Some nuclei are very hygroscopic.

Condensation begins on these particles at relative humidities less than 75%.

Their size increases as the nuclei acquire more water.

At relative humidities less than 100%, the tiny droplets do not grow to cloud droplet size.

Some particles are hygrophobic (water repelling).

• They do not attract water molecules.
• They remain small and tend to scatter blue light more so than red.
• The result is a bluish haze

Recipe for Cloud Formation

1. Plenty of water vapor
2. A way to reach saturation and remain there
3. Plenty of CCN

What about if the air temperature is below 0°C?

The Bergeron Process

Supercooled Water

It is possible for liquid water to remain liquid below temperatures of 0°C.

• It is unlikely that molecules will go from a quasi-random state (liquid) to a rigid lattice (ice) without a little nudge.
• Liquid water below 0°C is called "supercooled" or "subcooled" water.
• All water will freeze below -40°C.
• The water molecules need a ìgathering placeî in which to group in an ordered fashion.

These are Deposition Nuclei, i.e., water vapor deposits directly on the ice.

The best ice nuclei are those that have shapes similar to natural ice crystals.

Kaolinite (a clay) is one of   the most popular.  It ìactivates,î or begins to accept water molecules, at -9°C.
Silver Iodide, which activates at -4°C, has been used in ìcloud seedingî experiments.

This is about 100 times larger than a CCN and 100 times smaller than a typical rain drop.

How can a very small droplet grow to the size of a rain drop?

Typical Sizes (Radius)

CCN - 0.0002 mm
Typical Cloud Droplet - 0.02 mm
Typical Rain Drop - 2 mm

A droplet can grow only if the number of water molecules entering the drop exceeds the number of water molecules leaving the drop. Supersaturation!

The saturation vapor pressure is defined over a plane (flat) surface of water.

Each water molecule attracts its neighbor.  The combined attractive forces on the molecules at the surface make up the surface tension.

There are attractive forces from all the surrounding molecules.  These forces must be overcome for the molecule to escape into the vapor phase.

The saturation vapor pressure is defined over a plane (flat) surface of water.  Let us now consider a droplet.

The forces on the molecule are less than that for a plane surface of water.  To maintain equilibrium, the vapor pressure above the drop must be greater in order to keep the drop from evaporating.

This reduced surface tension and the larger saturation vapor pressure required for the drop to remain in equilibrium with its surroundings is called the curvature effect.

This effect is enhanced for smaller drops.  In other words, the smaller the drop radius, the larger the  curvature of the drop, and the larger the vapor pressure required to keep the drop in equilibrium at a given temperature.

CCN in cloud droplet formation

CCN give the water molecules a place to gather together.  This ìinstantlyî gives them a much larger size than without the CCN.

Some CCN are hygroscopic.  Some salts begin to collect water at RHs as low as 75%.  A solution is formed when water condenses onto a salt particle.

There are fewer water molecules at the surface.  So not as many water molecules can escape at a given temperature.  In equilibrium, the saturation vapor pressure over the salt water is less than that over the pure water.  This is called the solute effect.

Growth by condensation

Given:

CCN with an initial mass of 10^-12 g
RH greater than RH* (The critical RH)
How long will it take for a droplet to grow to the size of a cloud droplet (0.02 mm)?

5900 sec
~98 Minutes!

This is too slow for what we often observe!

To increase the volume of a single drop to the size of a rain drop, it must grow for a very long time.

A faster way is to get several small drops together to form a single large drop.

Larger drops fall faster than small drops.

Terminal Velocity

In the absence of any wind, the drop will begin to fall.

As soon as the drop falls, the air resists the fall of the drop.

The air resistance is dependent on the size of the drop and the velocity of the drop.  So the larger the drop will have a larger air resistance.

Eventually, there will be a balance between gravity, which wants to pull the drop down toward the ground, and the air resistance, which wants to retard the fall of the drop to the ground.

From Newton, when there is a balance of forces, the drop will continue in a straight line at constant speed.  This speed is called the Terminal Velocity.

The larger drops will have a larger terminal velocity than small drops so the large drops will fall faster.

In calm air, a typical rain drop will fall nearly 600 times faster than a typical cloud droplet.

If the large drops collide with the smaller drops and merge with them, this process is called coalescence.

Not all small drops will merge with the larger ones!

Coalescence Depends On:

• Cloud Water Content
• Large Droplet Size
• Small Droplet Size
• Updrafts (Relative speed of the large drop to the small drop.)
• Cloud Electricity

So when do drops make it to the ground?

The drops can make it to the ground if:

• The drop is large enough
• The cloud base is low enough
• The air between the cloud base and the ground is not too dry.

Fog

Fog is a cloud that has its base close to the ground.  There are five basic types of fog.

1. Radiation fog - caused by radiative cooling of the ground and adjacent air; requires clear skies, calm winds, and high humidity.  A cooling fog.
2. Advection fog - warm moist air moves over cold surfaces.  E.g., a warm moist air being blown offshore over cool ocean water.  Very common in coastal areas.  A cooling fog.
3. Upslope fog - warm, humid air moves upslope and cools adiabatically eventually reaching saturation and forming fog.  A cooling fog.
4. Steam fog - cool air flows over warm water, creating a rolling fog.  Very common on cold mornings in the fall (the water is still warm, the air is cold).  An evaporation fog.
5. Frontal (precipitation) fog - warm air is lifted over cold air by a weather front.  An evaporation fog.

Weather Modification

1. Use of energy to modify the weather, e.g., applying heat, or giant fans, to disperse fog.
2. Modification of land or water surfaces to change weather, e.g., applying oils to water to reduce evaporation or covering land with a dark material to increase solar energy absorption.
3. Triggering or intensifying a natural process, e.g., cloud seeding.