Thread: What an April

Originally Posted by Gasman

I hear the ugly weather is headed your way, Mike.

Been raining and 50° for the past three days.
Should see some sun, later today.

Originally Posted by STF

I don't know the science so i can't argue but, I live in the most southern town in Canada - south of Northern California and I'm a 5 minute walk from Lake Erie.

During the height of the summer when I have to mow the grass twice a week sometimes, it gets so hot that it hurts to be outside and it gets so humid with it that it's like sitting in a swamp.

I don't know about the humidity cooling it down but my personal experience is that it stops the sweat evaporating and makes me feel hot, damp and uncomfortable.

Water has a high specific heat capacity. What that means is that it take a lot of energy, in this case heat, the raise the temperature of it. Therefore when the air is very dry the sun can make the air very hot quite easily. If the air is very humid it takes a lot more energy from the sun to heat the air. That is why it keeps it from getting as hot which is a better way of saying it than makes it cooler. Also, water can hold a lot of heat so if a large body of water is close like say a lake, maybe a Great Lake which can hold a lot of water it can also hold a lot of heat. Conversely, if there is not as much energy from the sun because that side of the earth is tilted away from it then even the energy there is does not heat the air as much because of the amount needed to raise the temperature of the water in the air.

As a result humidity will keep the air from getting as hot but it feels hotter than it is because of waters heat retention potential and, as you pointed out the evaporation principal doesn't work as well. In addition humidity can keep it from getting as cold because it is holds what little heat there is but it feels colder than it is because the humidity in the air draws the heat out of your body.

So, humidity is sort of a double whammy here weather is concerned.

Of course we, and I am sure you in the States get Humidex levels as part of our summer forecast.

My question has to be, if humidity can keep it cooler, why does our weather forecasts show things like 35c, feels (humidex) like 40c ?

Originally Posted by PaulFLUS

Also, water can hold a lot of heat so if a large body of water is close like say a lake, maybe a Great Lake which can hold a lot of water it can also hold a lot of heat.

We live on Lake Superior, the largest of the Great Lakes, but we still get some hot and humid days and nights. The average summer surface temperature of Lake Superior is 10 C/50 F. Generally the surface temp does not go above 20 C/68 F. The shallower Great Lakes are slightly warmer. Normally our temperatures are not as high as those recorded slightly inland.

Bob

Originally Posted by STF

Of course we, and I am sure you in the States get Humidex levels as part of our summer forecast.

My question has to be, if humidity can keep it cooler, why does our weather forecasts show things like 35c, feels (humidex) like 40c ?

Because too much of a good thing is bad, even Ice-Cream.

When you overload the air with moisture it feels like a steam bath. In smaller quantities it does cool the air. It's very possible if you didn't have the moisture the heat would increase.

In coastal areas come the afternoon you often times (not always) get an onshore breeze in the afternoon or evening which helps to cool the land. This has to do with the heating or the land as opposed to the ocean absorbing the heat much better and the pressure differences it causes. There is this whole thing about on shore and offshore air flow.

Of course in a place like Florida I doubt any onshore breeze cools anything it just brings in much more humid air.

I suspect with the size of the Great lakes something similar happens.

Evaporation is a cooling process. When a water molecule turns into vapor the process lowers temperature of the remaining liquid. So high humidity slows down or stops the evaporation from sweating which is the body's primary method of cooling itself. You can definitely die of heat stroke in dry weather however.

100'F air with a RH of 20% has an enthalpy of 33 BTU holds the same amount of water as approximatly 60"F air at 75% RH with an enthalpy of 23 BTU.

The latent heat of vaporization is what makes a swamp cooler work. The change of state is where the heat energy is used.

Yes, if I recall correctly, it's been a long time since I studied the sciences, it is the change of state from a liquid to a gas using heat energy it draws from the surrounding matter that gives the cooling effect...if I'm saying that right.

Yea, that's why it's called "an evaporative cooler". It's the main way to cool things in this neck of the woods.

Basically you have a pad saturated with water and air is drawn through it and the air picks up the moisture. With 100 degree heat and 10% humidity it works almost as good as an air conditioner. However at 30% humidity it don't work so good.

My next older brother was born in Fort Worth. I've lived in Florida most of my life. It goes without saying that they have quite different climates. I remember my father talking about a fan they used with basically a wet sponge in front of it and thinking "huh???" Why would you want to add moisture to the air?