Temperature Humidity Index Forecast
About the Heat Load Index (HLI)
The heat load index (HLI) is a model that combines temperature, relative humidity, solar radiation and wind speed into a single index.
The HLI was developed and evaluated in unshaded cattle feedlots in Australia and the United States (Gaughan et al. 2018), with heat thresholds determined from animal respiration rates.
The HLI thresholds are dependent on factors such as cattle breed, coat colour, nutrition, access to water and animal acclimatisation to local conditions.For Angus cattle, the critical HLI threshold at which the animal will begin to accumulate heat is an HLI of 86. For Waygu, the threshold is 90 to 91 and for Brahman, the threshold is 96. These thresholds are shown on the forecast map colour legend.
The lower threshold at which an animal will dissipate its heat load is 77. If the hourly HLI values are between the low and high thresholds, then an animal's accumulated heat load will remain constant.
Cattle exposed to prolonged heat loads are likely to have a higher core body temperature, reduce their feed intake and impact their fertility rates. Calves and weak cattle are highly susceptible to high heat loads.
About the Cattle Thermal Stress Forecasts
The forecasts are derived from the Bureau of Meteorology's numerical weather prediction system, ACCESS-G4 (Australian Community Climate and Earth-System Simulator -- Global version 3). The horizontal resolution of ACCESS-G4 is approximately 12.5 km.
The Cattle Comfort Index, Heat Load Index and Accumulated Heat Load Unit prediction maps are calculated from forecasts of screen level temperature (°C), screen level relative humidity (%), average zonal and meridional winds at 10 metres (m/s) and the average downwards shortwave radiation at the surface (W/m2). The Temperature Humidity Index are calculated from only screen level temperature and relative humidity.
The ACCESS-G4 model is run every 6 hours and the forecasts shown above are from a run initialised at 12z (10pm AEST). The forecasts go out to approximately 10 days and are shown at a 3 hourly interval.
Further reading
- Gaughan et al., 2008: A new heat load index for feedlot cattle. J. Anim. Sci., 86, 226--234
- Mader et al., 2010: A comprehensive index for assessing environmental stress in animals. J. Anim. Sci., 88, 2153--2165
- Wang et al., 2018: A review and quantitative assessment of cattle-related thermal indices. J. Therm. Biol., 77, 24--37
- Cowan et al., 2024: Observed climatology and variability of cattle heat stress in Australia, J. Appl. Meteor. Climatol.
About the Cattle Comfort Index (CCI)
The Cattle Comfort Index (CCI) is used for monitoring cattle condition during hot and cold weather. The thermal effects of relative humidity (RHadj), wind speed (WSadj), and solar radiation (RADadj) are adjusted to conform to the units of surface mean air temperature (T; °C), where:
CCI = T + RHadj + RADadj + WSadj
The CCI thresholds for cattle are based on young and/or non-acclimatised animals with high susceptibility (for more information, please refer to Mader et al. 2010 and Wang et al. 2018):
| CCI stress category | Cool/Cold conditions | Warm/Hot conditions |
|---|---|---|
| No stress | > 5°C | < 25°C |
| Mild | 5 → 0 °C | 25 → 30°C |
| Moderate | 0 → -5°C | 30 → 35°C |
| Severe | -5 → -10°C | 35 → 40°C |
| Extreme | -10 → -15°C | 40 → 45°C |
| Extreme danger | < -15°C | > 45°C |
As cold stress levels have been developed by researchers using feedlot cattle data and weather observations in North America's Midwest, the thresholds for Australian tropical cattle breeds are likely to be at higher temperatures (i.e., in the 10-15°C range).
About the Cattle Thermal Stress Forecasts
The forecasts are derived from the Bureau of Meteorology's numerical weather prediction system, ACCESS-G4 (Australian Community Climate and Earth-System Simulator -- Global version 3). The horizontal resolution of ACCESS-G4 is approximately 12.5 km.
The Cattle Comfort Index, Heat Load Index and Accumulated Heat Load Unit prediction maps are calculated from forecasts of screen level temperature (°C), screen level relative humidity (%), average zonal and meridional winds at 10 metres (m/s) and the average downwards shortwave radiation at the surface (W/m2). The Temperature Humidity Index are calculated from only screen level temperature and relative humidity.
The ACCESS-G4 model is run every 6 hours and the forecasts shown above are from a run initialised at 12z (10pm AEST). The forecasts go out to approximately 10 days and are shown at a 3 hourly interval.
Further reading
- Gaughan et al., 2008: A new heat load index for feedlot cattle. J. Anim. Sci., 86, 226--234
- Mader et al., 2010: A comprehensive index for assessing environmental stress in animals. J. Anim. Sci., 88, 2153--2165
- Wang et al., 2018: A review and quantitative assessment of cattle-related thermal indices. J. Therm. Biol., 77, 24--37
- Cowan et al., 2024: Observed climatology and variability of cattle heat stress in Australia, J. Appl. Meteor. Climatol.
About the Temperature Humidity Index (THI)
The Temperature Humidity Index (THI) combines air temperature and relative humidity into one index, making it relatively easy to calculate. Although originally developed as an index for measuring human discomfort, the THI became useful in quantifying heat stress in cattle.
THI Calculation
The THI is calculated as:
THI = (1.8 × Ta + 32) - [(0.55 - 0.0055 × RH) × (1.8 × Ta - 26)]
where Ta is air temperature (°C) and RH is the relative humidity (%). This definition of THI was developed for measuring heat stress in cattle in the outdoors (Wang et al. 2018).
THI Thresholds for Cattle
| Cattle type | Heat stress category | THI range |
|---|---|---|
| Beef | Mild (all breeds) | 72 ≤ THI ≤ 77 |
| Significant (Bos taurus) | 78 ≤ THI ≤ 86 | |
| Significant (all breeds) | 87 ≤ THI ≤ 98 | |
| Severe (all breeds) | THI ≥ 99 | |
| Dairy | Low-mild | 68 ≤ THI ≤ 71 |
| Mild-moderate | 72 ≤ THI ≤ 79 | |
| Moderate-severe | 80 ≤ THI ≤ 89 | |
| Severe | THI ≥ 90 |
About the Cattle Thermal Stress Forecasts
The forecasts are derived from the Bureau of Meteorology's numerical weather prediction system, ACCESS-G4 (Australian Community Climate and Earth-System Simulator -- Global version 3). The horizontal resolution of ACCESS-G4 is approximately 12.5 km.
The Cattle Comfort Index, Heat Load Index and Accumulated Heat Load Unit prediction maps are calculated from forecasts of screen level temperature (°C), screen level relative humidity (%), average zonal and meridional winds at 10 metres (m/s) and the average downwards shortwave radiation at the surface (W/m2). The Temperature Humidity Index are calculated from only screen level temperature and relative humidity.
The ACCESS-G4 model is run every 6 hours and the forecasts shown above are from a run initialised at 12z (10pm AEST). The forecasts go out to approximately 10 days and are shown at a 3 hourly interval.
Further reading
- Gaughan et al., 2008: A new heat load index for feedlot cattle. J. Anim. Sci., 86, 226--234
- Mader et al., 2010: A comprehensive index for assessing environmental stress in animals. J. Anim. Sci., 88, 2153--2165
- Wang et al., 2018: A review and quantitative assessment of cattle-related thermal indices. J. Therm. Biol., 77, 24--37
- Cowan et al., 2024: Observed climatology and variability of cattle heat stress in Australia, J. Appl. Meteor. Climatol.
About the Accumulated Heat Load Unit (AHLU)
The continuous buildup of heat in cattle is measured by the Accumulated Heat Load Unit (AHLU). The AHLU is defined as the amount of heat accumulated in cattle over a day or longer, when the Heat Load Index (HLI) exceeds an upper critical threshold. These upper limits vary across cattle breeds and have been determined from feedlot cattle respiration rates (i.e., panting scores; Gaughan et al. 2008). The lower threshold is generally at an HLI = 77. For cattle, if the HLI stays above their upper threshold over the course of a day and night, then their AHLU will increase. When HLI values sit between the lower and upper HLI thresholds, then an animal’s AHLU will remain constant.
For Australia's most common cattle breeds, the upper HLI thresholds are shown below.
| Cattle breed | Extra information | Upper HLI threshold |
|---|---|---|
| Angus | 100% Bos taurus | 86 |
| Hereford, Poll Hereford, Shorthorn (southern adapted), Limousin, Gelbvieh | 86 | |
| Red Angus | 87 | |
| Murray Grey | 88 | |
| Simmental | Swiss breed | 88 |
| Senepol | 89 | |
| Limousin x Indicus | French breed | 89 |
| Shorthorn (northern adapted) | 90 | |
| Brahman x Angus | Not Brangus | 90 |
| Charbray | 91 | |
| Braford | 38% Bos indicus | 91 |
| Wagyu (Black) | 92 | |
| Ultrablacks | 92 | |
| Santa Gertrudis | 38% Bos indicus | 93 |
| Belmont Red | 93 | |
| Brangus | 38% Bos indicus | 93 |
| Senepol x Brahman, Droughtmaster x Brangus | 93 | |
| Droughtmaster | 50% Bos indicus | 94 |
| Brahman x Droughtmaster | 94 | |
| Brahman | 100% Bos indicus | 96 |
There are three main AHLU risk classifications:
- Moderate: 21 ≤ AHLU ≤ 50
- High: 51 ≤ AHLU ≤ 100
- Extreme: AHLU >100
As each risk category is exceeded, this increases the likelihood of heat-induced stress within cattle, leading to reduced feed intake, hyperthermia (e.g., high core body temperature), and eventual death.
About the Cattle Thermal Stress Forecasts
The forecasts are derived from the Bureau of Meteorology's numerical weather prediction system, ACCESS-G4 (Australian Community Climate and Earth-System Simulator – Global version 3). The horizontal resolution of ACCESS-G4 is approximately 12.5 km.
The Cattle Comfort Index, Heat Load Index and Accumulated Heat Load Unit prediction maps are calculated from forecasts of screen level temperature (°C), screen level relative humidity (%), average zonal and meridional winds at 10 metres (m/s) and the average downwards shortwave radiation at the surface (W/m2). The Temperature Humidity Index are calculated from only screen level temperature and relative humidity.
The ACCESS-G4 model is run every 6 hours and the forecasts shown above are from a run initialised at 12z (10pm AEST). The forecasts go out to approximately 10 days and are shown at a 3 hourly interval.
Further reading
- Gaughan et al., 2008: A new heat load index for feedlot cattle. J. Anim. Sci., 86, 226--234
- Mader et al., 2010: A comprehensive index for assessing environmental stress in animals. J. Anim. Sci., 88, 2153--2165
- Wang et al., 2018: A review and quantitative assessment of cattle-related thermal indices. J. Therm. Biol., 77, 24--37
- Cowan et al., 2024: Observed climatology and variability of cattle heat stress in Australia, J. Appl. Meteor. Climatol.
About the Livestock Chill Index
The Livestock Chill Index (LCI; kJ/m²/hour) is used to measure potential heat loss in sheep and cattle. It accounts for the effects of ambient temperature, wind speed and rainfall on unsheltered livestock:
LCI = [(2.8 + 0.75W0.5)(40 - T) + 70 + (1 – e(-R/25.4)) × 100] × 4.184
Where:
- W = daily average wind speed at 2 metres (m/s), averaged 13z-13z (2300 AEST).
- T = near surface daily average temperature (°C), averaged 13z-13z
- R = daily rainfall (mm), accumulated 15z-15z (0100 AEST).
The LCI was developed for newborn lambs and newly-shorn sheep and the thresholds shown below may not accurately represent the level of risk to northern Australian cattle.
| LCI stress category | Potential heat loss (kJ/m²/hour) |
|---|---|
| Moderate | 900 → 1000 |
| High | 1000 → 1100 |
| Very high | 1100 → 1200 |
| Extreme | >1200 |
The thermal tolerance of tropical and semi-arid breeds (Brahman, Droughtmaster) is likely to be closer to that of shorn sheep and lambs as opposed to southern breeds with longer coats.
About the Cattle Thermal Stress Forecasts
The forecasts are derived from the Bureau of Meteorology's numerical weather prediction system, ACCESS-G4 (Australian Community Climate and Earth-System Simulator – Global version 4). The horizontal resolution of ACCESS-G4 is approximately 12.5 km.
The Cattle Comfort Index, Heat Load Index and Accumulated Heat Load Unit prediction maps are calculated from forecasts of screen level temperature (°C), screen level relative humidity (%), average zonal and meridional winds scaled down to 2 metres (m/s) and the average downwards shortwave radiation at the surface (W/m2). The Temperature Humidity Index is calculated from only screen level temperature and relative humidity. The Livestock Chill Index is calculated from screen level temperature, average zonal and meridional winds, and daily rainfall (mm).
The ACCESS-G4 model is run every 6 hours and the forecasts shown above are from either the 12z (10pm AEST) or 00z (10am AEST) initialised runs. The heat stress forecasts go out to approximately 7 days and are shown at a 3 hourly interval. The Livestock Chill Index relies on daily averages and is shown at daily intervals.
Further reading
- Gaughan et al., 2008: A new heat load index for feedlot cattle. J. Anim. Sci., 86, 226--234
- Mader et al., 2010: A comprehensive index for assessing environmental stress in animals. J. Anim. Sci., 88, 2153--2165
- Wang et al., 2018: A review and quantitative assessment of cattle-related thermal indices. J. Therm. Biol., 77, 24--37
- Cowan et al., 2024: Observed climatology and variability of cattle heat stress in Australia, J. Appl. Meteor. Climatol.