March 10, 2008

Summary for cloud formation

n Most clouds form well above the ground surface. Air parcels near the Earth's surface contain water vapor, which evaporated from liquid surfaces, e.g., oceans, lakes. When these parcels move upward, they take this water in the form of water vapor with them. As parcels move upward, they expand and cool, i.e., the temperature in the air parcel decreases. This increases the relative humidity of the air in the parcel because as air temperature falls, the capacity for water vapor in the air decreases. If the air is cooled enough (down to its dew point temperature), the relative humidity becomes 100%. Once this point is reached, when air parcels rise higher, their temperature becomes so low that they cannot hold all of the water vapor that they contain. The excess water vapor condenses into liquid water, forming clouds. In fact, just enough water vapor condenses to liquid water droplets, so that the relative humidity remains 100% and does not exceed 100%.

Cloud in a bottle demonstration

n In this demonstration, we will cause a cloud to form in a large flask.

n Setup. Pour a small amount of liquid water into the flask and place a stopper over the open end of the flask. Evaporation of the liquid water in this closed system will lead to the air in the flask becoming nearly saturated with water vapor.

n The flask also has a small opening, which we will use to pump air into the flask. After pumping air into the flask, the air pressure in the flask will be greater than the air pressure in the room.

n Now pop the stopper off of the flask. This causes the high pressure air inside to expand suddenly as it pushes out of the top of the flask. This sudden expansion of the air, causes the air to cool (temperature lowers).

n This cooling causes the relative humidity to increase. If everything works well, the air temperature will fall below its dew point temperature, and a visible cloud will start to form as water vapor condenses onto cloud condensation nuclei.

n To demonstrate the influence of cloud condensation nuclei, we will repeat the demonstration with one modification. We will first add additional cloud condensation nuclei into the bottle by dropping in a burning match before pumping air into the flask. Smoke from the burning match will act as additional cloud condensation nuclei.

n This time the cloud that forms should appear thicker and be more visible. The reason for this is that with more cloud condensation nuclei, we now form a cloud that is composed of many more small droplets as opposed to fewer large droplets. A cloud composed of many small water droplets scatters visible light more effectively than a cloud composed of fewer large water droplets … in other words, visible light has a harder time penetrating straight through the second cloud as compared with the first cloud.

Cloud Development – Mechanisms that force air to rise vertically (Ch. 5, p. 117)

n Most clouds form as air rises, expands, and cools. There are 4 basic mechanisms that cause air to rise in the atmosphere:

1. Surface heating and free convection. We have already discussed this mechanism when we discussed energy transfer by convection and the development of thermals (see 2/25/08 lecture notes). As thermals rise, they cool by expansion. If a rising thermal cools to the points where it cannot hold all of the water vapor it contains, then a visible cloud will form.

· Figure 5.11 from textbook

2. Orographic Lifting. When horizontally moving air encounters a mountain, it is forced to rise upward. The rising air expands and cools, which may lead to cloud formation. In mountainous areas, it is very common to observe clouds over the mountains, while clear skies are observed over the valleys. Conversely, because air is forced sink on the leeward side of mountains, this region is often characterized by fair weather and lack of cloud cover (sometimes called a rain shadow because much of the water in the air may be taken out as precipitation over the mountains, leaving drier air for the leeward side of the mountain).

· Figure 5.14 from textbook. Point out climate differences between windward and leeward sides (example: western part of the US).

3. Surface convergence and/or upper level divergence. We have already discussed these mechanisms. Surface convergence occurs near surface low pressure areas. Upper level divergence occurs just downstream of 500 mb troughs.

4. Uplift along weather fronts. We have already discussed this as well. Remember that the warmer air mass is forced to rise near weather fronts.

Rules for Moving Air Parcels Up and Down in the Atmosphere

n In this course, you will have to work with numerical examples wherein we will move air parcels up and down in the atmosphere, while keeping track of the air parcel’s temperature, dew point temperature, and whether or not a cloud forms in the parcel. The rules:

1. The starting temperature and water vapor content (use the dew point) of the parcel is taken to be the measured conditions at ground level. You will always be given this information.

2. As long as the parcel is unsaturated (relative humidity < 100% or whenever the dew point temperature of the parcel less than the temperature of the parcel), the rate of cooling is 10° C for every 1000 meters the parcel is lifted.

· See figure 5.2 in textbook

3. As a rising parcel cools, its relative humidity increases. Once the relative humidity reaches 100% (determined when the parcel temperature cools down to its original dew point temperature), further lifting (and cooling) results in net condensation, forming a cloud. Remember that an air parcel will never contain more water vapor than its capacity or saturation mixing ratio. This rule allows us to determine the cloud base or the altitude at which a cloud forms in rising air.

4. Since condensation releases latent heat within the parcel, the rate of cooling is slower. Parcels which are saturated cool at a rate of 6°C for every 1000 meters the parcel is lifted. Also keep in mind that once a cloud begins to develop in a parcel, just enough water vapor will condense into liquid water so that the air in the parcel remains saturated (relative humidity = 100% and the dew point temperature equals the air temperature inside the parcel).

5. When lowering an air parcel in the atmosphere, the temperature changes are reversed. If there is no cloud (liquid water) in the parcel, the air temperature in the parcel increases at a rate of 10°C for every 1000 meters the parcel is lowered. If there is a cloud in the parcel, it will evaporate because as the parcel warms its capacity for water vapor increases. As long as there is still a cloud (liquid water) in the parcel, just enough water will evaporate to keep the relative humidity at 100% and the dew point temperature equals the air temperature. Since it takes energy to evaporate water, the rate of heating is slower. Parcels which contain an evaporating cloud warm at a rate of 6°C for every 1000 meters the parcel is lowered until the entire cloud has evaporated.

n Numerical example for lifting an air parcel upward. We will use a tabular method to perform these exercises. In practice, you will be given the information printed in black in the table below, and you will have to fill in the values shown in red. The comments are there to aid in understanding. Comments will not be provided on a quiz.

Elevation (meters)	Parcel Temperature (°C)	Parcel Dew Point Temperature (°C)	Comments
5000	-8	-8	Latent heat release; condensation occurring
4000	-2	-2	Latent heat release; condensation occurring
3000	4	4	Latent heat release; condensation occurring
2000	10	10	Saturated; level where cloud begins to form
1000	20	10	Unsaturated
0	30	10	Unsaturated

n Hopefully, by going through some of these simplified numerical examples, you will gain some understanding about cloud formation in the atmosphere.

Atmospheric Stability

n We will now add the concept of atmospheric stability. An important reason for discussing stability is that thunderstorms, tornadoes, and hurricanes form when the atmosphere is unstable. The more unstable the atmosphere, the higher the potential for severe weather.

n To determine the stability of the atmosphere (with regard to vertical air motion), we will consider this question: Suppose an air parcel is initially forced to move upward in the atmosphere, and then is no longer forced upward. How will the parcel react?

o If after the parcel is lifted, it tends to sink back downward to its original position, then we say that the atmosphere is stable. Push a parcel upward and it will return back to its original position.

o If after the parcel is lifted, it continues to move upward (in most cases it will actually accelerate upward), we say that the atmosphere is unstable. Push a parcel upward and it will continue moving upward away from its original position.

n The stability of the atmosphere influences the type of clouds that form.

o Under stable atmospheric conditions, air will only rise as long as it is forced to rise. Once the air reaches an altitude where it is no longer forced to rise, it will spread out horizontally. Thus if clouds are forming under stable atmospheric conditions, the clouds will be stratiform-type clouds.

§ Draw a simple diagram showing this.

o Under unstable atmospheric conditions, if the air is pushed up to a level where it becomes unstable, then it will accelerate upward. Thus, in unstable atmospheric conditions, the clouds will be cumuliform-type clouds.

§ Draw a simple diagram showing this.

n We are going to extend the tabular method for lifting air parcels to include a determination of atmospheric stability. The bottom line is this … if a lifted air parcel ever becomes warmer than the air surrounding the parcel, the atmosphere is said to be unstable for lifted parcels. If a lifted parcel does not become warmer than the air surrounding the parcel, the atmosphere is said to be stable for lifted parcels.

o Before going through a numerical example, we will briefly explain the physics behind the italicized statements above.

§ Recall that air parcels always adjust their sizes so that the air pressure inside the parcel equals the air pressure outside the parcel.

§ Recall the gas law that was introduced in the 1/30/08 lecture

Pressure = (Temperature) x (Number Density) x (Constant of proportionality)

or we can say that Pressure is proportional to (Temperature) x (Number Density)

§ Since the air pressure inside the parcel is the same as the outside pressure

{(temperature) x (number density)}_{inside parcel} = {(temperature) x (number density)}_{inside parcel}

§ If the temperature inside the parcel is greater than the temperature outside the parcel, then the number density inside the parcel must be smaller than the number density outside the parcel. This means that a parcel of warm air surrounded by cooler air, is less dense than the surrounding air, and will rise upward.

§ Basically, warm air rises!

§ This works the same way in any fluid. For an object to float or rise upward in a fluid, the object must be less dense than the surrounding fluid. I think you all have an idea about what does and does not float in water. It is basically the same principle to decide whether or not an air parcel will move up or down.

o Try a demonstration or two (Wednesday, maybe)

§ Try to float a can of Pepsi and a can of diet Pepsi

§ Helium filled balloons, cooled by liquid nitrogen

n Numerical Example for stability. The environmental temperature is the temperature of the air surrounding the air parcel. This will always be given to you. In practice this information is know based on data collected by radiosondes. Again black print indicates what you will be given and red indicates what you are expected to fill in. The stability column will not be provided on quizzes. It is up to you to figure that out.

Elevation (meters)	Environmental Temperature (°C)	Stability?	Parcel Temperature (°C)	Parcel Dew Point Temperature (°C)	Comments
7000	-19	Stable	-22	-22	↑
6000	-15	Unstable	-14	-14	↑
5000	-10	Unstable	-8	-8	↑
4000	-4	Unstable	-2	-2	↑
3000	5	Stable	4	4	Latent heat release; condensation occurring
2000	14	Stable	10	10	Saturated; level where cloud begins to form
1000	22	Stable	20	10	Unsaturated
0	30	Neutral	30	10	Unsaturated

Some notes on Predicting Violent Storms

Meteorologists use a similar method to assess the potential for thunderstorms and severe weather:

n Lift parcels upward to see if they become unstable. This must be done for the current measured atmospheric conditions as well as for forecasted conditions later in the day.

n The more unstable the atmosphere, the greater the potential for violent storms.

o The depth of the unstable layer is important.

o The greater the positive difference between the parcel temperature and the surrounding air temperature, the more unstable the atmosphere.

n Question to answer. If the atmosphere is unstable, will parcels be lifted high enough (by one of the four mechanisms listed on the previous page) to reach the unstable layer? This must occur to “release the instability” if storms are to occur.