1. Relative Humidity Ms. Bankoff
Hello and welcome to today’s video. I am Ms. Bankoff and today we will be talking about relative humidity.
Ms. Bankoff

2. Learning Goals
Understand how clouds, rain, and other forms of precipitation form.
Understand the relationship between specific humidity, relative humidity, and capacity.

3. Low Pressure  High Pressure 
We should start by talking about the atmosphere if we are going to talk about weather. You Have spent your entire life in the troposphere and that is where all weather occurs. It’s this layer right next to the surface of the earth. The only time you might have left the troposphere is if you have flown in a plane. Planes want to avoid weather for obvious reasons so commercial planes’ cruising altitude is just above the troposphere. The atmosphere is about three hundred miles thick, but most of the air is within 10 miles of the surface. It sinks, due to the gravity of the earth. Actually, you should imagine the atmosphere like a giant pool. If you dive down to the bottom of the deep end, the pressure here is really great.** The pressure gets lower and lower as you up. **That’s an important relationship to remember. As you go up, the pressure decreases.

4. Temperature Pressure Volume
As the pressure decreases ** other things happen as well. The air gets to expand and The volume of air increases ** as the volume of air increases**… it gets cooler!

5. As air rises and expands
it Cools.
We can see this phenomena if we release some of the gas from the canister. The remaining gas in the can gets to spread out – the sides of the can get really cold! **There is even a warning on the side – do not spray at exposed skin, risk of frostbite!

6. Air rises because..
There are a number of reasons why air rises. First, as the sun heats the surface, the air next to the surface is warmed and becomes less dense than the surrounding air. Hot air rises! Second, if moving air runs into mountains, the air must rise to get over them. The third and fourth reason air rises have to do with fronts. Third, if two fronts of similar temperature converge or come together, the air in between them will be forced up. And forth, a cold front can force the warm air in a region to rise because it is more dense than the warm air.

7. Facts: As pressure decreases, volume increases
Facts: As pressure decreases, volume increases. As volume increases, temperature decreases. Hot air can hold more water vapor than cold air.
To recap… read the screen.

8. The amount of water that is actually in the air.
Specific Humidity
The amount of water that is actually in the air.
Measured in g/m3
Before we go any further, you need to know some vocabulary. Specific humidity is the amount of water vapor in the air. Its measured in g/m^3

9. Capacity
The maximum amount of water the air can hold. Depends on the temperature of the air.
The capacity describes the maximum amount of water vapor that the air could hold if it were completely full. The capacity of the air depends on its temperature. ** if you look at this graph, the capacity of air goes up as the temperature goes up. Remember we said a minute ago that hot air can hold more water vapor than cold air?

10. Grams of water that a 1 m3 of air can hold.
How much water can 50⁰C air hold?
Grams of water that a 1 m3 of air can hold.
Capacity
Here’s a bar graph that describes the amount of water vapor that air at different temperatures can hold. Capacity is on this side and temperature is on this side.** Read the graph and tell me how much water vapor 50 degree air can hold in your notes.

11. Grams of water that a 1 m3 of air can hold.
How much water can 30⁰C air hold?
Grams of water that a 1 m3 of air can hold.
Capacity
**How much can 30 degree air hold?

12. Saturation / Saturated air
Describes air that cannot hold any more water. The air is FULL of water.
Specific Humidity = Capacity
The next vocab word is saturation or saturated air. **This describes air that CANNOT hold any more water vapor. The air is completely full no more evaporation could occur in saturated air. If you stepped out of the shower this morning to a steamy bathroom, you probably experienced saturated air. **When air is completely full of water, the specific humidity**, which is how much water is in the air ** is equal to it’s capacity, which is the max amount that the air could hold.
How much water is in the air 
Max amount of water the air can hold 

13. The air in this picture is saturated
The air in this picture is saturated! Can you think of some things that would start to occur at saturation? You got it! Fog, Clouds, rain, dew.. Etc.

14. Relative Humidity Always measured as a %
Another way to describe the moisture in the air is relative humidity. It’s always measured in a percent. Its like saying the air in ** Lakewood at this time was 82 percent full. That’s pretty full for Colorado!

15. Specific Humidity x 100 Capacity .
Relative Humidity
Specific Humidity x 100 Capacity .
= % Relative Humidity
Here’s how we arrive at that percent. Specific humidity divided by capacity times 100 is equal to the relative humidity. Again, that’s how much is IN the air divided by how much it COULD hold times 100 to make it a percent.

16. How to measure humidity
Psychrometer
thermometer 
Wet
Dry thermometer 
Measures the relative humidity by comparing the temperature difference between the wet/dry bulbs to a chart.
Greater difference means lower humidity.
This is an old school tool that can be used to measure relative humidity. A Psychrometer consists of two thermometers. One is damp and one is dry. The thermometer that is damp will be cooler than the one that is dry. The greater the difference between the bulbs, the more evaporation is occurring and thus, the lower the relative humidity.** We will talk about this guy a lot more when you watch the humidity calculations video.

17. Dew point The temperature at which air reaches 100% relative humidity.
What happens if the relative humidity reaches 100%? Remember we said that cold air can hold less water? If we cool this 30 degree air down to 20, the capacity decreased. The amount of water vapor in the air didn’t change, just the capacity of the air. The air went from 28% relative humidity at 30 degrees to 52% relative humidity at 20 degrees. Cool it down even more and the capacity will eventually be equal to the amount of water vapor in the air. Now the relative humidity is 100%. This temperature is called the DEW Point. If you cool the air down even more, the air will have to lose some if its water vapor. Can you think of some ways the atmosphere can lose water vapor? … right! First Clouds or fog will form, then rain, snow, hail, dew, sleet, etc.

18. Skip this slide

19. Grams of water that a 1 m3 of air can hold.
At what temperature would that air be “saturated” if it has
50 g/m3 of water vapor?
Grams of water that a 1 m3 of air can hold.
Capacity
Read the graph and tell me what temperature the air would have to be to be saturated if it has 50 g/m^3 of water vapor in it. Remember, when air is saturated, the specific humidity is equal to the capacity.

20. Let’s put it all together now
Let’s put it all together now. Maybe this air is rising because of surface warming. Hot air is less dense, so it rises. As it rises, the pressure decreases, volume increases, and it gets cooler and cooler and cooler. The relative humidity in this air gets higher as the air cools because it can hold less water if it is cooler. Eventually, the temperature reaches the dew point and the relative humidity reaches 100%. When it does, you start to see condensation in the air in the form of clouds! As the air continues its trip upwards, and condensation accumulates, it may be enough for droplets to form or snow to form!

21. Adiabatic Rate – The rate at which the temperature of the air decreases as it rises.
The rate at which the temperature of the air decreases as it rises is called the adiabatic rate. As it rises from the surface, it cools at the dry adiabatic rate – 10 degrees cooler for every 1000 meters it rises. The rate of cooling is a little faster down here before condensation has occurred. Because condensation is an exothermic process, it slows down the rate of cooling a little. The dry adiabatic rate happens as air rises before condensation, The air reaches the dew point at this altitude we can see that in the form of clouds. After that, the air cools at the wet adiabatic rate – 5 degrees cooler for every 1000 meters it rises.

22. Thanks for watching!
That’s all for today! Thanks for watching

23. Measuring Dew Point
Measure the temperature at which the air in the classroom will produce condensation.

24. Video How to measure the relative humidity using a sling psychrometer:

25. Video
How climate change is related to recent extreme weather phenomena: