Liquid water content

Key takeaways

Liquid water content 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 content to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Liquid water content from memory before moving on to harder problems.

Reference excerpt

The liquid water content (LWC) is the measure of the mass of the water in a cloud in a specified amount of dry air. It is typically measured per volume of air (g/m3) or mass of air (g/kg) (Bohren, 1998). This variable is important in figuring out which types of clouds are likely to form and is strongly linked to three other cloud microphysical variables: the cloud drop effective radius, the cloud drop number concentration, and the cloud drop size distribution (Wallace, 2006). Being able to determine the cloud formations that are likely to occur is extremely useful for weather forecasting as cumulonimbus clouds are related to thunderstorms and heavy rain whereas cirrus clouds are not directly associated with precipitation.

Characteristics The liquid water content of a cloud varies significantly depending on the type of clouds present in the atmosphere at a given location. The classification of the cloud is highly related to the liquid water content as well as the origin of the cloud. The combination of these two allows a forecaster to more readily predict the types of conditions that will be in an area based on the types of clouds that are forming or have already formed.

Relation to classification of clouds Clouds that have low densities, such as cirrus clouds, contain very little water, thus resulting in relatively low liquid water content values of around .03 g/m3. Clouds that have high densities, like cumulonimbus clouds, have much higher liquid water content values that are around 1-3 g/m3, as more liquid is present in the same amount of space. Below is a chart giving typical LWC values of various cloud types (Thompson, 2007).

Maritime vs. continental Maritime clouds tend to have fewer water droplets than continental clouds. The majority of maritime clouds have droplet concentrations between 100 drops/cm3 and about 200 drops/cm3 (Wallace, 2006). Continental clouds have much higher droplet concentrations ranging up to around 900 drops/cm3. (Wallace, 2006). However, the droplet radius in maritime clouds tend to be larger, so that the result is that the LWC is relatively similar in both types of air masses for the same types of clouds (Linacre, 1998).

Measuring techniques There are several ways that can be used to measure the liquid water content of clouds. One way involves an electrically heated wire. The wire is attached to the power supply and is situated on the outside of the airplane. As it moves through a cloud, water droplets hit the wire and evaporate, reducing the temperature of the wire. The resistance caused by this is measured and is used to determine the power needed to maintain the temperature. The power can be converted to a value for the LWC. (Wallace, 2006). Another way involves an instrument that uses scattered light from a large number of drops. This value is then converted to a value for the LWC. (Wallace, 2006). A cloud chamber can also be used to simulate adiabatic ascent in the atmosphere through the decrease of pressure by removing air inside the chamber. A series of equations shown in the section below show how the LWC is obtained in this procedure. (Thompson, 2007).

Equations/relations Various equations are useful in determining LWC and the effects that influence it. One of the most significant variables related to the LWC is the droplet concentration of a cloud.

Cloud droplet concentration The droplet concentration of a cloud is the number of water droplets in a volume of cloud, typically a cubic centimeter (Wallace, 2006). The formula for the droplet concentration is as follows.

n = N / V {\displaystyle n=N/V}

In this equation, N is the total number of water droplets in the volume, and V is the total volume of the cloud being measured. Converting this to a LWC gives an equation that is shown below.

L W C = ( m w ⋅ n ) / N {\displaystyle LWC=(m_{w}\cdot n)/N}

In this equation, mw is the mass of the water in the air parcel.

Cloud chamber A common type of experiment is one that involves a cloud chamber that is de-pressurized to simulate adiabatic ascent of air parcels. Determining LWC is a simple calculation shown below (Thompson, 2007).

L W C = m w / V c {\displaystyle LWC=m_{w}/V_{c}}

Mw is the mass of the water in the cloud chamber and Vc is the volume of the cloud chamber. Obtaining the mass of the liquid water in the cloud chamber is possible through an equation involving the latent heat of condensation (Thompson, 2007).

m w = − m a ⋅ c p ⋅ Δ T a L c ( T ) {\displaystyle m_{w}={\frac {-m_{a}\cdot c_{p}\cdot \Delta T_{a}}{L_{c}(T)}}}

In the equation above, Lc(T) is the latent heat of condensation of water at temperature T, ma is the mass of the air in the cloud chamber, cp is the specific heat of dry air at constant pressure and Δ T a {\displaystyle \Delta T_{a}} is the change in the temperature of the air due to latent heat.

Worked examples

Example 1 — a first encounter with Liquid water content

Start with the simplest possible case. Write down what Liquid water content 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 content 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 content 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 content

In research: Liquid water content 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 content 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 content is common in secondary-school and first-year university syllabi. It links to neighbouring topics Atmospheric thermodynamics, Cloud and fog physics, so understanding it makes those chapters shorter.
In everyday life: Look for Liquid water content 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 content in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Liquid water content 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 content out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Liquid water content in simple terms?

The liquid water content (LWC) is the measure of the mass of the water in a cloud in a specified amount of dry air. It is typically measured per volume of air (g/m3) or mass of air (g/kg) (Bohren, 1998).

Why does Liquid water content 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 content?

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 content.