|Clouds are formed whenever the air is cooled to or below its dew point. For almost all clouds this cooling is associated with vertical motions in the atmosphere. However, fog is also a form of cloud, and is commonly created without vertical motions. We will look at that first and then consider the various types and processes forming the true clouds in the atmosphere..|
Photo 3.3.1: A variety of clouds seen across Bogue Sound on a winter morning.
| Fog, as with any cloud, consists of
tiny droplets of water suspended in the atmosphere. For most of us the
main characteristic of fog is the resultant reduced visibility. The
various types of fog are defined by the amount of the reduction. Fog
occurs when visibility is less than 1 km (0.62 miles). Heavy
fog has visibility less than 0.25 miles. Mist
is less rigorously defined, but refers to any conditions where visibility is obscured,
but where it is possible to see more than 1 km.
Most of our official fog observations until recently came from simple visual observations. At each weather station an upstanding object was identified which was close to 1 km away from the station. If it was visible, there was no fog. If it wasn't there, we had fog. Now we have an instrument, a transmissometer, which measures the change in intensity of a light beam along a path through the atmosphere and converts this to a visibility measure.
With the moist air and calm conditions common in North Carolina, we are most likely to encounter radiation fog. On a clear night the earth's surface cools because of the loss of energy by long-wave radiation. The cooling may be sufficient to drop the temperature below dew point. In non-calm conditions the cooling will be restricted to a very shallow layer, and only dew will form. However, if it is calm a deeper layer is cooled and fog is likely. It generally starts to form around midnight, getting thicker and deeper as the night progresses. Soon after dawn, once the sun is up and the solar radiation begins to warm the top layer of the fog, evaporation occurs and the fog dissipates.
|Photo 3.3.2: Radiation fog in the Durham basin. In this case the cold air has tended to drain (flowing just like water) into the lowest lying areas, so that the patches of densest fog are in the valleys. (TO COME)|
Advection fog commonly occurs when a warm, moist air stream blows over a cooler surface, so that the air is cooled by energy exchange with that surface (Fig 3.3.1). Theoretically this can occur anywhere that these conditions are met. In practice, we tend to see this most commonly along the coast in winter, when a warm moist air stream is blowing onshore onto the colder land. We also see this type of fog when there is snow around, creating the very cold surface needed to cool the overlying air below its dew point. In all cases a rather slow and steady air stream is needed, so that the near-surface cooling can occur without mixing in warmer air from aloft.
|Fig 3.3.1: Energy exchanges leading to the creation of advection fog. As the warm moist air moves over the cold surface the near-surface air layer is cooled as heat is conducted downwards. The amount and depth of cooling increases as the air stream moves farther over the cold surface, encouraging a deeper, denser fog.|
The evaporative fog shown in Photo 3.3.3 occurred in the evening as the air temperature was dropping. The water, warmed during the preceding sunny day, continued to evaporate moisture into the air. The calm conditions ensured that the air wasn't stirred, so that the moisture content continued to increase at the same time as the dew point was dropping. The fog would persist for a few hours while the lake continued to evaporate. Eventually sometime during the night the slowly cooling lake water would cease to evaporate. The moisture of the fog was then likely to diffuse steadily upwards and mix with the drier air above. The fog would probably dissipate before dawn.
|Photo 3.3.3: A thin layer of evaporative fog on Cheoah Lake, between Graham and Swain Counties.|
For most parts of the state, heavy fog is a winter time phenomenon, and infrequent even then (Fig 3.2.2). At Hatteras the common high wind usually prevents fog formation. Just inland of the coast, in areas such as those around Wilmington, lower wind speeds gives a summer frequency akin to that at Charlotte. Charlotte receives a mixture of advection and radiation fog during the winter. Fog at Asheville airport, where the observing station is located, is almost entirely a result of radiative cooling in the calm conditions within the bowl in the hills where the airport is located. Usually the fog is a very short-lived morning phenomenon, so that the impact is much smaller than the number of days with fog would suggest.
|Fig 3.3.2: The monthly average number of days upon which heavy fog is reported at selected North Carolina observing stations.|
Cloud formation processes
|Clouds form because upward motion causes the air to cool. When 'parcels' of air ascend, they expand as pressure decreases and cool as they expand. Cool them enough, and they will start to condense and form clouds. Rising parcels of air cool at a fixed rate, depending on whether they are saturated or not. If we know the air temperature and dew point temperature of a parcel, we can forecast whether any predicted lifting, by whatever processes, will lead to cloud formation. Indeed, this is a major advantage in considering humidity in terms of the dew point temperature.|
|A 'parcel' is a convenient term for a block of air moving vertically in the atmosphere. It is useful to think of a parcel as being represented by the small white, fluffy (cumulus) cloud of a summer afternoon, which we can see moving upwards as it grows into a thundercloud. All of the following processes assume that there are such blocks, or a constant stream of such blocks, available.|
|Photo 3.3.4: Air parcels of varying size rising at about noon on a summer day.|
|There are three major ways in which the air parcels rise to form clouds (Fig 3.3.3). First, it can be forced to rise, simply to get over a mountain barrier. Second, it will rise if two air streams converge, the air being squeezed upwards like toothpaste out of a tube. Thirdly, in certain circumstances the air will rise spontaneously.||
|Fig 3.3.3: Processes creating clouds - the ways in which air can be made to rise.|
|When air is forced to rise, the process is known as orographic uplift (Fig 3.3.4). The NC mountains are a major cause of such uplift. The air slides up the mountain side, cooling as it ascends. Eventually it cools below the dew point and a cloud forms. In many instances precipitation, orographic precipitation, occurs. Once over the top the descending air warms and the cloud droplets evaporate back into the air and the cloud dissipates.|
|Fig 3.3.4: Schematic diagram for the formation of orographic cloud.|
|We associate orographic effects with our western mountains. The Appalachians, however, are very unusual. Most mountain ranges have air flow almost exclusively from one direction, so that the upwind side is wet day after day, while the lee side is continuously dry, and may even be a desert. This dry lee side is in the rain shadow of the mountains. The Appalachians have no such preferred wind direction. One day the wind may come from the west, giving rain on the 'Tennessee-facing' sides of the hills. The next day the wind may be coming from the east, and those same hill sides are now dry, the slopes facing the NC Piedmont now being the wet ones. The only places where there is a more-or-less guaranteed rain shadow are the mountain basins. The Asheville basin is the best known. There is still plenty of cloud - even if away on the distant hillsides - but it is one of the driest places in the state.|
|Any slope which causes air to rise and cool below its dew point will give an orographic effect. In some cases, for instances, very humid air blowing in off the Gulf Stream has to rise as it crosses Coastal Plain and enters the Piedmont. The lift may be enough to give a cloud. In this case the low angle of slope gives a cloud which is more horizontal and extended in appearance than the vertical clouds associated with the mountain effects.|
|Photo 3.3.5: A line of clouds resulting from orographic uplift, looking to the west from the Blue Ridge Parkway in Haywood County.|
| There are two
possible consequences when two air streams come together. If they
are at more or less the same temperature, we will have simple convergence,
and a general uplift over a broad area. However, if the two air streams
are at different temperatures, the warm, less dense air will ride up over
the colder, denser air, producing frontal uplift (Fig
Both conditions are common in our area, and are often responsible for the rainy weather we get. They are usually associated with the storms we call depressions and which we consider in section 4.5.
|Fig 3.3.5: A TV weather map (top) showing a front off the Carolina coast, with warm air off the ocean rising over the cold air coming from the west. As a result of the uplift, there is a cloud band extending westward over the eastern portion of the state. The radar image (bottom) shows the bands of rain associated with the fronts and its clouds.|
| The final type of uplift is called convection.
In its simplest form this is very similar to what happens when you put a
pan of water on the stove to boil. Heat from below causes the bottom
water to become less dense than the overlying layers, the deep water moves
upwards and colder water replaces it at the surface, setting up a
convection current. Because air, unlike water, can be squashed, we have to
take into account air pressure differences as well as temperature, but the
end result is the same.
We commonly see convection occur on what starts out as a cloudless summer morning. As the day progresses more and more solar energy is absorbed at the ground. The ground heats and eventually becomes warm enough to start an upward convection current. Small clouds develop. As the afternoon progresses these clouds may get bigger, eventually leading to a late afternoon thunderstorm (see section 5.2). However, by evening the ground is beginning to cool, and there is no longer the the energy available to keep the convection going. The cloud dissipates.
|Photo 3.3.6: Air parcels rising by convection - this is the situation about 1 hour after the view in Photo 3.3.4, when the convection has been established longer.|
|Like fog, clouds are created by the cooling of air below its dew point. Every cloud is unique in appearance, but we can divide them into four general classes. Two, stratus and cirrus, represent clouds having mainly horizontal development, one class, cumulus, is for vertical clouds, while the fourth, alto-, can be either horizontal of vertical. Some of the various classes frequently occur in combination, and Fig 3.3.6 shows schematically the types we are likely to see.||
|Fig 3.3.6: The major types of clouds, grouped according to their general appearance.|
|Photo 3.3.7: Cloud types|
|a) Cumulus: clouds with marked vertical development, ranging from the small cauliflower-like clouds of a summer afternoon to the towering cumulonimbus of the thunderstorm.||b) A more ominous, well-developed cumulus, grading into a cumulonimbus, over the mountains. This cloud was the result of a combination of orographic and convective actions.||c) Alto-: These are middle level clouds, commonly either altocumulus, having the same characteristics as cumulus but with a base at a much higher level, or altostratus, (shown here) higher level stratus, often ligher and less threatening than regular stratus.|
|d) Stratus: low horizontal clouds, frequently grey and stretching from horizon to horizon. Drizzle is often associated with them.||e) A thicker stratus, and the resultant rain||f) Cirrus: thin, whispy high level clouds, which may occur in streaks, as here, or may spread out to cover the sky almost completely. Diffuse sunlight readily passes through them The fibrous appearance occurs because these clouds are composed of ice crystals, not water droplets.|
|All of these cloud types occur frequently in North Carolina, although we have relatively few records of the occurrence of the individual cloud types. Rather, we have records of the total cloud amount. This is a visual observation, with the observer estimating the amount of the sky covered by clouds, to the nearest tenth of sky covered. The results are similar for each station (Fig 3.3.7). There is a fairly even distribution of amounts throughout the year except for a marked dip in October and November, which also tend to be the driest months of the year. The cloud amount is closely associated with the sunshine duration shown in Fig 2.1.2.||
|Fig 3.3.7: Monthly average sunrise to sunset cloud amounts (oktas) for selected stations.|
| The variation in
cloud from day-to-day within the
various months is also similar from station to station. Fig 3.3.8
shows it for the Charlotte station. Winter has the greatest number of clear days, Fall the
least. For Fall, with its relatively low total cloud amount, the
number of cloudy days is at a maximum, but with low amounts on other
These patterns reflect the varying importance of the different types of weather systems which influence the state at each season, a topic we look at in the next chapter.
|Fig 3.3.8: Average number of days each month with cloudy, partly cloudy and clear conditions at Charlotte.|
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