Need urgent help: air velocity and precipitation…

[Winkiepinkie]

Dear all, another one of my seemingly useless questions.

Right, driving home last night in foggy (i.e. high jolly humid) Milan and when I got out I noticed that condensation/water had formed on the tip of the bonnet (corner where vertical bit of headlight area leads to horizontal bonnet), top of the leading edge (where windscreen becomes roof) and sides of the mirrors. Obviously this is where the air was travelling fastest as it was met by the on-coming car at 180km/hr. I thought "ah yes, air is compressed by on coming car and relative humidity goes through the roof and hey presto, precipitation (water/rain etc). Physics in action" I thought smugly to myself wishing my girlfriend was there so I could show her how smart I was.

And this is where my world started to come a little unstuck at the sides. As air is accelerated, pressure drops, hence reducing relative humidity and reducing the chance of precipitation/rain, i.e. the given volume of air is able to hold more moisture. So why did I get precipitation on the areas of the car where the air had passed at its fasted?

Can anyone shed any light on this because its really upsetting me and I’m heading for a full blown nervous breakdown. I couldn’t sleep last night.

Thanks in advance.

PS: Wasn’t quite sure where to put this so it went in "wannabes" coz I thought all you clever wannabes would know the answers, what, with all that ATPL swotting etc…

[M.85]

was it a full moon night?

M.85

[Winkiepinkie]

Nice try M.85

Don’t remember, or perhaps I should say…… DUNNO, COZ IT WAS SO BL00DY FOGGY I COULDN’T SEE A DAMN THING!!!

My my, M.85, you almost caught me out. Quick check on the net reveals moon is almost ¾ if that’s any help…

W

[High Wing Drifter]

As air is accelerated, pressure drops, hence reducing relative humidity and reducing the chance of precipitation/rain, i.e. the given volume of air is able to hold more moisture. So why did I get precipitation on the areas of the car where the air had passed at its fasted?

Its the other way around. As the fastest air has the lowest pressure, then RH increases...in this case to 100%. RH deacreases with an increase in pressure.

[Dick Whittingham]

Well, on the old adiabatic expansion, air gets colder, which increases RH, and at the same time the lower pressure reduces RH. Of the two effects, the temp effect is dominant, so reducing pressure eventually leads to condensation.

Think explosive decompression, so graphically demonstated in the movies. Bang, and cabin fills with fog! Also think dew point at height, which is a lower temp than at the surface for the same balloon of air as it rises, expands in the low pressure and cools. Notwithstanding this, the temp drop quickly catches up and cloud forms.

Also the vortex trails you see from wingtips etc, which is water condensing out in the low pressure at the vortex centre

Dick W

[Winkiepinkie]

My stupidity just amazes me sometimes.

Thank you both DW and HWD for your answers. All the best.

[High Wing Drifter]

No problem, but looks like I needed reminding too! Thanks Dick Doubleya