climate and weather

GEO 344 Weather and Climate Prof. Stuart Evans

Lecture 6 Humidity and Condensation

Updates

• Homeworks will get graded as quickly as possible

• Next week’s reading quiz will be posted tonight

Phases of water

Gas Liquid Solid

You can’t actually see water vapor – it’s invisible

Clouds are made of tiny water drops We’re pretty familiar with this

Water: if you can see it, it’s not a gas

Phases of water

What is inside the bubbles? What phase do you see here?

Measures of Water Vapor

We have three ways to define the amount of water vapor in the air: • vapor pressure, e • relative humidity, RH • dew point, TD

Not actually a measure of water vapor in the air: • saturation vapor pressure

Vapor pressure

• Pressure review: • Pressure = Force / area • Pressure is additive (Dalton’s Law) • Total pressure = pressure from dry air

+ pressure from water vapor

• What if we JUST considered the water vapor? • This is what we call water vapor pressure, e • e tells you the actual amount of water in the air

Column of air (both dry air and water vapor)

nitrogen, oxygen, etc.

Vapor pressure

• Pressure review: • Pressure = Force / area • Pressure is additive (Dalton’s Law) • Total pressure = pressure from dry air

+ pressure from water vapor

• What if we JUST considered the water vapor? • This is what we call water vapor pressure, e • e tells you the actual amount of water in the air

Column of air (both dry air and water vapor)

nitrogen, oxygen, etc.

Vapor pressure • Total pressure = pressure from dry air

+ pressure from water vapor

What if we JUST considered the water vapor? • This is what we call water vapor pressure, e

• Above example: e = 10 hPa • What if there were twice as many vapor molecules? e = 20 hPa

1000 hPa of pressure from dry air + 10 hPa of pressure from water vapor

= 1010 hPa total pressure

Vapor pressure

Vapor Pressure: pressure due to the water vapor in the air, e

Saturation vapor pressure: pressure due to water vapor in the air, in the hypothetical scenario that the air is saturated, es

What is saturation?

Saturation

• Saturation is an equilibrium situation where: evaporation = condensation

• Consider dry air over a body of water…

Dry, no water vapor Water vapor increases Water vapor stays same

evaporation condensation

water SATURATION!

Saturation

Water evaporating out of glass into the air (and some being condensed into it)

All the water evaporated means there was no equilibrium means air was not saturated

Later

What does saturation look like? Saturation means liquid water must exist

Vapor pressure

Vapor Pressure: pressure due to only water vapor, e

Saturation vapor pressure: pressure due to only water vapor, if the air were saturated, es

es depends only on temperature!

Vapor pressure

Saturation vapor pressure: pressure due to only water vapor, if the air were saturated, es

es is how much water vapor there could be

It’s a property of air: for a particular temperature, there’s a maximum amount of water vapor possible.

How much water there could be

Relative humidity

Relative humidity tells you how close the vapor pressure, e, is to saturation

RH = 100 * e / es

RH tells you how close to the red line you are.

How much water there could be

Relative humidity

Examples from the reading quiz

es for air that is 30 °C = 42 hPa e for air that is 30 °C and 50% RH = 42*50% = 21 hPa es for air that is 10 °C = 12 hPa e for air that is 10 °C and 100% RH = 12*100% = 12 hPa

Warm air can have more water in it even when not close to saturated than totally saturated cold air

How much water there could be

Relative humidity You get more snow when it’s only a little below freezing

How much water there could be

Where is there water vapor?

Satellite image of upper atmosphere (about 300 hPa) water vapor.

Bright shades are more water vapor, dark is less.

The world, animated

Dew Point

Dew point: the temperature at which RH = 100% would occur

Dew point tells us how much water is in the air (another way of getting e).

T – TD is called the dew point depression, and tells you how much you would need to cool to create condensation

When the temperature equals the dew point

If the dew point is below 0 °C you get frostCondensation occurs on surfaces at TD

When the temperature equals the dew point

Morning fog over a field

• By what process did the ground cool at night? • By what process did the air cool? • What is the dew point depression right now? • Why does this only happen first thing in the morning?

Steam fog in Chicago

Lake water – right about freezing. Air A result of “warm” water exposed to cold air.

Dew point changes with weather

Warm air mass

High dew point

Cool air mass

Low dew point

Dew point changes with height

Which air mass is more humid?

At what pressure levels would you be most likely to find clouds?

When the temperature equals the dew point

At what altitude does T = TD?

As air cools

cooling air Why doesn’t this change?

Why is it dropping now?

es only depends on temperature. No formula for it, just something you need to look up in a table like this.

As air cools

cooling air

Why doesn’t this change?

Why is it dropping now?

A sample box of air

Box of air starts here… then starts cooling

Temperature

Dew Point

Humidity

Pressure

Cloud in a bottle!