The Purrfect Nanny & Purrfect Pet Health Blog

Useful guidelines for riding in this heat:

From an article about the death Saturday night of a horse in a show in Tennessee:

Add air temperature and relative humidity and subtract wind speed if your total adds up to 180 or above don’t ride, if it is 130-170 use caution, 130 or below ride!

Determine: Temperature (F) + relative humidity (%) ­ wind speed
For example:
Temperature (F) 79
Relative Humidity (%) 58
Wind Speed 4.6 (MPH)
Answer = 132.4

Less than 130: All ­horses can function to cool themselves assuming
adequate hydration.
130 ­ 170: Caution horse’s cooling mechanisms can only partially
function as intended. Some cooling management procedures will need to be performed.
180 or above: Stop­ horse’s cooling systems cannot and will not
function adequately. All cooling procedures will need to be utilized
to keep the horse out of serious trouble.

Why is it an issue for the horse when heat and humidity combine to
equal 180+? What doesn’t work and why? What are some of the
physiological ramifications? What are some of the symptoms?
Heat is produced by muscles in the metabolic conversion of chemical
energy to the mechanical energy required for muscle contraction and
limb movement. Seventy-five to eighty percent of the chemical energy is converted to heat, which moves from the contracting skeletal muscles to the surrounding tissues by the flow of lymph and blood.
Assuming a comparable rate of exercise intensity, the rate of
cooling, or heat loss is affected by air temperature, wind velocity
and humidity. (Werner, 1993). Heat can also be lost in a fourth way,
conduction, which is a direct transfer of heat from the skin or feet
to surfaces in direct contact (such as an ice bag on the skin).

So, thinking about the chart and the equation:
Temperature (F) + relative humidity (%) ­ wind speed (MPH), we see
how the ability of the horse to cool itself in these four different
ways will be affected:

 In cool temps with low humidity, heat loss through convection and
conduction can be as much as 50%. Heat can also be lost through
radiation, with as much as 60% of a body’s heat lost in this way when
air temperatures are cool. The numbers in our equation would add up
to much less than 180, and the horse would have no difficulty cooling
itself.

As temperatures rise, the thermal gradient for heat dissipation is
reduced, resulting is less convective, conductive and radiative heat
loss and more evaporative cooling. The evaporation of water from the
skin surface is the most important means of heat dissipation in high-
heat/low-humidity conditions. So, when we get a high temperature
reading with low humidity, a horse may still not have difficulty
cooling, but if temperatures are extremely high with no wind, we
might get a result above 140, which would means our horse needs our help cooling off.

With high humidity, sweat cannot evaporate as easily and so the
ability of the horse to cool itself in this important way is reduced.
When high humidity is combined with high temperatures, (which we just saw reduce the effectiveness of radiant, conductive and convective cooling), the horse has now lost all four means to cool itself and is in a dangerous situation, subject to a greater rate of heat
accumulation within his body.