climate and weather
GEO 344 Weather and Climate Prof. Stuart Evans
Lecture 7 Latent Heat Hurricane Florence (2018), powered by latent heat
Announcements
• Reading Quiz for Tuesday (6.1 – 6.5) • This will be the end of material for the first midterm
• Midterm coming up (we’ll do some review on Tuesday) • About 30 multiple choice, about 3 short answer • Nothing that requires a calculator • More details Tuesday
Announcements
In-class Activities are available as a study tool
Announcements
Answers to reading quizzes are available via the Gradebook
This is a link to the questions/answers
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
Relative humidity tells you how close the vapor pressure, e, is to saturation
RH = 100 * e / es
How much water is in the air How much water there could be (hypothetical, depends on temperature)
You can change RH by: • Changing e
o more water → higher e → higher RH o less water → lower e → lower RH
• Changing es o Higher temperature → higher es → lower RH o Lower temperature → lower es → higher RH
Why is the air dry in an airplane cabin?
• Air brought in from outside the plane.
• Outside air is ~ -50 °C • Air is warmed up to ~ 20 °C
• es for -50 °C is 0.06 hPa • es for 20 °C is 2.4 hPa • es increases by factor of 40!
• If outside is 100% RH, inside would be 2.5%.
• Airplanes have to humidify the air they bring in
Dew Point
Dew point: the temperature at which RH = 100% would occur
Dew point tells us how much water is in the air (you can get it from e).
T – TD is called the dew point depression, and tells you how much you would need to cool to create condensation
Why do the inside of your car windows fog when it’s cold? The air in the car is humid
Imagine 16 °C and 67% RH. 16 °C means es = 18 hPa (Table 5.1) 18 hPa * 67% = 12 hPa = e
How cold does it need to get to reach saturation? For what temperature would 12 hPa be saturated?
12 hPa is the es for air that is 10 °C → 10 °C is the dew point
If the windows are cold enough to cool the cabin air to 10 °C you’ll get condensation.
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 • 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 • The more water vapor in the air, the more pressure it exerts
• It can be odd to think of an amount of water as a pressure – but we can visualize the pressure from water vapor if we make it really big.
Visualizing vapor pressure
A really big can: https://www.youtube.com/watch?v=JsoE4F2Pb20
Total pressure = pressure from dry air +
pressure from water vapor
Total pressure outside
Total pressure outside
Total pressure inside
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
Phase changes
These get most of our attention
What is energy?
Energy is that which allows work to be done
Make something move
Heat something up Make an object change shape
Make a chemical reaction happen
Make something melt or evaporate
Energy has to be added to make any of the happen
• The molecules of a liquid are more energetic than those of a solid → melting takes energy to make it happen
• The molecules of a gas are more energetic than those of a liquid → evaporating takes energy to make it happen
Why does it take energy?
Make something melt or evaporate
• If you go the other way (condensation or freezing) you get the energy back (energy is released)
The energy you have to put in or get back is called latent heat
Latent heat
Energy is required to evaporate water
Using energy for evaporation takes that energy away from the environment, cooling it.
Latent heat is why dogs pant and people sweat
Water evaporates off the dog’s tongue. Energy was needed to make the evaporation happen
That energy came from the dog’s body heat à The dog’s tongue is cooled down by the evaporation
Evaporating sweat off our skin takes energy out of our skin, cooling it.
If you were coated in 0.1 mm water and evaporated it all in 5 minutes, that would require ~1000 W!
As plants photosynthesize, they evaporate water out of pores in their leaves. You can see the water
being released if you trap it inside a bag
Latent heat is why trees and plants keep a place cool
• Evaporation uses up energy, cooling the leaves (and thus the air around them) • Evaporation of water in the soil cools the ground
As plants photosynthesize, they evaporate water out of pores in their leaves. You can see the vapor
being released if you trap it inside a bag
Latent heat is why trees and plants keep a place cool
• Evaporation uses up energy, cooling the leaves (and thus the air around them) • Evaporation of water in the soil cools the ground
New York City temperature map à Central Park is several degrees cooler
In hot dry places, evaporative cooling is very effective
Swamp coolers are popular in the Southwest DIY “air conditioning” is popular?
Air is cooled by evaporating water in the pad Air is cooled by evaporating condensation on the bottles
Latent heat
Condensing water releases energy
You get the energy back when you condense water vapor to liquid water.
Latent heat release warms up your drink
• Air next to the cold can cools by conduction
• Colder air → lower saturation vapor pressure → higher relative humidity
• Eventually saturation is reached, which causes condensation
• A 0.1 mm thick layer of condensation warms a 12 oz. drink by 4.9 °C
Latent heat release is why steam burns you, even though it’s only 212 °F
You could put your hand in a 500 °F oven and not get burnt Your hand over a boiling pot or kettle gets burnt quickly
The water vapor condenses on your skin and releases energy.
Latent heat release helps power thunderstorms (more later in the course)
Phase changes
Use energy, cool the environment
Release energy, heat the environment
A weather example
Sometimes rain falls through a dry layer and evaporates before it hits the ground. This is called virga.
A weather example
What is happening to the air temperature where the evaporation happens?
A weather example What is happening to the air temperature where the evaporation happens?
Evaporation requires energy, which it takes from the air, so:
COOLINGCOOLING
A weather example
Cold air sinks because it is dense…
COOLINGCOOLING
A weather example
Cold air sinks because it is dense and then spreads out…
COOLINGCOOLING
We call this a gust front, and is the cold wind you feel before an intense rain happens
The gust front picks up dirt/debris which shows up on radar