper technique to see whether they were accurate enough to be used for water potential measurement of samples.

What Do We Do Now?

Certainly, there’s no need to get rid of the filter paper method. If I didn’t have anything else, I would use it. It will give you a rough idea of what the water potential or soil suction is. But the idea that I think has to die is that you would ever check your fundamental methods (dewpoint or psychrometer) against the filter paper method to see if they were accurate. Of course they’re accurate. They are based on first principles. The dew point or psychrometer methods are a check to see if your filter paper technique is working, which it quite often isn’t (watch this video to learn why).

Which scientific ideas do you think need to be revised?

Download the “Researcher’s complete guide to water potential”—>

Download the “Researcher’s complete guide to soil moisture”—>

Get more information on applied environmental research in our

Do the Standards for Field Capacity and Permanent Wilting Point Need to Be Reexamined?

We were inspired by this Freakonomics podcast, which highlights the book, This Idea Must Die: Scientific Problems that are Blocking Progress, to come up with our own answers to the question:  Which scientific ideas are ready for retirement?  We asked METER scientist, Dr. Gaylon S. Campbell, which scientific idea he thinks impedes progress.  Here’s what he had to say about the standards for field capacity and permanent wilting point:

Canola Field right next to an eroded soil cliff

A layered soil, a soil that has a fine-textured horizon on top of a coarse-textured soil, will hold twice as much water as you’ll predict from the -⅓ bar value.


The phrase, “this idea must die,” is probably too strong a phrase, but certainly some scientific ideas need to be reexamined, for instance the standard of -⅓ bar (-33 kPa) water potential for field capacity and -15 bars (-1500 kPa or -1.5 MPa) for permanent wilting point.

Where it came from:

In the early days of soil physics, a lot of work was done in order to establish the upper and lower limit for plant available water.  The earliest publication on the lower limit experiments was by Briggs and Shantz in 1913. They planted sunflowers in small pots under greenhouse conditions, letting the plants use the water until they couldn’t recover overnight, after which they carefully measured the water content (WC).  The ability to measure water potential came along quite a bit later in the 1930s using pressure plates.  As those measurements started to become available, a correlation was found between the 15 bar pressure plate WCs and the WCs that were determined by Briggs and Shantz’s earlier work.  Thus -15 bars (-1.5 MPa) was established as the lower limit of plant available water.  The source of the field capacity WC data that established a fixed water potential for the upper limit is less clear, but the process, apparently, was similar to that for the lower limit, and -⅓ bar was established as the drained upper limit water potential in soil.

Sunflowers against a blue sky

Briggs and Shantz planted sunflowers in small pots under greenhouse conditions, letting the plants use the water until they couldn’t recover overnight, after which they carefully measured the water content (WC).

Damage it does:  

In practice, using -15 bars to calculate permanent wilting point probably isn’t a bad starting point, but in principle, it’s horrible. Over the years we have set up experiments like Briggs and Shantz did and measured water potential. We have also measured field soils after plants have extracted all the water they can.  Permanent wilting point never once came out at -15 bars or -1.5 MPa.  For things like potatoes, it was approximately -10 bars (-1 MPa), and for wheat it was approximately -30 bars (-3 MPa).  We found that the permanent wilting point varies with the species and even with soil texture to some extent.

Of course, in the end it doesn’t matter much as the moisture release curve is pretty steep on the dry end, and whether you predict it to be 10 or 12% WC, it doesn’t make a huge difference in the size of the soil water reservoir that you compute.

However, on the field capacity end of the scale, it matters a lot.  If you went out and made measurements of the water potentials in soils a few days after a rain, it would be an absolute accident if any of them were ever -⅓ bar (-33 kPa).  I’ve never seen it.  A layered soil, a soil that has a fine-textured horizon on top of a coarse-textured soil, will hold twice as much water as you’ll predict from the -⅓ bar value.  On the other hand, if you’re getting pretty frequent rains or irrigation, that field capacity number becomes irrelevant. Thus, -⅓ bar may be a useful starting point for determining field capacity, but it’s only a starting point.

Why it’s wrong:

Field capacity and permanent wilting point are dynamic properties.  They depend on the rate at which the water is being extracted or the rate at which it’s being applied.  They also depend on the time you wait to sample after irrigation. Think of the soil as a leaky bucket.  If you were trying to carry water in a leaky bucket and you walked slowly, the bucket would be empty by the time you get the water where you want it. However, if you run fast, there will still be some water left in the bucket.  Similarly, if a plant can use water up rapidly, most of it will be intercepted, but if a plant is using water slowly, the water will move down past the root zone and out the bottom of the soil profile before the plant can use it.  These are dynamic phenomena that you are trying to describe with static variables.  And that’s where part of the problem comes.  We need a number to do our calculations with, but it’s important to understand the factors that affect that number.

Rye Field

Rye field

What do we do now:

What I hope we can do is better educate people. We should teach that we need a value we call field capacity or permanent wilting point, but it’s going to be a dynamic property.  We can start out by saying: our best guess is that it will be -⅓ bar for finer-textured soils and -1/10 bar (-10 kPa) for coarser-textured soils. But when we dig a hole and find out there is layering in the profile or textural discontinuities, we’d better adjust our number.  If we’re dealing with irrigated farmland, the adjustment will always be up, and if we’re dealing with dryland or rain-fed agriculture where the time between water additions is longer, we’ll use a lower number.

Some Ideas Never Die:

One of the contributors to the book, This Idea Must Die, Dr. Steve Levitt, had this to say about outdated scientific ideas, and we agree:  “I love the idea of killing off bad ideas because if there’s one thing that I know in my own life, it’s that ideas that I’ve been told a long time ago stick with me,  and you often forget whether they have good sources or whether they’re real. You just live by them. They make sense. The worst kind of old ideas are the ones that are intuitive. The ones that fit with your worldview, and so, unless you have something really strong to challenge them, you hang on to them forever.”

Harness the power of soil moisture

Researchers measure evapotranspiration and precipitation to understand the fate of water—how much moisture is deposited, used, and leaving the system. But if you only measure withdrawals and deposits, you’re missing out on water that is (or is not) available in the soil moisture savings account. Soil moisture is a powerful tool you can use to predict how much water is available to plants, if water will move, and where it’s going to go.

In this 20-minute webinar, discover:

  • Why soil moisture is more than just an amount
  • Water content: what it is, how it’s measured, and why you need it
  • Water potential: what it is, how it’s different from water content, and why you need it
  • Whether you should measure water content, water potential, or both
  • Which sensors measure each type of parameter

Take our Soil Moisture Master Class

Six short videos teach you everything you need to know about soil water content and soil water potential—and why you should measure them together.  Plus, master the basics of soil hydraulic conductivity.

Watch it now—>

Download the “Researcher’s complete guide to soil moisture”—>

Download the “Researcher’s complete guide to water potential”—>