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November-December 2017

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Weather Queries

Middle and higher clouds travel with weather systems, but do the lower clouds near the surface do the same or develop and redevelop as the system moves through any one location? Also, here in Southern California, I hear the term “low clouds and fog spreading inland during the night and mornings.” Do these low stratus and stratocumulus clouds actually move inland or do they develop as the sea breeze front moves inland?

Robert Price

Sun City, California

Robert, your note indicates to me that you are a careful observer of the weather. I attempt to answer both of your questions here, but sequentially, specializing the general principles and ideas used in answering the first to help answer the second in the context of the particular Southern California setting.

The terms “middle” or “high,” when used as a descriptor for clouds, refer to their vertical location within the troposphere—that portion of the atmosphere that contains most of its mass and where we all live. The troposphere extends from the ground surface up to a height of typically 5–8 miles above mean sea level over the conterminous United States. (It can be lower in the polar regions and commonly extends to 9–10 miles near the equator.) It is within the troposphere that one finds almost all clouds (the exception being rare noctilucent clouds) and where the air temperature typically gets colder as one climbs to higher altitudes.

Ultimately the atmosphere gains all its moisture by evaporation from the earth's surface, whether it be land or ocean, vegetation or lakes, or ice. Since the evaporation process involves the conversion of water in its liquid or solid form to water vapor, for clouds to form, something else must happen.

Evaporation increases the water-vapor content of the air in contact with the evaporating surface. However, in practice there is a limit on the amount of water that can exist in vapor form, and this limit (equivalent to a relative humidity of 100%) is a strong function of the temperature of the air containing the water vapor: warm air can contain much more water vapor than cold air. Roughly speaking, if the temperature of the air increases by 18°F, it can contain almost twice as much water in vapor form as before.

This principle, together with the decrease in pressure with height in the atmosphere, is critical to cloud formation. In our atmosphere, the atmospheric pressure, as measured by a barometer at a particular location in the atmosphere, is approximately proportional to the weight of the atmosphere above that point. Thus, as one goes higher in altitude, there is less air above and pressure decreases. A volume of air that rises experiences a decrease in pressure, and therefore its volume expands and its temperature decreases. Likewise, a volume of air that descends experiences a pressure increase and its temperature warms up.

So long as the relative humidity of this air remains below 100%, its temperature will change approximately according to the dry adiabatic lapse rate as it moves vertically, that is, about 5.4°F per thousand feet. Because cold air can hold less water in vapor form than warm air, clouds will eventually form in rising air unless that air is extremely dry.

In a typical weather system such as may approach California from the west in wintertime, the air is typically rising over and ahead of the surface low (to the east, if the surface low center is moving eastward). It is in this rising stream of air that clouds form. Behind the low center, the air is subsiding, and cloud cover is less. Because in a typical moving storm system the air at upper levels is moving faster than the weather system itself, the approach of the storm is generally heralded by cirrus clouds at high levels (often near 30,000 feet altitude) that have formed by rising air within the storm and have moved out ahead of it as they reach higher altitudes. Then, as the storm system approaches, lower, thicker clouds appear. But, at low levels, the air motion, and particularly the locations of upward-moving air where clouds are likely to form, is controlled not only by the dynamics of the storm system itself, but particularly by mountain ranges, hills, and coastlines. Proximity of sources of moisture, such as the ocean or large lakes, is also important.

As an example, consider what happens when onshore flow sets in as a storm system approaches Southern California from the west. This onshore flow, having been moistened by contact with the ocean surface for some time, is typically moist. As it crosses the coast and encounters the coastal hills and the higher mountains of the Peninsular Ranges down toward San Diego or the San Gabriel or San Bernardino mountains closer to Los Angeles, the air will rise, and clouds are likely to form, or if clouds are already present, they will thicken and become more extensive. This usually leads to greater precipitation in the mountains than at lower elevations. On the desert side of the mountains, where the air is sinking, the low clouds will dissipate.

Another factor governing the lifetime of clouds is the degree of small-scale motions or turbulence in the airflow. At low levels, the airflow is typically more turbulent. (If you travel by air you may have noticed that the air tends to be more “smooth,” with occasional notable exceptions, at cruise altitudes than on climbout and approach to landing.) Near the ground there tend to be pockets of upward and downward motions that may be enough to lead to formation of cumulus or stratocumulus clouds where the air is rising. If you watch the clouds under these conditions, you can often see them forming, changing shape, or dissipating over a few minutes time. So, unlike most clouds at higher levels (particularly clouds that are more than, say, about 10,000 feet above ground), the lifetimes of individual low-level clouds, particularly in active weather situations, are typically fairly short.

Now to the second part of your question. As a native of Southern California, I am no stranger to the phrase “low clouds and fog spreading inland during the night and mornings,” or similar descriptions of the daily rhythm (particularly in summer) of low-cloud coverage. Particularly in summer, the air aloft over Southern California is typically sinking, and so is dry and, below 8,000–10,000 feet above sea level, is usually warmer than the surface temperature of the ocean. Over the ocean, below this dry and sinking air is a “marine layer” of air based at the ocean surface. This air is relatively cool and moist by comparison, is typically only roughly 1,300–2,600 feet) thick, and usually has a layer of stratocumulus or stratus clouds near its top. Vertically separating these two air masses is an “inversion layer”—a thin layer (often no more than 300 feet thick) of transition where the temperature increases sharply and relative humidity decreases sharply with height.

The daily cycle of clouds and clearing is highly repeatable, but the physical processes involved are complex, involving not only the late morning and afternoon sea breeze (and weaker land breeze at night), but also including heating or cooling by radiation processes in the atmosphere and effects of the coastal mountains on the wind field. To illustrate this, I will attempt to walk through a typical summer day, noting the processes that contribute to changes in cloud cover near the coast and farther inland.

Typically in the early morning there are overcast stratus or stratocumulus clouds covering the land areas near the coast. The top of this cloud deck intersects the coastal mountains at the altitude of the base of the temperature inversion that separates the marine layer from the dry, warm air above. Where this cloud deck intersects the mountains it is, of course, foggy. As the morning progresses, the land areas begin to warm. Even though the land areas below the altitude of the top of the marine layer are cloudy early, the clouds are generally thin, letting some solar energy through and warming up the land surface.

The warming land surface heats up the atmosphere near the ground. This heating of the atmosphere is never completely uniform because of hills, buildings, streets, and varying vegetation cover. As a result, the air that warms the most will tend to rise off the surface in the form of “thermals,” or bubbles or pockets of rising air. If these make it to the top of the marine layer, as some of them certainly will, air within them will penetrate into the inversion layer and mix with the warmer, drier air within it. This will cause the low clouds to locally dissipate so that the low-cloud layer develops holes that allow direct sunlight to reach the surface, accelerating the heating of the marine layer air. A feedback effect ensues, with more sun warming the land areas more rapidly, resulting in more thermals that mix warmer, drier air down into the marine layer, resulting in rapid clearing of the low clouds at the marine-layer top. There is also a much smaller contribution to the heating of the marine layer air over land from absorption of sunlight by the water vapor in the marine layer.

As this happens, a sea-breeze circulation begins to develop as the land continues to heat up but the ocean temperature stays essentially constant. Places on land may experience a fairly abrupt onset of flow inland from the ocean (the sea-breeze front). Contributing to this circulation also is that the layer of cool marine air from overnight is thinner where elevations are well above sea level but still below the top of the marine layer. Where the marine layer is thinner, it will heat up faster, leading to flow upslope toward higher ground where it is warmer. This augments the sea breeze so that, by midday, near the ground there is flow directed inland over much of Southern California west of the major coastal mountain ranges. Within a few miles of the coast, the sea breeze may be strong enough to counteract the mixing of drier air from above noted earlier, bringing a layer of stratocumulus inland. However, on a typical day, individual clouds in this layer can be observed to disappear as they move inland.

This situation continues until sunset approaches and sunshine is no longer so effective at heating up the land and maintaining the sea-breeze circulation. Therefore, the marine-layer clouds from offshore have more opportunity to avoid dissipation, as there is less mixing of dry air into the marine layer from the inversion above it. At the same time, the sea-breeze circulation begins to weaken, although this lags the sun by a few hours. By dusk, however, the cooler marine-layer air from offshore has had a chance to spread inland and temperatures begin to cool down appreciably. By this time another process begins to assert itself, and that is radiative cooling of the air at the top of the marine layer. This is independent of the short wave solar radiation of daytime; this radiation is in the infrared part of the spectrum, and water vapor plays a crucial role. In particular, water vapor is a good radiator and absorber of radiation. Because the air above the inversion at the top of the marine layer is typically very dry (relative humidity under 25% usually and sometimes below 10%), there isn't much water vapor to absorb radiation from the top of the moist marine layer and radiate it back down. As a result, the top of the marine layer cools, contributing to the formation or thickening of stratus clouds at its top. At the elevation where the top of the marine layer intersects the coastal mountains or hills, this is seen as formation of fog. So, the return of clouds in the late afternoon and evening is due to a combination of factors: the remains of the sea breeze physically carrying clouds inland, the cooling by infrared radiation at the top of the marine layer brought in by the sea breeze contributing to formation of new clouds, and the thickening of those already present.

Soon after dark, the sea breeze typically quits and may actually be replaced by a “land breeze” toward the ocean. The land breeze is nearly always much weaker than the daytime sea breeze, and is often shallower: unlike the sea breeze it does not usually extend through the depth of the moist marine layer. During the night, the infrared cooling of a thin layer at the top of the stratus or stratocumulus continues, leading to what often is a solid cloud deck over the ocean and land areas as far inland as the elevation contour corresponding to the top of the marine inversion. This can be quite a spectacular sight at sunrise as viewed from an airplane flying above the clouds—individual ranges of hills or mountains (depending on the height of the top of the clouds) can be seen poking above the clouds, appearing as islands in a sea of white. 


JOHN M. BROWN was born and raised in San Diego, California, where he became fascinated by clouds, wind, and rain from his earliest years. Since completing his education at UCLA and MIT, he has worked as a meteorologist in various capacities. He is currently a research meteorologist at NOAA’s Earth System Research Lab in Boulder, Colorado.  


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