Basic weather question

[C172Heavy]

Got a question that has had me puzzled for a while. Does a cloud always form when the temp of the air reaches just below the dewpoint?

For example on the ground the temp is 12 and the dewpoint is 8. Assuming a standard lapse rate of 2 deg C per 1000' your air temp would be 8 at 2000' assuming a sea level departure. Shouldn't a cloud form at that point or close to it?

Anyone have any good insight as to why no cloud forms? Has the dewpoint changed at that altitude or something?

Just wondering.

 

[banned username 2]

First off, the temp can never be below the dewpoint. Basically the dewpoint temp is the temp a parcel of air has to be cooled to in order for that same air to be saturated (100% humidity).

Remember as you go up, the air changes. the air at 2000 AGL might have less water content (hence a lower dew point) than the air on the surface.

Clouds also need some type of condensation nuclei... whether it be a dust particle or something else... 100% humidity in itself won't develop a cloud.

Hope this helps.

 

[trainerjet]

First off, the dewpoint can never be below the temp.

I thought the dewpoint was always below (or equal to, in the case of 100% relative humidity) the temp.

 

[banned username 2]

I thought the dewpoint was always below (or equal to, in the case of 100% relative humidity) the temp.

Oooppssss... Typo!

 

[Vladimir Lenin]

Why couldn't the dew point be less than the temp? Dew point is the stage where a known quantity of air cools to a point where water vapors become visible, because cold air holds less water than warm air. So, as the air cools, the volume becomes more constricted and moisture becomes visible, sorta like wringing towels.

 

[trainerjet]

Oooppssss... Typo!

Yep..I thought you meant it the other way around...that the temp can never be below the dewpoint.

 

[Vladimir Lenin]

Oh, and you have to have sufficient moisture to form a cloud in a given parcel of air, or otherwise, it would have to be cooled to a higher degree (higher altitude, due to expansion cooling, among others) for the water to become visible.

 

[banned username 2]

Why couldn't the dew point be less than the temp?

The dewpoint is ALWAYS less than or equal to the air temp, it can never be higher than the air temp.

 

[A Squared]

C172heavy,

It's a rule of thumb, a useful one, but an approximation. Often you *can* use it to predict the height of the cloudbases, sometimes it doesn't work. THe humidity aloft may be less (dewpoint lower) or the lapse rate may not be standard due to a temperature inversion or some other reason.

remember all these sorts of things assume nice uniform standard conditions, and in reality the atmosphere is anything but standard. Take it in the context of being approximations and generalities, not hard and fast rules.

regards

 

[Singlecoil]

I thought clouds only form when a 747 on a chemtrail clearance from the CIA flies over and emanates their "exhaust".