LAPSE RATES?

[Mike1956]

Hoping someone can straighten this out for me.

Lapse rate for dry air is -1 degree C/100m (ALR or DALR)

Lapse rate for wet air is +/- -0.5 to -0.8 degree C/100m (SALR)

What I am not clear about is whether the SALR depends on condensation occuring?

Is SALR appropriate just because air is wetter

OR

Is SALR only relevant if condensation occurs

OR

Mixture of the 2 (Is higher figure for SALR correct if no condensation and lower figure correct if condensation does occur.

Thanks

Mike

[a64pilot]

I think what you want to do is google "Adiabatic lapse rate"
what are you trying to determine?
Never heard a sailor care about this, seems to be an aviating thing

[SFS]

Saturated in SALR means 100% relative humidity. How much beyond "wet" that is, from a meterology standpoint, I don't know. Typically 100% relative humidity is a pre-condensation state.

If I recall my meteorology classes correctly, you would not use a mixture of the two values.

What are you trying to determine? The only thing I can think of that is related to your question (other than aircraft performance issues for things like takeoff in non-STP or non-sea level airports) is instability of airmasses. Potential (or convective) instability occurs when dry air travels over warm wet air closer to the surface. The lapse rate increases when lifting happens because the humid air at lower altitudes cools at the SALR while the mid-level air cools at the dry adiabatic lapse rate. Are you wondering about convective activity over high-altitude lakes?

[StuM]

Originally Posted by SFS

Saturated in SALR means 100% relative humidity. How much beyond "wet" that is, from a meterology standpoint, I don't know. Typically 100% relative humidity is a pre-condensation state.

If I recall my meteorology classes correctly, you would not use a mixture of the two values.

What are you trying to determine? The only thing I can think of that is related to your question (other than aircraft performance issues for things like takeoff in non-STP or non-sea level airports) is instability of airmasses. Potential (or convective) instability occurs when dry air travels over warm wet air closer to the surface. The lapse rate increases when lifting happens because the humid air at lower altitudes cools at the SALR while the mid-level air cools at the dry adiabatic lapse rate. Are you wondering about convective activity over high-altitude lakes?

Foehn winds?

They have a major effect on winds on the East Coast of the South Island of New Zealand for example. Moist air crosses the Tasman and dumps its moisture as it rises to cross over the Southern Alps. This can result in very strong, hot, dry "Nor'westers." on the east coast in summer.

[StuM]

To the OP's question. Dry rate applies to to a rising air mass until the air mass reaches its Dew Point. Once condensation starts to occur, the Saturated rate is applicable. Similarly, once the water depleted air mass starts to descent and its temperature rises above its new dew point, the dry rate again applies.

So which rate applies depends on the the temperature of the air mass, its dew point and the altitude of that air mass.

[SFS]

Originally Posted by StuM

Foehn winds?

They have a major effect on winds on the East Coast of the South Island of New Zealand for example. Moist air crosses the Tasman and dumps its moisture as it rises to cross over the Southern Alps. This can result in very strong, hot, dry "Nor'westers." on the east coast in summer.

Standard example of a rain shadow arid region caused by orographic lifting. Happens lots of places - western Washington state is basically a desert formed in the rain shadow of the Cascades.

I was thinking more of a frontal boundary, where two air masses are colliding.

Still hope to hear what was on the OP's mind.

[Mike1956]

Originally Posted by StuM

To the OP's question. Dry rate applies to to a rising air mass until the air mass reaches its Dew Point. Once condensation starts to occur, the Saturated rate is applicable. Similarly, once the water depleted air mass starts to descent and its temperature rises above its new dew point, the dry rate again applies.

So which rate applies depends on the the temperature of the air mass, its dew point and the altitude of that air mass.

Thanks StuM

Just for reassurance.......

1. The dry rate applies initially irrespective of the moisture content of the air?

2. The SALR kicks in only when dew point is reached and condensation commences which as far as I understand is due to LATENT heat being released?

If you can confirm the 2 statements I have made above are correct you have sorted it for me.

Thanks

Mike

[StuM]

Originally Posted by Mike1956

Thanks StuM

Just for reassurance.......

1. The dry rate applies initially irrespective of the moisture content of the air?

2. The SALR kicks in only when dew point is reached and condensation commences which as far as I understand is due to LATENT heat being released?

If you can confirm the 2 statements I have made above are correct you have sorted it for me.

Thanks

Mike

Correct, the difference in the rates is caused by the latent heat of the water vapour in the air being released once the dew point is reached.

[belizesailor]

Whild lapse rate is an important weather concept, Ive never found it practically useful for marine weather.

OP: are u just trying to understand the calcs or do you have a use in mind?