Flight Environment
CLOUDS
To a pilot knowledgeable in the science of meteorology, clouds are an indication of what is happening in the atmosphere. The location and type of cloud are evidence of such weather phenomena such as fronts, turbulence, thunderstorms, and tell the pilot what type of conditions may be expected during flight.
CLASSIFICATIONS OF CLOUDS
Clouds are classified into four families: high clouds, middle clouds, low clouds and clouds of vertical development. As well, clouds are identified by the way in which they form. There are two basic types: cumulus and stratus. Cumulus clouds form in rising air currents and are evidence of unstable air conditions. Stratus clouds form in horizontal layers and usually form as a layer of moist air is cooled below its saturation point. Clouds from which precipitation falls are designated nimbus clouds. The cloud heights referred to below are for the temperate regions. In the polar regions, clouds tend to occur at lower heights and in the tropics at greater heights.
HIGH CLOUDS
The bases of high clouds range from 16,500 feet to 45,000 feet and average about 25,000 feet in the temperate regions. They’re composed of ice crystals.
Cirrus (Cl). Very high, Thin, wavy sprays of white cloud, made up of slender, delicate curling wisps or fibers. Sometimes takes the form of feathers or ribbons, or delicate fibrous bands. Often called cats' whiskers or meres' tails.(left)
Cirrocumulus (Cc). Thin clouds, cotton or flake-like. Often called mackerel sky. Gives little indication of future weather conditions.(right)
Cirrostratus (Cs). Very thin high sheet cloud through which the sun or moon is visible, producing a halo effect. Cirrostratus is frequently an indication of an approaching warm front or occlusion and therefore of deteriorating weather. (left*)High clouds have little effect on flying. Some moderate turbulence may be encountered.
MIDDLE CLOUDS
The bases of middle clouds range from 6500 feet to 23,000 feet. They are composed of ice crystals or water droplets, which may be at temperatures above freezing or may be supercooled.
Altocumulus (Ac). A layer or series of patches of rounded masses of cloud that may lie in groups or lines. Sometimes they indicate the approach of a front but usually they have little value as an indication of future weather developments.(right)
Altocumulus Castellanus (Acc). Altocumulus with a turreted appearance. Instability is a characteristic. Altocumulus castellanus may develop into cumulonimbus.(left*)
Altostratus (As). A thick veil of grey cloud that generally covers the whole sky. At first, the sun or moon may be seen through the cloud, but they disappear as the cloud gets thicker. The presence of altostratus indicates the near approach of a warm front. Some light rain or snow may fall from thick altostratus. Icing may occur in this cloud. (right)There is usually little turbulence associated with middle clouds unless cumulus clouds are embedded in them or unless altocumulus is developing.
LOW CLOUDS
The bases of low clouds range from surface height to about 6500 feet. They are composed of water droplets which may be supercooled and sometimes of ice crystals.
Stratus (St). A uniform layer of cloud resembling fog but not resting on the ground. Drizzle often falls from stratus. When stratus cloud is broken up by wind, it is called stratus fractus.
Stratocumulus (Sc). A layer or series of patches of rounded masses or rolls of cloud. It is very often thin with blue sky showing through the breaks. It is common in high pressure areas in winter and sometimes gives a little precipitation.(left*)
Nimbostratus (Ns). A low layer of uniform, dark grey cloud. When it gives precipitation, it is in the form of continuous rain or snow. The cloud may be more than 15,000 feet thick. It is generally associated with warm fronts.(right)Little turbulence occurs in stratus. The low cloud bases and poor visibility make VFR operations difficult to impossible.
CLOUDS OF VERTICAL DEVELOPMENT
The bases of this type of cloud may form as low as 1500 feet. They are composed of water droplets when the temperature is above freezing and of ice crystals and supercooled water droplets when the temperature is below freezing.
Cumulus (Cu). Dense clouds of vertical development. They are thick, rounded and lumpy and resemble cotton balls. They usually have flat bases and the tops are rounded. They cast dense shadows and appear in great abundance during the warm part of the day and dissipate at night. When these clouds are composed of ragged fragments, they are called cumulus fractus.(left*)
Towering Cumulus (TCu). Cumulus clouds that build up into high towering masses. They are likely to develop into cumulonimbus. Rough air will be encountered underneath this cloud. Heavy icing may occur in this cloud type.(right*)
Cumulonimbus (Cb). Heavy masses of cumulus clouds that extend well above the freezing level. The summits often spread out to form an anvil shaped top that is characteristic of thunderstorm.(left*)
Clouds form when the invisible water vapor that is present in the air changes into its visible form as water droplets or ice crystals.
The process by which water vapor changes into water droplets is called condensation and occurs when the relative humidity is high, when condensation nuclei are present in the air and when there is cooling of the air.
The level at which water vapor condenses and becomes visible is known as the recondensation level. This level is in practice the base of the clouds. If the cloud forms at ground level, it is called fog rather than cloud.
Except at temperatures well below freezing, clouds are composed of very small droplets of water which collect on microscopic water absorbent particles of solid matter in the air (such as salt from evaporating sea spray, dust, and smoke particles). The abundance of these particles, called condensation nuclei, on which the droplets form, permits condensation to occur generally as soon as the air becomes saturated. If the condensation nuclei are particularly abundant, condensation may occur at less than 100% relative humidity.
Clouds which form at temperatures well below freezing are usually composed of small particles of ice known as ice crystals which form directly from water vapor through the process of sublimation. When the temperature is between freezing and about -15°C, clouds are composed largely of supercooled water droplets with some ice crystals as well.
Saturated warm air holds much more water vapor than does saturated cold air. Cooling saturated warm air will result in more water vapor condensing into visible water droplets than is the case when cooling saturated cold air. Denser, thicker cloud formations occur when condensation occurs in a warm air mass.
Clouds are formed in two ways. (1 ) Air, in which water vapor is present, is cooled to its saturation point and condensation occurs. The cooling process will occur as warm air comes in contact with a cold surface or with a surface that is cooling by radiation or as air is affected by adiabatic expansion. (2) Air, without a change in temperature taking place, may absorb additional water vapor until its saturation point is reached with the result that clouds are formed. Of these, the most common cause of cloud formation is adiabatic expansion, that is, cooling due to expansion brought about by lifting.
Stability of the air, of course, is one of the major factors which determines the strength and extent of vertical motion and therefore cloud formation. In stable air, the cloud forms in horizontal sheets of stratus cloud. In unstable air, cumulus clouds develop. The lifting process is initiated by a number of different phenomena.
Air blowing against a range of hills or mountains is forced upward, reaches a region of lower pressure, expands and cools. Condensation will occur when the dewpoint is reached. The type of cloud formed will depend on the moisture content and on the stability of the air. The slope and height of the terrain and the strength of the wind component that produces the upslope flow also have an effect.
If the air is dry, very little cloud will form. Stratus cloud will form if the air is moist and stable, cumulus and cumulonimbus, if the air is moist and unstable.
A long bank of cloud from which rain may fall forms on the windward side and on the upper parts of the hills or mountains. Due to the fact that the rising ground is always in the same place, orographic clouds and rain are typically persistent and usually widespread. The descending air on the leeward side of the mountains will be compressed and heated, causing dissipation of the clouds.
CONVECTION
Warm air rises. Owing to the heating of the ground by the sun, rising currents of air occur. The upward movement of air is known as convection. (The downward movement of air is known as subsidence. ) As currents of air rise due to convection, they expand. The expansion is accompanied by cooling. The cooling produces condensation' and a cumuliform cloud forms at the top of each rising column of air. The cloud will grow in height as long as the rising air within it remains warmer than the air surrounding it. The height of the cloud, however, is also dependent on the stability of the air in the mid levels of the troposphere. Convection also occurs when air moves over a surface that is warmer than itself. The air is heated by advection and convective currents develop. Warming of air by advection does not depend on daytime heating. Convection will, therefore, continue day or night so long as the airflow remains the same.
FRONTAL LIFT
When a mass of warm air is advancing on a colder mass, the warm air rises over the cold air on a long gradual slope. This slope is called a warm frontal surface. The ascent of the warm air causes it to cool, and clouds are formed, ranging from high cirrus through altostratus down to thick nimbostratus from which continuous steady rain may fall over a wide area.
When a mass of cold air is advancing on a mass of warm air. The cold air undercuts the warm air and forces the latter to rise. The slope of the advancing wedge of cold air is called a cold frontal surface. The clouds which form are heavy cumulus or cumulonimbus. Heavy rain, thunderstorms, turbulence and icing are associated with the latter.
When a strong wind blows over a rough surface, the friction between the ground and the air produces mechanical turbulence, or eddy motion. The intensity of the turbulence is dependent on the roughness of the underlying surface, the strength of the wind and the instability of the air. The eddy motion consists of irregular up and down currents. The air in the upward current cools, and if sufficient moisture is present and if the turbulence is vigorous, condensation may take place in the upper pad of the turbulent layer. The cloud layer has an undulating base which is lower in the rising eddies than in the sinking eddies. The top of the cloud marks the top of the turbulent layer and tends to be very flat. Very often an inversion exists above the turbulent layer and it is this which blocks further vertical motion. The cloud layers tend to be stratocumulus in form. Sometimes convection occurs in combination with the mechanical turbulence and then cumulus clouds will develop and will be embedded in the stratocumulus layer. If the air is very stable, the mechanical turbulence is dampened and confined to very low levels.
CONVERGENCE
When air piles up over a region as at the center of a low pressure area, convergence is said to be occurring. The excess air is forced to rise; it expands and cools and, when the condensation level is reached, clouds form. Since all fronts lie in regions of low pressure, convergence is often a contributing factor to frontal weather.
PRECIPITATION
Precipitation occurs when the water droplets (visible as a cloud) grow sufficiently in size and weight to fall due to gravity. In clouds with temperatures above freezing, vertical air currents cause the droplets to move about. As a result, they collide with other drops and gradually grow in size, as they absorb those drops with which they collide, and they gain momentum until they fall through the air as rain. A single water droplet must grow enormously in order for precipitation to take place. The average raindrop is about one million times larger than a cloud water droplet. This process is known as coalescence. Precipitation due to coalescence alone generally occurs only in warm climates.
In a stable cloud such as stratus, there is very little vertical motion, not even enough to sustain small water droplets. They frequently escape and drift slowly to the earth. This form of precipitation is called drizzle.
A second mechanism by which precipitation occurs requires that ice crystals and water droplets exist side by side in a cloud at temperatures below freezing. The ice crystals grow at the expense of the water droplets. The droplets tend to evaporate and the resulting water vapor sublimates on the ice crystals. The ice crystals grow in size and weight. They are sustained in the cloud until they grow large enough that their terminal velocity exceeds the updraft velocity in the cloud. They then fall as precipitation. If the temperature below the region of formation is above freezing, the crystals will melt, coalesces with other drops and will arrive at the earth as rain. If the temperatures are cold all the way to the ground, the ice crystals will aggregate into snowflakes. In the US and Canada, heavy rainfall usually occurs as a result of a combination of sublimation on ice crystals and coalescence.
Two facts are therefore significant. If the ice crystals are necessary for the occurrence of heavy precipitation, the cloud from which the rain is falling must have built up well above the freezing level. Since the size of a raindrop is a function of the turbulence in the parent cloud, large drops and heavy precipitation are an indication of strong vertical motion.
Steady precipitation falls from a layer of stratus cloud. A shower, a sudden heavy burst of precipitation, falls from a well developed cumulus or cumulonimbus cloud, which may be embedded in a stratus layer. Precipitation may take many forms.
DRIZZLE
Precipitation in the form of very small drops of water which appears to float is called drizzle. At temperatures at or below the freezing level, drizzle will freeze on impact with objects and is known as freezing drizzle.
RAIN
Precipitation in the form of large water droplets is called rain. Freezing rain is composed of supercooled water droplets that freeze immediately on striking an object which is itself at a temperature below freezing.
HAIL
Observations have revealed the fact that water drops, certainly in the liquid form, can exist with temperatures as low as -40°C. It is clear, then, that small drops can be supercooled a long way without freezing. Most big clouds formed as a result of an upward current of air are divisible into three well defined regions. First there is the lowest layer where the cloud particles are in the form of water drops.
Next there is a region where some of the water droplets are frozen into ice crystals (snow) but some are still liquid but supercooled. Third there is the highest region of the cloud where the water vapor sublimates into minute ice crystals. There is no sharp dividing line between the snow and supercooled water regions. For some distance the ice crystals and supercooled water drops are co-existing. When a supercooled water drop collides with an ice crystal, it at once freezes on the latter, imprisoning a little air which causes it to freeze in the form of soft ice. As it falls through the supercooled region, more soft ice is deposited on it, increasing its size. The ball of soft ice so formed then falls through the water region. Water freezes on it in the form of hard, transparent ice. Finally the ball falls out of the base of the cloud as a hailstone, a hard transparent layer of ice covering a soft, white core.
Sometimes gusts carry hailstones back up to the top of the cloud, in which case the whole process is repeated perhaps several times. In this way, very large hailstones are formed. The vertical gusts which produce very large hailstones may have speeds in excess of 85 knots. The conditions which produce hail are very similar to those in which thunderstorms originate. Hence hail is often encountered in a cumulonimbus thundercloud.
SNOW PELLETS (SOFT HAIL)
If the water region lying below the supercooled region of the cloud is not of great depth, the hailstone does not acquire the hard. transparent covering and arrives at the ground as the original soft white ice. It is then known as a snow pellet or soft hail.
SNOW
In the formation of snow, the invisible water vapor in the air sublimates directly into ice crystals, without passing through any intermediate water stage. Snow flakes are formed of an agglomeration of ice crystals and are usually of a hexagonal or star like shape. Snow grains are tiny snow crystals that have acquired a coating of rime. They fall from non-turbulent clouds.
ICE PRISMS
Ice prisms are tiny ice crystals in the form of needles. They may fall from cloud or from a cloudless sky. They exist in stable air masses and at very low temperatures.
ICE PELLETS
Ice pellets are formed by the freezing of raindrops. They are hard, transparent, globular, grains of ice about the size of raindrops. They generally rebound when striking the ground.
PRECIPITATION AND CLOUD TYPE
Each of the various forms of precipitation is associated with a particular type of cloud.
FOG
*
Fog is, in fact, a cloud, usually stratus, in contact with the ground. It forms when the air is cooled below its dewpoint, or when the dewpoint is raised to the air temperature through the addition of water vapor. To form a water drop in the atmosphere (the basis of fog formation), there must be present some nucleus on which the water may form. Dust, salt, sulphur trioxide, smoke, etc. provide this function.
Given a sufficient number of condensation nuclei, the ideal conditions for the formation of fog are high relative humidity and a small temperature dewpoint spread and some cooling process to initiate condensation. Light surface winds set up a mixing action which spreads and increases the thickness of the fog. In very still air, fog is unlikely to form. Instead dew will collect.
Fog is most likely to occur in coastal areas where moisture is abundant. Because of the high concentration of condensation nuclei, it is also common in industrial areas.
Smoke and dust in the air over large cities produce the "pea soup" fogs characteristic of large industrial centers. The carbon and dust panicles cause such fogs to be dark. Otherwise, when composed of water drops only, fog is white in color.
Fog is usually dissipated by sunlight filtering down through the fog or stratus layer. This results in heating from below.
TYPES OF FOG
RADIATION FOG is formed on clear nights with light winds. The ground cools losing heat through radiation. The air in direct contact with the earth's surface is cooled. If this air is moist and the temperature is lowered below the dew point, fog will form. The ideal conditions for the formation of radiation fog are a light wind which spreads the cooling effect through the lower levels of the air, clear skies that permit maximum cooling and an abundance of condensation nuclei. This type of fog is commonly called ground fog, since it forms only over land. Radiation fog normally forms at night but sometimes it thickens or even forms at sunrise as the initial slight heating from the sun causes a weak turbulence. Radiation fog tends to settle into low areas, such as valleys and it is usually patchy and only a few hundred feet thick. It normally dissipates within a few hours after sunrise as the sun warms the earth and radiation heating causes the temperature to rise.
ADVECTION FOG is caused by the drifting of warm damp air over a colder land or sea surface. This type of fog may persist for days and cover a wide area. It occurs most frequently in coastal regions. Widespread fog forms when moist air from a warm region of the ocean moves over colder waters. It will persist for lengthy periods since the water surface is not affected by daytime heating. Advection fog will spread over land if the circulation is from the sea to a colder land surface and will persist until the direction of the wind changes. Although it may dissipate or thin during the day from daytime heating, it will reform at night. The warm sector of a frontal depression is also favorable for the formation of advection fog.
UPSLOPE FOG is caused by the cooling of air due to expansion as it moves up a slope. A light upslope wind is necessary for its formation.
STEAM FOG forms when cold air passes over a warm water surface. Evaporation of the water into the cold air occurs until the cold air becomes saturated. The excess water vapor condenses as fog. Steam fog occurs over rivers and lakes, especially during the autumn.
PRECIPITATION-INDUCED FOG is caused by the addition of moisture to the air through evaporation of rain or drizzle. This type of fog is associated mostly with warm fronts and is sometimes known as frontal fog. The rain falling from the warm air evaporates and saturates the cooler air below.
ICE FOG forms in moist air during extremely cold calm conditions. The tiny ice crystals composing it are formed by sublimation and are often called needles. Ice fog is caused by the addition of water vapor to the air through fuel combustion. The very cold air cannot hold any additional water vapor and the excess sublimates into visible ice crystals. Ice fog may appear suddenly when an aircraft engine is started.
HAZE
Haze is composed of very small water droplets, dust or salt particles so minute that they cannot be felt or individually seen with the unaided eye. Haze produces a uniform veil that restricts visibility. Against a dark background, it has a bluish tinge. Against a bright background, it has a dirty yellow or orange hue.
Smoke, industrial pollutants and smog from vehicular exhausts are responsible for the thick blanket of haze that severely restricts visibility in some urban and industrial areas. When flying in such conditions, visibility is very poor, especially when flying into the sun. Haze is a problem only in very stable air. In unstable conditions, the panicles scatter.
A thunderstorm is a weather phenomenon whose passage creates extremely serious hazards to flying. It has aptly been described as a cumulus cloud gone wild. It is always accompanied by thunder and lightning, strong vertical drafts, severe gusts and turbulence, heavy rain and sometimes hail. It is a weather condition of which a pilot should be enormously respectful. Since over 44,000 thunderstorms occur daily over the earth, every pilot is sure occasionally to come in contact with one.
The basic requirements for the formation of a thunderstorm are unstable air, some form of lifting action and a high moisture content. Since these are also the requirements for the formation of a harmless cumulus cloud, it follows that the intensity of the conditions is the key to development of a thunderstorm. These violent weather factories occur when an air mass becomes unstable to the point of violent overturning. Such unstable atmospheric conditions may be brought about when air is heated from below (convection), or forced to ascend the side of a mountain (orographic lift) or lifted up over a frontal surface (frontal lift). The resulting buoyancy causes air which is warmer than its environment to push up in the form of convection currents, like drafts up a chimney flue.
If a mass of superheated moist air rises rapidly, an equal amount of cooler air rushes down to replace it.
When these conditions lead to the development of a thunderstorm, the area in which the rising and descending currents are active is called a thunderstorm cell. A thunderstorm may be composed of a number of such cells. As a storm develops, each successive cell grows to a greater height than did the previous one.
There are three distinct stages in the life cycle of a thunderstorm. Every thunderstorm begins life as a cumulus cloud. The cloud starts growing upward, driven by the latent heat as water vapor condenses. Strong updrafts prevail throughout the cell and it rapidly builds up into a towering cumulonimbus cloud. Temperatures within the cell are higher than temperatures at the same level in the surrounding air, intensifying still more the convective currents within the cell. There is usually no precipitation from the storm at this stage of its development since the water droplets and ice crystals are being carried upwards or are kept suspended by the strong updrafts.
In its mature stage, the buildup of a towering cumulonimbus thunderstorm may reach heights as great as 60,000 feet. The updratts may attain speeds of 6000 feet per minute. As the water droplets grow large enough to fall, they drag air down with them, starting a downdraft in the middle region of the cell that accelerates downward. The speed of the downdraft, although not as great as the updrafts, may nevertheless be as high as 2000 feet per minute. Violent turbulence is associated with the up and down drafts. The appearance of precipitation on the ground is evidence that the thunderstorm cell is in its mature stage. The mature stage lasts for about 15 to 20 minutes, although some thunderstorms have been known to last as long as an hour. Lightning, microbursts, gust front wind shear, hail and tornadoes are all phenomena associated with a thunderstorm in its mature stage. Then the cell begins to dissipate. The cool precipitation tends to cool the lower region of the cloud and the cell loses its energy. The downdraft spreads throughout the whole area of the cell with the exception of a small portion at the top where updrafts still occur. The rainfall gradually ceases. The top of the cell spreads out into the familiar anvil structure.
Individual thunderstorms are usually no more than 10 nautical miles in diameter. However, they do tend to develop in clusters of two or more. Such clusters, with individual thunderstorms in various stages of development, may cover vast areas and last for many hours, travelling great distances across the country during their life cycle. There are two main types of thunderstorm: air mass thunderstorms and frontal thunderstorms. Air mass thunderstorms usually form either singly or in clusters on hot summer days in warm moist air. Being scattered, there is usually VFR weather around them and they are therefore easy to avoid. They form either as a result of convection or orographic lift.
Frontal thunderstorms are associated most commonly with an advancing cold front, but do develop in warm fronts as well. They usually form a line that may extend for hundreds of miles. They pose a special hazard to pilots because they are often embedded in other cloud decks and are therefore impossible to see. An advancing line of frontal thunderstorms should be avoided. Squall line thunderstorms rolling along in advance of a cold front are particularly violent.
THUNDERSTORM WEATHER
Thunderstorms produce very complex wind patterns in their vicinity. Wind shear can be found on all sides of a thunderstorm cell, in the downdraft directly beneath the cell and especially at the gust front that can precede the actual storm by 15 nautical miles or more.
In the cumulus stage of a thunderstorm's development, there’s an inflow of air (updraft) into the base of the cloud. As the thunderstorm matures, strong downdrafts develop and the cold air rushing down out of the cloud spreads out along the surface of the ground well in advance of the thunderstorm itself undercutting the warm air in such a manner as to resemble a cold front. This is the gust front. Turbulence within this lower level of spreading air is severe.
The severe downward rush of air and its outburst of damaging winds on or near the ground is commonly called a downburst. Downbursts can be classified as either macrobursts or microbursts. A macroburst is a large downburst with a diameter of 2 nautical miles or more when it reaches the earth’s surface and with damaging winds which last from 5 to 20 minutes. The most intense of these cause tornado like damage.
Smaller downbursts with a surface outflow diameter of less than 2 n. miles and peak winds that last less than 5 minutes are called microbursts. Wet microbursts occur in the presence of storm clouds with precipitation reaching the ground. Dry microbursts originate in moisture laden cumulus clouds. The downward flowing column of air contains precipitation (called virga) which evaporates before reaching the ground. The evaporation of the water appears to further cool the air, increasing the intensity of the microburst.
Downdrafts in microbursts can have vertical speeds as great as 6000 feet per minute. As they near the ground, these downdrafts spread out to become horizontal winds with speeds as high as 80 knots. They rapidly change direction and even reverse, causing very severe and dangerous wind shear. Because of the rapidly changing winds, both in speed and direction, an airplane encountering a microburst may lose so much lift that it cannot remain airborne. Depending upon the intensity of a thunderstorm, microbursts may occur anywhere up to 10 miles from the storm cell itself. Sometimes microbursts are concentrated into a line structure and, under these conditions, activity may continue for as long as an hour. Once microburst activity starts, multiple microbursts in the same general area are common.
Lightning flashes are the visible manifestation of the discharge of electricity produced in a thunderstorm. Areas of positive and negative charge accumulate in different parts of the cloud until the difference in electrical potential reaches a critical value and the air breaks down electrically.
In an active thunderstorm, the positive charge usually collects in the upper portion of the cloud and the negative charge in the cloud base. Flashing in a single thunderstorm cell may reach a peak of five discharges in a minute. Besides the activity within the individual cell, lightning may travel from one cloud to another or from the cloud to the ground. Occasionally, it will travel from the ground to the cloud. It means a negative charge has accumulated in the ground.
Thunder is the noise which accompanies a lightning flash. It is attributed to the vibration set up by the sudden heating and expansion of the air along the path of the lightning flash.
Hail may be regarded as one of the worst hazards of thunderstorm flying. It usually occurs during the mature stage of cells having updrafts of more than average intensity. The formation of hail has been covered earlier in this section.
Icing in a thunderstorm is encountered at or above the freezing level in the areas of heaviest turbulence during the mature stage of the storm. The altitudes within a few thousand feet of the freezing level, above or below, are especially dangerous.
The barometric pressure ahead of a thunderstorm falls abruptly as the storm approaches then rises quickly when the rain comes, and returns to normal when the storm subsides. Occasionally after a storm, the pressure falls below normal. Then rises to near normal again. All this can happen in a matter of 10 to 15 minutes.
