Материал готовится,

пожалуйста, возвращайтесь позднее

пожалуйста, возвращайтесь позднее

PROF JOHN GYAKUM: We need to understand a little bit more about instability.

We need to define a term.

And that term is a lapse rate.

The lapse rate itself is defined to be an upward decrease

in atmospheric temperature.

Now as we will see, there's actually two kinds of lapse rates.

The first kind that we will consider here

is a rate of temperature decrease following an air parcel.

In order to do this, let us note the circle — this very small circle

that we see at the very base of this figure.

And as this circle rises, what we see is that there's a bit of an expansion.

There's an expansion of this circle that actually represents

what we call an adiabatic expansion of an air parcel against a lower pressure.

What is happening is that this air parcel

is, in fact, doing work against its environment.

And according to the first law of thermodynamics,

that air parcel will be losing temperature.

Its temperatures will be falling as a consequence

of the work being done against its environment.

Now what we mean by adiabatic is that there's

no heat that's being exchanged between the air parcel and the surrounding

environment.

So if we can go back to the beginning of this figure, and as it rises,

we see at the very bottom there is a lapse rate,

or a temperature decrease, that amounts to approximately 10 degrees

Celsius per kilometer.

That straight line that we see, that goes up

to what we call the level of condensation,

illustrates the rate at which an air parcel will have its temperature fall.

By the time it gets to the level of condensation, at this point,

something happens.

And there's a sufficient amount of moisture

that's embedded within the air parcels so

that the relative humidity at this point becomes 100%.

Now as this air parcel continues its lift, or rise upwards,

what we see happening is that the lapse rate itself is changing.

This lapse rate is now, instead of a dry adiabatic lapse rate —

which is representative of relative humidities less than 100% —

we now have a relative humidity of about 100%.

In which case, the lapse rate itself is different from the earlier one.

It's modulated by the fact that there is latent heat of condensation

occurring as this air parcel rises.

As the temperatures fall, the temperatures

fall to their saturation vapor pressure, and we have condensation processes.

That heating is actually acting to mitigate

the cooling that would otherwise be occurring

at a rate of 10 degrees Celsius per kilometer.

What is happening here is that this lapse rate

is, instead of being about 10 degrees Celsius per kilometer,

it's now about six degrees Celsius.

That's why the curve itself is a curve and not a straight line.

And as you can see in the illustration very well,

clouds are beginning to form.

So this is the definition of lapse rate following an air parcel.

The wet adiabatic lapse rate tends to be variable,

but approximately 6 degrees Celsius per kilometer.

And the dry adiabatic lapse rate is a constant value

at a rate of 10 degrees Celsius per kilometer.

So now, let's consider putting all this together,

and go back to the concept of stability.

In the leftmost picture, what we see here

is a direct comparison of two different lapse rates.

In the first element, what we see is the this DALR,

which is the dry adiabatic lapse rate.

Remember this is 10 degrees Celsius per kilometer.

Now what we actually see in the right part of this picture

is something called the environmental lapse rate.

We need to define a term.

And that term is a lapse rate.

The lapse rate itself is defined to be an upward decrease

in atmospheric temperature.

Now as we will see, there's actually two kinds of lapse rates.

The first kind that we will consider here

is a rate of temperature decrease following an air parcel.

In order to do this, let us note the circle — this very small circle

that we see at the very base of this figure.

And as this circle rises, what we see is that there's a bit of an expansion.

There's an expansion of this circle that actually represents

what we call an adiabatic expansion of an air parcel against a lower pressure.

What is happening is that this air parcel

is, in fact, doing work against its environment.

And according to the first law of thermodynamics,

that air parcel will be losing temperature.

Its temperatures will be falling as a consequence

of the work being done against its environment.

Now what we mean by adiabatic is that there's

no heat that's being exchanged between the air parcel and the surrounding

environment.

So if we can go back to the beginning of this figure, and as it rises,

we see at the very bottom there is a lapse rate,

or a temperature decrease, that amounts to approximately 10 degrees

Celsius per kilometer.

That straight line that we see, that goes up

to what we call the level of condensation,

illustrates the rate at which an air parcel will have its temperature fall.

By the time it gets to the level of condensation, at this point,

something happens.

And there's a sufficient amount of moisture

that's embedded within the air parcels so

that the relative humidity at this point becomes 100%.

Now as this air parcel continues its lift, or rise upwards,

what we see happening is that the lapse rate itself is changing.

This lapse rate is now, instead of a dry adiabatic lapse rate —

which is representative of relative humidities less than 100% —

we now have a relative humidity of about 100%.

In which case, the lapse rate itself is different from the earlier one.

It's modulated by the fact that there is latent heat of condensation

occurring as this air parcel rises.

As the temperatures fall, the temperatures

fall to their saturation vapor pressure, and we have condensation processes.

That heating is actually acting to mitigate

the cooling that would otherwise be occurring

at a rate of 10 degrees Celsius per kilometer.

What is happening here is that this lapse rate

is, instead of being about 10 degrees Celsius per kilometer,

it's now about six degrees Celsius.

That's why the curve itself is a curve and not a straight line.

And as you can see in the illustration very well,

clouds are beginning to form.

So this is the definition of lapse rate following an air parcel.

The wet adiabatic lapse rate tends to be variable,

but approximately 6 degrees Celsius per kilometer.

And the dry adiabatic lapse rate is a constant value

at a rate of 10 degrees Celsius per kilometer.

So now, let's consider putting all this together,

and go back to the concept of stability.

In the leftmost picture, what we see here

is a direct comparison of two different lapse rates.

In the first element, what we see is the this DALR,

which is the dry adiabatic lapse rate.

Remember this is 10 degrees Celsius per kilometer.

Now what we actually see in the right part of this picture

is something called the environmental lapse rate.

Загрузка...

Выбрать следующее задание

Ты добавил

Выбрать следующее задание

Ты добавил