Forms of Condensation and Precipitation

• Chapter 5, pg. 130-131, 137-150
• Mult. Choice Question

Condensation

• Mult. Choice Answer:
• It turns out that saturation is not always enough to get net condensation
• Need a surface on which water vapor can condense
• ____________ __________

Condensation

• _____________ _________ are tiny atmospheric particles that serve as surfaces on which water vapor condenses
• They are ___________ in the atmopshere
• Dust, smoke, salt particles, sulfates, nitrates
• The most effective ___________ ________ are hygroscopic—makes it easy for water vapor to condense on them

How Precipitation Forms

• Mult. Choice Question

How Precipitation Forms

• Mult. Choice Answer:
• Cloud droplets are too tiny and fall too slowly to ever make it to the ground without evaporating
• Since condensation nuclei are quite abundant, the available moisture is divided up between all the small cloud droplets
• So none of them can grow big enough to be raindrops by condensation alone
• How do we get from cloud droplet to raindrop?

How Precipitation Forms—Fig. 5-13

How Precipitation Forms

• Two main methods:
• The ____________ Process (_______ Clouds)
• The Collision-Coalescence Process (_______ Clouds)

The Bergeron Process

• Mult. Choice Question

Bergeron

• Mult. Choice Answer:
• When water is liquid at temps below freezing, we call it "________________"
• Usually, though, we ________ have pure water droplets (condensation nuclei dirty things up) in the atmosphere
• So freezing of the _____________ drops usually starts between only ________________

Bergeron

• Depending on the season, you have to go fairly high in the troposphere to get temps that cold
• And just like how condensation happens easier if there is a condensation nucleus, freezing of water droplets happens easier if there are freezing nuclei present

Bergeron

• Freezing nuclei are not as ___________ as condensation nuclei
• They are ______ particles that have a crystal form similar to ___ (example: silver iodide)
• Freezing nuclei generally don’t become active until the temp is lower than ______

Bergeron

• So cold clouds are mainly composed of:
•  _____________ water droplets when temps are between 0º and -10ºC
• A mixture of _______ droplets and ____ ________ when temps are between -10º and -20ºC
• Almost entirely ____ _________ when temps are lower than -20ºC—like _______ clouds

Bergeron--See Fig. 4-5

• Recall from Chapter 4, when air is saturated between vapor and liquid, condensation = evaporation
• When air is saturated between vapor and solid, deposition = sublimation

Bergeron--See Table 5-2

• Water molecules more easily evaporate from liquid droplets than they sublimate from rigid ice crystals
• This means when air is saturated between vapor and liquid, it is supersaturated between vapor and solid

Bergeron—Table 5-2

Bergeron--See Fig. 5-14

• So, at the same temp for which condensation = evaporation, deposition > sublimation
• Thus, for each water molecule condensing, one is also evaporating
• But more than one water molecule is depositing for each one sublimating
• Ice crystals can then grow

Bergeron—Fig. 5-14

Bergeron--See Fig. 5-14

• Snow crystals usually grow rapidly this way because they’re soaking up all this evaporated water from the nearby liquid droplets
• When they become large enough (heavy enough), they start falling through the cloud
• They intercept cloud droplets which then freeze to the ice crystals—increasing their volume and making them fall faster

Bergeron

• The growing snow crystals may also collide with one another and join together
• They then will become snowflakes
• Large snowflakes may consist of 10 to 30 individual crystals
• Well, we learned how snow forms, but what about RAIN???

Bergeron

• As the ____________ fall to warmer temps, they may melt, but they’re now big enough that they melt into ______ large enough to continue falling without totally evaporating
• Thus, sometimes the ___________ that keep falling on your head were initially ____________ high in the sky

Bergeron

• Summary:
• The Bergeron process can form raindrops and precipitation—even year-round in the _____________—provided at least the upper portions of clouds are cold enough to generate ___ ______
• Gotta have those ____ ________

Collision-Coalescence

• We know that many clouds form at temps too warm to generate ice crystals
• We also know that sometimes lots of rain can fall from clouds that do not have ice crystals
• Thus, the Bergeron process does not explain how 100% of the raindrops form

Collision-Coalescence

• Some condensation nuclei are larger and/or more ____________ than others
• ____________ particles (like sea salt) begin to condense water vapor at RH ______ than 100%
• The large _____________ nuclei will be able to grow larger cloud droplets through ________________

Collision-Coalescence

• The larger drops will fall ________ (or rise _______ in an updraft)
• Since they will have different velocities than smaller droplets regardless, their chances of colliding with them ___________

Collision-Coalescence—Table 5-3

Collision-Coalescence

• When a larger drop collides with a smaller drop, the two might _________ (join together)--increasing the size of the larger drop
• About ___________ cloud droplets would have to _________ with a large droplet for it to grow to raindrop size

Collision-Coalescence

• Therefore, clouds of high vertical extent with some _________ cloud droplets are needed for the collision-coalescence method to produce precipitation
• It also helps if there’s a strong __________ because it will allow the large droplets to cycle through the cloud over and over to collect smaller droplets

Collision-Coalescence

• Once we have the raindrop, it can only get so large before frictional drag of air overwhelms its surface tension
• When this happens, the drop breaks up into numerous smaller drops that each start picking up cloud droplets to grow bigger again
• Notice falling raindrops do NOT look like teardrops

Collision-Coalescence—Fig. 5-15

Collision-Coalescence

• The collision-coalescence process is more common in the ________--where there are ______ condensation nuclei (air is cleaner) and ______ water vapor available--leads to ________ initial drops
• ___________________ clouds in mid-latitudes can create precip with a combo of Bergeron and collision-coalescence processes