Chapter 12, Vertical Motion and Clouds 12-6
Figure 12-4. Orographic Effects Example
The air parcel begins with a temperature of 15°C, dewpoint of 10°C, and a relative humidity of 80 percent
at 2,000 ft. As the parcel is lifted on the windward slope, the temperature cools at the dry adiabatic lapse
rate of 3°C per 1,000 ft, and the dewpoint cools at a rate of 0.5°C per 1,000 ft until it becomes saturated at
the LCL at 4,000 ft. Then, the air parcel’s temperature and dewpoint both cool at the moist adiabatic lapse
rate of 2°C per 1,000 ft until the parcel reaches the summit at 12,000 ft. At that altitude, the parcel ’s
temperature is -7°C, the dewpoint is -7°C, and the relative humidity is 100 percent. As the air parcel
descends the leeward slope, the temperature increases at a rate of 3°C per 1,000 ft while the dewpoint
increases 0.5°C per 1,000 ft. The air parcel ends with a temperature of 23°C, dewpoint of -2°C, and a
relative humidity of 33 percent at 2,000 ft, much warmer and drier than at the beginning.
Orographic effects are especially apparent from west to east across the Pac ific Northwest, where the
north–south Cascade Range intercepts the prevailing flow of humid air from the Pacific Ocean.
Exceptionally cloudy, rainy weather prevails western slopes, whereas semiarid weather characterizes the
eastern slopes and areas farther east.
12.4.2 Frictional Effects
In the Northern Hemisphere, the surface wind spirals clockwise and outward from high pressure, and
counterclockwise and inward into low pressure due to frictional force. The end result is that winds diverge
away from surface high pressure, causing the air to sink, compress, and warm, which favors the dissipation
of clouds and precipitation. Conversely, winds converge into surface l ow pressure, causing the air to rise,
expand, and cool, which favors the formation of clouds and precipitation given sufficient moisture
(see Figure 12-5).
Chapter 12, Vertical Motion and Clouds 12-7
Figure 12-5. Frictional Effects
12.4.3 Frontal Lift
Frontal lift (see Figure 12-6) occurs when the cold, denser air wedges under the warm, less dense air,
plowing it upward, and/or the warmer air rides up and over the colder air in a process called overrunning.
Clouds and precipitation will form given sufficient lift and moisture content of the warm air.
Figure 12-6. Frontal Lift
12.4.4 Buoyancy
Air near the ground can warm at different rates depending on the insular properties of the ground with
which it is in contact. A newly plowed field will warm faster than an adjacent lake. These temperature
differences result in different densities, allowing the warm air to become buoyant. The denser cool air will
tend to push (i.e., lift) the less dense warm air aloft. On a grand scale, the tendency of air to rise due to
heating, and how high it will rise, is referred to as stability and is covered i n Chapter 13, Atmospheric
Stability.
Chapter 12, Vertical Motion and Clouds 12-8
12.5 Cloud Forms
There are four basic cloud forms (appearances) observed in the Earth’s atmosphere (see Table 12-2). See
Appendix A, Cloud Types, for cloud types.
Table 12-2. Cloud Forms
Cirri-form
High-level clouds that form above 20,000 ft (6,000 m) and are usually
composed of ice crystals. High-level clouds are typically thin and white in
appearance but can create an array of colors when the Sun is low on the
horizon. Cirrus generally occur in fair weather and point in the direction of air
movement at their elevation.
Nimbo-form
Nimbus comes from the Latin word meaning “rain.” These clouds typically
form between 7,000 and 15,000 ft (2,100 to 4,600 m) and bring steady
precipitation. As the clouds thicken and precipitation begins to fall, the bases
of the clouds tend to lower toward the ground.
Cumuli-form
Clouds that look like white, fluffy cotton balls or heaps and show the vertical
motion or thermal uplift of air taking place in the atmosphere. The level at
which condensation and cloud formation begins is indicated by a flat cloud
base, and its height will depend upon the humidity of the rising air. The more
humid the air, the lower the cloud base. The tops of these clouds can reach
over 6
0,000 ft (18,000 m).
Strati-form
Stratus is Latin for “layer” or “blanket.” The clouds consist of a featureless
low layer that can cover the entire sky like a blanket, bringing generally gray
and dull weather. The cloud bases are usually only a few hundred feet above
the ground. When stratus clouds move over hills and mountains, they are able
to reach ground level and are thus called fog. Also, as fog lifts off the ground
due to daytime heating, the fog forms a layer of low stratus clouds.
Source: NWS JetStream – Online School for Weather
12.6 Cloud Levels
By convention, the part of the atmosphere in which clouds are usually present has been divided into three
levels: high, middle, and low (see Table 12-3). Each level is defined by the range of heights at which the
cloud of a certain type occurs most frequently. The levels overlap, and their limits vary with latitude. The
approximate heights of the limits are included in Table 12-3.
Chapter 12, Vertical Motion and Clouds 12-9
Table 12-3. Approximate Height of Cloud Bases Above the Surface
Level Polar Regions Temperate Regions Tropical Regions
High Clouds 10,000–25,000 ft (3–8 km) 16,500–40,000 ft (5–13 km) 20,000–60,000 ft (6–18 km)
Middle Clouds 6,500–13,000 ft (2–4 km) 6,500–23,000 ft (2–7 km) 6,500–25,000 ft (2–8 km)
Low Clouds Surface–6,500 ft (0–2 km) Surface–6,500 ft (0–2 km) Surface–6,500 ft (0–2 km)