Humidity

Clouds, fogs, precipitation

The percentages don’t add up! Water vapor is considered an add-on and can be as much as 4%.

That’s called absolute humidity.

Relative Humidity

The capacity of air to carry water vapor depends on temperature. At 10°C the air can only carry 9 grams in each cubic meter of air…

Water vapor is invisible. If you are not at capacity, skies are perfectly clear.

How close is the air to being at full capacity to hold water as vapor?

That depends on (1) the actual amount of vapor and (2) the capacity amount. This ratio is called the Relative Humidity.

RH = w/ws x 100% alternately,

RH = e/es x 100%w is called the mixing ratio ws is the saturation

mixing ratio.

e is the vapor pressure, es is the saturation vapor pressure.

When w = ws or e=es, then the RH is 100%. We call that

Saturation

When air is at saturation, you see clouds or fog.

Use the chart. If the temperature is 50°F which is 10°C, what is the saturation mixing ratio (ws)?

Now suppose the actual water vapor content (w) is only 4.5 g in the same volume. What is the RH?

Answer: at 50°F, ws = 9 grams. If w = 4.5 g,

RH = w/ws x 100% = 4.5/9 x 100% = 50%

Since 1 cubic meter of air is around 1 kg in mass, we express (saturation) mixing ratio in g/kg.

It’s hard to measure w and ws

In a moment we’ll see an easy way to do it.

This is a fog in the Susquehanna River Valley

You can’t see the valley here because of thick fog. RH = 100% in this fog.

Same view on a foggy day and a clear day.

In this fog the visibility is around ¼ mile (you can’t see farther than that)

Dew is another way you see 100% relative humidity. Water vapor has condensed onto the grass.

The dew point is the temperature at which dew forms. RH = 100% when the temperature reaches the dew point temperature (call it Td)

Since ws depends only on Temperature, T, each one has its own ws:

Here’s the fabulous part:

Since RH = 100% when T = Td, and RH = w/ws x 100%, then it follows that

at saturation, RH = 1 x 100% so w = ws.

Since T = Td at saturation and each T has its own ws, then at saturation, the dew point also shows you ws.

Furthermore, since w = ws, the mixing ratio (w) at saturation has its own Td.

The dew point allows you to easily measure w, which tells you exactly how many grams of water vapor is actually in the air.

Remember, T ≠ ws and Td ≠ w, so NEVER calculate Relative Humidity with the ratio T/Td. Always use

RH = w/ws x 100%

Find w and ws by using the Dew Point and Temperature. We have tables for this, as you’ve seen.

Here’s the average distribution of fog in the U.S. over the course of a year:

Not very foggy in Arizona

Really foggy in NY and New England

Why is it often foggy in New England but rarely foggy in Arizona?2. It’s hot in Arizona, so ws is high

1. It’s humid in New England so w is high.

What’s the difference between a fog and a cloud?

Fog Cloud

Clouds

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