Liquid water path

Key takeaways

Liquid water path belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Liquid water path to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Liquid water path from memory before moving on to harder problems.

Reference excerpt

Liquid water path - in units of g/m2 is a measure of the total amount of liquid water present between two points in the atmosphere. LWP is an important quantity in understanding radiative transfer in the atmosphere. It is defined as the integral of liquid water content between two points in the atmosphere. For nadir observations and whole atmospheric column we have

L W P = ∫ z = 0 ∞ ρ a i r r L d z ′ {\displaystyle LWP=\int _{z=0}^{\infty }\rho _{air}r_{L}dz'}

where rL is the liquid water mixing ratio and ρair is the density of air (including water loading). The atmosphere is in approximate hydrostatic equilibrium and hydrostatic equation for atmospheric pressure is given by

d p d z = − ρ a i r g {\displaystyle {\frac {dp}{dz}}=-\rho _{air}g}

which gives

L W P = ∫ 0 p = p 0 r L d p / g {\displaystyle LWP=\int _{0}^{p=p_{0}}r_{L}dp/g}

where g is gravitational acceleration, dp is the pressure increment between two layers in the atmosphere and integration is between surface and top of the atmosphere. Liquid water path can also be defined between any two selected points. The liquid water path can be approximately retrieved from passive and active remote sensing such as microwave radiometer instruments, for example SSM/I. Typical values of liquid water path in marine stratocumulus can be of the order of 20-80 g/m2. The liquid water path also contributes to important cloud properties. As the value of the liquid water path increases, so does the albedo of the cloud. This increase in albedo is seen most quickly at the lower end of the liquid water path spectrum, that is, the smaller the amount of total water, the quicker the albedo increases. The radiative absorption of clouds is also dependent on the liquid water path. An increase of liquid water path leads to an increase in absorption. Again, the largest increase is seen with lower levels of liquid water path. These connections are due to the proportionality between the liquid water path and the optical depth of the cloud.

Worked examples

Example 1 — a first encounter with Liquid water path

Start with the simplest possible case. Write down what Liquid water path claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Liquid water path before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Liquid water path ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Liquid water path

In research: Liquid water path appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Liquid water path in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Liquid water path is common in secondary-school and first-year university syllabi. It links to neighbouring topics Atmospheric science stubs, Atmospheric thermodynamics, so understanding it makes those chapters shorter.
In everyday life: Look for Liquid water path outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Liquid water path in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Liquid water path means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Liquid water path out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Liquid water path in simple terms?

Liquid water path - in units of g/m2 is a measure of the total amount of liquid water present between two points in the atmosphere. LWP is an important quantity in understanding radiative transfer in the atmosphere.

Why does Liquid water path matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Liquid water path?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Liquid water path.