Water in the Atmosphere

The Hydrologic Cycle Also known as the water cycle ` We will begin here

This lesson is all about the gas represented in pink in the lower pie graph Recall

We are talking about water vapor Can you see any water vapor in this picture? Remember, if you can see it, its not water vapor

This model represents an extremely small parcel of air Notice air is composed of several gases including water vapor The amount of water vapor in the air at any moment in time is referred to as the humidity

High temperatures increase evaporation Also, don’t clothes dry better on a hot day?

Windy conditions increase evaporation Also, don’t clothes dry better on a windy day?

Dry conditions (low humidity) increase evaporation Unfortunately there is no water here to evaporate

Greater surface area increases evaporation Which bowl will evaporate more water in an hour? When you hang out sheets to dry, to you spread them out or hang them up all folded?

Once water evaporates into the atmosphere, we can measure the humidity of the air. 2 ways of expressing or describing the humidity of the air. 1. Specific humidity This is the actual grams of water vapor per kg. of air

The 2 nd way to express humidity is as a percent 2. Relative Humidity What do you think a relative humidity of 50% means? Well, what does it mean if you get 50% on a test? Apply the same thinking. Relative humidity = the actual amount of water in the air compared to the amount the air can hold.

The maximum amount of water the air can hold Is called the Capacity of the air So if the air was holding 5 grams of water vapor and The capacity of the air is 10grams, the relative humidity = 5/10 Which equals 50% Unfortunately the capacity of the air changes. In fact it is entirely dependant on the air temperature. Here is a table which shows the capacity of the air at various temperatures

What is the relationship between air temperature and it’s capacity to hold water vapor? If the air at 28ºC has a specific humidity of 12.3 g/kg of water vapor, what Is the relative humidity? RH = Spec.Hum./Capacity RH = 12.3 / 24.6 RH = 0.5 = 50% *

* Now let’s think about this. What if the air still had a specific humidity of 12.3 g/kg but the air temperature was 17ºC instead of 28ºC, what would be the relative humidity? RH = spec. hum. / capacity RH = 12.3 / 12.3 = 1.0 = 100% So what effect did lowering the temperature from 28ºC to 17ºC have on the relative humidity? So what effect did lowering the temperature from 28ºC to 17ºC have on the specific humidity? Yes, R.H. increased No Effect

Let’s recap: Lowering the temperature causes the relative humidity to go up but has no effect on the specific humidity. Also recall that the relative humidity was 100% at 17º C. As you lower the air temperature the relative humidity increases until it finally reaches 100%, in this case at 17º C. The temperature at which you reach 100% relative humidity is called the dew point temperature In our example the dew point temperature was 17º C

Further recap : As temperature is lowered closer to the dew point, relative humidity ____________ (inc. or dec.?) As temperature increases away from the dew point, relative humidity ___________(inc. or dec.?) And in both cases specific humidity remains the same. In fact, the specific humidity is always The capacity of the air at the dew point

What actually happens at the dew point? Well, since the air is holding all that it can hold at the dew point, some of the moisture held as vapor by the air, condenses on to solid surfaces as a liquid. You can see this as dew on plants and grass in the early morning hours in summer. Can you tell why dew only forms when the air cools off?

You might find condensation of cold water pipes on a hot humid summer day

Or on the outside of a cold beverage taken out of the fridge on a hot humid day In all of these examples, why did dew or condensation form?

Using the same idea, can you tell why the relative humidity In houses and buildings in very low in winter? Remember R.H. = Spec. Hum. / Capacity Also remember that changing the temperature of the air does not effect the specific humidity. The air in your home in winter came from outside where is was cold. therefore the air has a very low specific humidity (say 3.0g/kg) The air in your home in winter is pretty warm, say 20º C, and therefore A high capacity. (15.0 g/kg) What type of relative humidity would you get with that combination? Yes, a very low relative humidity. Ex. R.H. = 3.0/15.0 = 0.2 = 20% Which results in dry skin and cracked lips So when you go outside, you can expect the relative humidity to increase

What instrument do we use to measure Relative Humidity? It is called a sling psychrometer Wet bulb Dry bulb

Several things are all true at the dew point temperature 1. The air is saturated 2. The specific humidity and the capacity are equal 3. The relative humidity is 100% 4. condensation occurs

When this condensation occurs on the ground and the Temperature is above freezing it is called dew All condensation must occur on a solid surface

When condensation occurs on the ground and the temperature is below freezing it is called frost

If condensation occurs in the air just above The ground, it is called fog

In the case of a fog droplet or a cloud droplet, the solid surface needed for condensation is a dust particle, or the scientific name condensation nuclei

There is a hydrostatic force that holds The water droplet to the dust particle (Size exaggerated) Hydrostatic force

Some sources of Condensation Nuclei would be: 1. Smoke from forest fires

2. Volcanic ash from eruptions 2. Ash from volcanic eruptions

3. Particles from smoke stacks

Salt from breaking waves

If the condensation occurs higher up in the atmosphere it is called a cloud

Other common examples of condensation would be: 1. When you see your breathe outside on a cold morning 2. When you take a hot shower and the mirror gets all “fogged up” 3. When you boil water in a tea kettle, the white stuff coming out of the spout is called steam In all of these examples, water has gone from a vapor to a liquid except for the frost

For all types of condensation you need 2 things: 1.A solid surface 2.Air cooled to it’s dew point In the atmosphere the most common method of cooling the Air is to lift it. Remember air expands as it rises and cools. Because there are always plenty of condensation nuclei present in our atmosphere, air just needs to be lifted until it cools to it’s dew point and a cloud forms.