Wind chill factor and orchids

[ArronOB]

Early this morning the temperature was described as being 11.7 degrees Celsius, “but feels like 7.5 degrees”.

I assume the difference of 4 degrees is due to wind chill factor, as it quite windy here today.

Something I’ve always wondered about. Does windchill factor affect things like plants, or is it only relevant to endothermic things like mammals?

Thinking it through, without the benefits of a scientific background, I’m thinking it could only affect something with internal heat, which could be lost to the ambient in greater or lesser quantities due to environmental factors like wind.

Obviously this is relevant to orchids growing outdoors, or in my case in shadehouses not screened from the wind.

Can anyone with a better grasp of physics clarify this please.

Cheers
Arron

[estación seca]

I would agree with you.

[Subrosa]

Plants transpire water through their stoma via evaporation. Evaporation is a cooling process. All else being equal, increased air movement will increase evaporation, and therefore cooling. Plants are susceptible to wind chill. Ectotherms are more susceptible than endotherms because they can't replace lost heat without an external source.

[ArronOB]

Originally Posted by Subrosa

Plants transpire water through their stoma via evaporation. Evaporation is a cooling process. All else being equal, increased air movement will increase evaporation, and therefore cooling. Plants are susceptible to wind chill. Ectotherms are more susceptible than endotherms because they can't replace lost heat without an external source.

I don’t understand this. Isn’t evaporation only a cooling process if the body in question is of higher temperature then the ambient ?

Put another way, if the body is the same temperature as the ambient air, then why does loosing moisture reduce its temperature ?

[plantzzzzz]

Originally Posted by ArronOB

I don’t understand this. Isn’t evaporation only a cooling process if the body in question is of higher temperature then the ambient ?

Put another way, if the body is the same temperature as the ambient air, then why does loosing moisture reduce its temperature ?

I think you're correct.

If it's 10c outside, but it feels like 5c due to windchill, that means that your body is losing heat at a rate that feels like it is 5c outside.

(living) humans will never actually reach 5c, but plants will, at which point the rate of heat loss stops mattering.

[Subrosa]

Leaves have more thermal mass than air, and will tend to maintain their temperature at least somewhat as the temp of the surrounding air changes. Consider also that evaporating water can cool it enough to freeze.

[ArronOB]

OK, I did what I should have done at the start (but I never seem to think of it) and typed ‘plants and wind chill’ into Google and got a lot of responses showing that wind chill doesn’t matter to plants, like this one from Michigan University

Wind chill doesn’t really matter to a plant |


MSU Extension

At least it doesn’t matter in terms of temperature, but it seems it matters in terms of desiccation. That’s timely knowledge because I was looking at my plants today and noticed that they were looking dried out - at least the leaf surfaces were - much more so then could be explained by the watering cycle. I guess I know why now.

[Ray]

Originally Posted by ArronOB

Put another way, if the body is the same temperature as the ambient air, then why does loosing moisture reduce its temperature ?

It isn't the loss of moisture that causes the cooling, it is the physical process of evaporation that does.

In order for liquid water to become vapor, it must absorb sufficient energy for the molecules to vibrate so actively that the bonds that keep it a liquid are broken. That is heat energy, and it will come from either the body losing the water, cooling it, and/or from the environment around it.

Back to "wind chill" for a moment, I think that If there is a sufficient wind velocity to cause a significant chill, the bigger issue is the drying of the plant tissues, not the cooling of them.

[ArronOB]

Originally Posted by Ray

It isn't the loss of moisture that causes the cooling, it is the physical process of evaporation that does.

In order for liquid water to become vapor, it must absorb sufficient energy for the molecules to vibrate so actively that the bonds that keep it a liquid are broken. That is heat energy, and it will come from either the body losing the water, cooling it, and/or from the environment around it.

Back to "wind chill" for a moment, I think that If there is a sufficient wind velocity to cause a significant chill, the bigger issue is the drying of the plant tissues, not the cooling of them.

Sooo, spelling out a conclusion for someone who never paid attention during science class, you’re saying that strong winds DO lower the temperature of plants towards the wind chill minimum, though the bigger problem might be desiccation. Is that right ?

[Puja]

Originally Posted by ArronOB

Sooo, spelling out a conclusion for someone who never paid attention during science class, you’re saying that strong winds DO lower the temperature of plants towards the wind chill minimum, though the bigger problem might be desiccation. Is that right ?

Essentially the article you linked boils down to this. Strong winds do chill a plant if it were warmer than the air that is blowing past it. Mammals including people need a very high body temperature to stay alive, and so are far warmer than cold air. Hence wind chill factor, which essentially just increases the speed with which things exposed to that wind are cooled off approaching air temperature. The bigger the difference between the object's temperature and the air temperature, the faster the cooling (just like boiled water cools to drinking temperature within minutes, while it remains lukewarm for over an hour before cooling to room temperature).
Because plants have a much lower temperature than people and also can tolerate much bigger differences in temperature, the cooling effect of wind chill is much less serious for plants. There would only be a serious risk of cooling the plant below what it could tolerate if the air itself was already a dangerous temperature (such as freezing temperatures for orchids).
Or to explain it more simply; plants will only tolerate air temperatures to which they themselves can cool down - because wind chill won't cool an object to below air temperature, it isn't a serious problem unless you place the orchid in a place with temperature too low for them to handle anyway (strong winds or not).
Wind chill can be a problem for people, however, because we need to maintain body temperatures way higher than some of the cold temperatures in which we can still live (e.g. humans can get used to swimming in ice water without special gear), so for us cooling down towards air temperature faster forces the body to work much harder to counteract that cooling effect.

The passing winds do absorb moisture whereas still air gets more saturated and so the plant does dry out much faster with increased winds. Hence at any temperature (but especially when air is dry) strong winds can be problematic for sensitive plants.
This is also true for people, actually. If you've ever been up high in the mountains in a dry climate/season, you may have noticed you are likely to suffer chapped lips, fingers and scabby dried out patches of skin on your face and neck which is due to the strong, dry winds.