How sensitive is the HLI?

Kite Feedlot Team

December 1, 2023

Tips for Summer

The HLI is called a heat stress index and does not have any units. It is derived from several different weather variables, namely humidity, wind speed and black globe temperature (which is a measure of temperature and radiation). The HLI reacts to changes in these underlying variables in different ways.

Let’s look at wind speed first. Across a yard, there will be many areas that are more exposed, or sheltered than others. The chart below may look complicated but when you know how to read them they can give a large amount of information about how the HLI changes.

Let’s look at the 1 m/s chart first (the middle one). Select the temperature you are interested in from the button axis (for example select 30°C) then select the humidity you are interested in from the vertical axis (for example select 50%). Now follow along the lines until they intersect (in the example it shows the HLI will decrease from 98 to 90 and then 85 as the wind speed increases from calm to 1 m/s and then 5 m/s.

Remember if your weather station is located in a sheltered proposition (or the bearings have not been replaced recently in your anemometer and it is stalling at low wind speeds, which means it will be recording zero and the winds are maybe 2 m/s) then the HLI recorded on your weather station may be significantly higher than what the animals are experiencing.

Now let’s look at humidity. As you all know humidity in the pens can be significantly higher than outside the pens if you have received recent rain. If your pens are also carrying a deep manure pack, the humidity in the pens will be even greater.
In the example below for a wind speed of 2 m/s and temperature of 26°C, the HLI will increase by 4 HLI units every 10% the humidity is increased.

Some sites use 15 mm of rain as a trigger to drop the HLI threshold to account for the difference in humidity in the pens and their weather station (or forecast). Therefore if you normally assess your risk by looking at the AHLU86 you may want to assess AHLU83 or even AHLU80 to account for the different conditions.

Related Articles

Assessing the heat risk at your Processing Plant

Assessing the heat risk at your Processing Plant

To make it easier for the feedlot operators to understand the heat risk at a processor, we have introduced a new feature to start the AHLU calculation every weekday morning at 6 am. Read more.

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