i dont get it the humidty is 64% now and at nite its gonna be 77%, isnt it suposed to be more humid in the day than at nite


http://www.weather.com/outlook/recre...i/detail/48160

 

typically not . . . cooler temps support moisture much easier. the heat/sun will dry it out over the course of the day. unless there is a supporting weather pattern that brings in moister air, it's usually more humid at night.

Ruf

 

thats funny, my car pulls alot better at nite...

 

that's because the air is still cooler, ie more dense, thus more power . . . however the degree of your benefit is balanced by how humid it gets.

So generally speaking, cooler air is more important than humidity. Its those hot humid days that are the killer.

Ruf

 

I'll tell ya one thing its fawkin humid as shit here!! not to mention windy...i almost got blown off the road today on my bike

 

Actually, cooler air has the potential to hold less water vapor. Warmer air can hold more water vapor. This is why looking at RH (Relative Humidity) is often misleading. An RH of 60% at 60 and 80 degrees F is not the same amount of water vapor. RH is just a percentage of the amount of water vapor in the air versus the amount it can potentially hold.

Below is a simple chart showing a how the potential amount of water vapor in air decreases with temperature.

Air temperature in degrees C- Water vapor air can hold at this temperature.

30 degrees - 30 grams per cubic meter of air
20 degrees - 17 grams per cubic meter of air
10 degrees - 9 grams per cubic meter of air

So not only is cooler air denser to begin with, it also cannot hold as much O2 displacing water vapor.

The value to actually be somewhat aware of is the Relative Density. This takes into account the barometric pressure, temperature and humidity at the least. This change in relative density can have nearly, and in some cases a greater, a direct impact on power changes.

Here are a few examples of relative densities; I'll use a Baro of 30.00 inHg in all cases below:

RH 75%, Temp 60 F = 99.6%
RH 65%, Temp 75 F = 96.6%

That is a change of 3% or about 9 - 10 HP on a CL-S with typical mods.

Now let’s look at extreme cases:

RH 45%, Temp 45 F = 102.9%
RH 85%, Temp 85 F = 94.2%

This is almost 10%, or about 30 HP on the same said CL-S.

Now let’s factor barometric pressure into the same example:

RH 45%, Temp 45 F, Baro 30.5 inHg = 104.6%
RH 85%, Temp 85 F, Baro 29.0 inHg = 91.1%

This is a 13.5% difference or about 40 hp.

And without taking into account track differences, can you understand why it is pointless to compare time slips run at different tracks at different times. There can be a huge difference in power with the change of conditions alone.

 

You're in Schenectady, what do you expect..

 

And thanks for the indepth details.

I was thinking of the hot summers in Texas where all the weather guys talk about the high heat squeezing out the moisture of the air and then at night with the cooler air the humidity begins to increase.

Ruf

 

Nice post Scalbert! As an added note to the cooler air holding less moisture. Most nights in the summer the humidity gets over 100% This is dewpoint when the cooler air cannot hold as much water as the warmer air and the excess moisture drops out.

 

Here is link to play with: http://wahiduddin.net/calc/calc_hp_abs.htm

 

Sounds like you need to do your racing in the daytime only.

 

humidty has alot to do with quckness in the car.