Once saturation is reached, condensation can occur in many forms. On clear, calm nights water droplets condense directly onto the ground, forming dew. If the temperatures are below freezing, the water vapor deposits itself in the form of a light coating of frost. Much more annoying is the formation of frozen dew. In this case, dew forms and then the temperatures drop, forming a thick coating of clear ice as the liquid drops freeze.

Condensation also occurs in the form of fog. Precipitation fog forms when falling raindrops evaporate, adding more water vapor near the ground so the air becomes saturated and fog forms. With steam fog, warm moist air mixes with cold dry air, causing saturation. Steam fog usually occurs above water, and its formation is similar to seeing your breath on a cold day. Radiation fog forms on clear nights with light winds. The air just near the surface cools off rapidly and night and is then stirred gently by the wind, allowing a shallow layer of air to cool to the dew point and thus reach saturation and form fog. Later in the day, sunlight penetrates the fog, warming the surface and evaporating the water droplets and dissipating the fog. However, if the sun angle is low, sunlight can be scattered back instead of reaching the ground, allowing the fog to persist for some time. With advection fog, warm moist air moves over a cool surface, such as Gulf of Mexico air moving over Oklahoma during the winter. The cold ground causes the warm moist air to lose heat, enough so it reaches the dew point and fog forms. Upslope fog forms when air moves up the slope of the Great Plains, cooling adiabatically as it goes. Eventually it cools down to the dew point and fog forms.

• Why heterogeneous nucleation makes the formation of cloud droplets possible
• The difference between dew, frost, and frozen dew, and what conditions they form under
• The types of fog, and how their formation differs

In order to get clouds, we need two things, moisture and rising air. We’ve discussed moisture previously, so this chapter deals with what makes air rise. There are four basic processes that lift air. There is orographic uplift where air running into mountain ranges is forced to rise. The rising air causes an enhancement of precipitation on the windward side of mountains and a rainshadow on the leeward side. For example there is plenty of precipitation on the west side of the Sierra Nevadas, and a desert on the east side. The second lifting mechanism is due to the presence of fronts. With fronts there is a large temperature difference over a small area, and thus a large density difference. This density difference causes cold air to "plow up" warm air, causing air to rise along the frontal boundary. Air also rises due to areas of convergence. In this case, air moving horizontally along the earth’s surface meets in certain places. Where the air comes together, it rises. The final lifting mechanism is free convection. With warm temperatures, some local pockets of air become heated. The warm air is buoyant (think hot air balloons) and begins to rise.

Now that air has been lifted, will it continue to rise once the lifting has stopped, or will it return to where it originally was? This the question behind stability. In general, if a parcel of air is warmer than the air around it, it will rise, and if the parcel is cooler than the air around it, it will sink. In order to determine stability from a sounding we want to compare the plotted temperature from the weather balloon, which represents the environmental lapse rate (ELR), to the dry adiabatic lines and saturated adiabatic lines, which represent a parcel cooling off at the dry adiabatic lapse rate (DALR) or the saturated adiabatic lapse rate (SALR). (Look at the handouts I gave you in class to determine which lines these are on a skew-T.) If the environment is cooling off rapidly with height, the plotted temperature on the sounding will be tilted over to the left in comparison to the dry and saturated adiabatic lines. So if a parcel in this case is lifted upwards, it will cool either at the DALR or SALR. This cooling is not as rapid as the ELR, so the lifted parcel is always warmer than the environment around it, and will continue to rise. This type of air is called absolutely unstable. If on the other hand, the plotted temperature line on the sounding is tilted over to the right of both the dry and saturated adiabatic lines, the environment is not cooling off very rapidly with height. The lifted parcel still cools off at the DALR or the SALR, following the dry or saturated adiabatic lines. In this case the cooling of the parcel is more rapid than the ELR, so the lifted parcel is cooler than the environment, and sinks back down once the lifting has ended. This is absolutely stable air. If the plotted temperature line on the sounding is between the dry adiabatic line and the saturated adiabatic line, the air is conditionally unstable. In this case a lifted parcel of dry air cools off faster than the ELR, making the parcel cooler than the environment, and thus stable. However, if the parcel has saturated air, it cools slower than the ELR, making the parcel warmer than the environment, and thus unstable.

In some cases, the temperature does not decrease with height as usual, but instead increases with height in certain layers. These layers with increasing height are called inversions. Since the air in inversions are increasing with height, any parcel traveling through them would be cooler than the environment, so inversions are always stable. An inversion right at the surface during the morning is caused by the loss of heat from the ground at night and is called a radiation inversion. Inversions higher up in the atmosphere where the temperature increases with height, but the dew point decreases rapidly are called subsidence inversions. In this case sinking air causes a layer of air to warm and dry. Frontal inversions are caused by a front where cold air lies underneath warm air. Unlike subsidence inversions, the dew point increases with height similar to the temperature.

Besides stability, the skew-T’s can be used to determine cloud bases and heights. One just has to follow a parcel up various lines. A parcel lifted from (near) the surface temperature cools at the DALR, and thus goes up the dry adiabatic line. The dew point also changes as the parcel is lifted, represented by going up the saturation mixing ratio lines. The dry adiabat lifted from the temperature and the mixing ratio line coming from the dew point will eventually meet. At that point the RH= 100% and the air becomes saturated. This is the lifted condensation level (LCL) and represents the base of a cloud. If the air is still being lifted, it now saturated and now cools at the SALR and goes up the saturated adiabatic lines. In some sounding there may be a point where the lifted parcel becomes warmer than the environment (the saturated line the parcel is following will cross the plotted temperature sounding). At this point, called the level of free convection (LFC) the air no longer need a lifting mechanism, but is now conditionally unstable and will rise on its own. The area below the LFC is called negative area, on Convective Inhibition (CIN). The area above the LFC between the curved saturated adiabat line the parcel is following and the plotted temperature line is called positive area or Convective Available Potential Energy (CAPE). This represents the amount of unstable energy available. In general the greater positive area (CAPE) there is, the stronger the storm. The top of the cloud is indicated by the point where the parcel again becomes colder than the environment, the equilibrium level.

The previous paragraph describes the conditions needed for the towering clouds that produce thunderstorms, but clouds can take a variety of shapes and sizes. Clouds are classified by height and four basic forms. High clouds (> 6000 m) include the thin, wispy cirrus, the layer of ice crystals known as cirrostratus, and the small puffs of cirrocumulus. Middle clouds (2000 – 6000 m) include layered liquid water clouds called altostratus, and the larger, gray, puffy altocumulus. The basic low cloud (< 2000 m) is the layered stratus. If the stratus cloud is producing rain it is known as nimbostratus. Stratocumulus clouds are lumpy stratus clouds that have a small amount of vertical development and can form in rows. Cumulus clouds are clouds with true vertical height. The fair weather cumulus clouds (cumulus humilis) resemble cotton balls, while cumulus congestus form large towering clouds. Cumulonimbus clouds are thunderstorms. They reach high into the atmosphere, with the tops spreading out into an anvil as they reach the stratosphere. There are also some unusual clouds that are not covered in the basic classification system. Lenticular clouds form with waves of air coming off of mountains, and often resemble UFO’s. Banner clouds form as a "scarf" around a mountain top. Mammatus are bag-like clouds that hang down from the underside of severe thunderstorms. There are even rare clouds that form high in the atmosphere and can only be seen at certain times at high latitudes. Nacreous clouds form in the stratosphere, while noctilucent clouds are even higher in the mesosphere.

• The four methods of lifting air
• What makes air stable or unstable, and how to tell from looking at a diagram (similar to homework #3)
• What the basic lines are on a skew-T
• Be able to look at the path of a parcel traced out on a skew-T and tell where the LCL is, be able to find the LFC (if any) and tell if there is any CAPE (again similar to homework #3)
• Know the three basic types of inversions and what causes them
• Understand the cloud classification system
• Be able to identify some of the basic cloud types by sight (similar to homework #4)
• Know the difference between some of the unusual cloud types: lenticular, mammatus, and banner

A cloud droplet has certain forces acting on it. Gravity is pulling the droplet down, but the resistance of the air (drag) is pushing the droplet back up. The cloud droplet’s fall is slowed to a constant speed known as the terminal velocity. Large droplets have a much larger terminal velocity than small droplets, and thus can fall fast enough to get out of the cloud and become precipitation. Raindrops usually need to grow to be 100 times bigger than typical cloud droplets. Condensation initially deposits water onto the drops to make them larger, but only up to a certain point. Droplet growth by condensation alone is not enough to make it rain.

In clouds found in the tropics (entirely above 0^oC), droplets grow by collision-coalescence. A large collector droplet falls at a faster speed, collecting smaller droplets as it goes. The smaller drops can’t be too small or they get swept out of the way, nor too large, or else the collector drop would never catch them. The collision efficiency gives the percentage of collisions a droplet has. Once the droplets collide, they either bounce apart or combine into a larger droplet. The coalescence efficiency gives the percentage of droplets that combine instead of bouncing apart.

When clouds are in the middle latitudes, there is a combination in the cloud of water droplets, supercooled water droplets, and ice crystals. In order to get precipitation in the mid-latitudes, we need to get ice crystals to grow. In the Bergeron process, ice crystals grow larger at the expense of supercooled water droplets due to the difference in saturation vapor pressure over ice. Ice crystals also can collide with the supercooled water droplets, causing riming, where the water freezes onto the ice crystal. Two or more ice crystals can also collide and form a larger crystal by aggregation. Since the ice crystals grow larger easier than just water droplets, all precipitation in the mid-latitudes starts out as snow.

Snow can form in various shapes: columns, plates, and dendrites. The shape depends on temperature and moisture conditions, so any change can cause very complicated shapes (no two snowflakes alike). Relatively warmer clouds form dense, wet snow, while very cold clouds produce a dry, powdery snow. Most of the snow in the western U.S. falls in the mountains, while snow distribution in the east is by latitude. Snow can be enhanced by winds blowing over the warm Great Lakes, picking up moisture and instability and producing heavy, intense, Lake Effect Snows.

When it is warm enough, the snow falling the mid-latitudes melts into rain. Raindrops start off large since the larger drops fall out of the cloud faster. The rain that falls intermittently with large variations in intensity (like it is Sunday when I’m typing this) is called showers and comes from cumuliform clouds. Raindrops are never shaped like teardrops, but instead turn from a sphere into a slowly flattening shape as they fall, until it breaks apart.

Riming on an ice crystal can often cause it to grow into a larger, smooth, spongy piece of ice called graupel. Graupel can form a nucleus for hail, growing as the ice piece moves up and down through a thunderstorm. The updrafts in a thunderstorm push the ice upwards. The ice collects water on its outside as it falls down though the cloud. If the updraft pushes the graupel piece up again, the water layer freezes, forming ring of ice and making it larger. Hailstones have concentric circles on their insides, showing their trips up and down in the cloud. The stronger the updraft, the larger the hailstone, since it is able to keep the stone in the cloud collecting ice layers. Large hail is very large and can fall very fast, causing a great deal of damaging, particularly to crops in the western Great Plains were it is most common.

Another form of frozen precipitation is sleet. Snow falling from a cloud falls into a warm layer of air, melting into rain. The rain then encounters a layer of cold air before it reaches the surface, causing the water to refreeze into sleet. In order for the rain to refreeze, the inversion layer with the cold air need to be fairly deep. If the cold air near the surface is very shallow, the rain won’t form sleet pellets, but will only freeze once it has formed a layer around ground objects, a phenomenon called freezing rain.

Humans have a great interest in precipitation and where it falls. We measure precipitation in a variety of ways including raingages, tipping bucket raingages, radar, snow courses, and snow pillows. Coverage of rain measurement is particularly a problem, especially in undeveloped countries and over oceans. Humans also can try and affect precipitation by cloud seeding, the adding of dry ice or silver iodide to a cloud to try and force ice crystal growth and make it rain. However conditions have to be favorable and it is difficult to tell whether the seeding made any difference.

Water droplets and ice crystals can refract (bend) and reflect light, producing a variety of optical effects. The most familiar is the rainbow, formed by reflection of different visible wavelengths by raindrops. Halos are formed by ice crystals between the observer and the sun or moon. The crystals refract the light so a ring around the sun or moon is visible. Different positions of ice crystals can produce bright spots called sundogs, or columns of light called sun pillars. Coronas and glories are optical effects produced by the bending of light (diffraction) around cloud droplets.

• Understand what forces affect cloud droplets, and how big they need to be to fall
• Collision-coalescence process and what type of clouds it occurs in
• How ice crystals grow in cool clouds
• The types of precipitation and how they form
• What lake effect snow is
• Measurement of precipitation and its problems
• Cloud seeding and its controversies