IVAO i-Pack

L7 - Advanced Information about Meteorology

25-11-2005

For basic instructions about METAR see L1-MetarSpeci

Content

1. The Atmosphere

2. Atmospheric Circulation

3. Moisture, Precipitation, and Stability

4. Clouds

5. Air mass

6. Fronts

7. High Altitude Weather

1. The Atmosphere

Almost all of the earth’s atmosphere (See Fig 1) is contained within 50 km (164,000 feet) of the surface. Likewise, 90% of the atmospheric mass exists below 16 km (53,000 feet). One of the best ways of classifying the atmosphere is according to its thermal characteristics.

The troposphere is a layer from the surface to an altitude that ranges from 24,000 to 50,000 feet. This average height in the middle latitudes is 36,000 to 37,000 feet. This average height tends to vary with the season and location over the earth; tends to be higher where it is warmer and lower where it is colder. This layer is characterized by a decrease in temperature with increasing altitudes. The top of the troposphere is called the tropopause (the interface between the troposphere and the stratosphere.), which is characterized by an abrupt change in temperature lapse rate and thus acts like a lid that confines most water vapor and the associated weather, to the troposphere.

The next layer is the stratosphere. One of the few weather phenomena that may extend into this layer is thunderstorms. The next two layers, which contain almost no atmospheric gases, are the mesosphere and thermosphere.

Fig 1 Different Regions of the Earths Atmosphere

2. Atmospheric Circulation

Atmospheric circulation patterns are a result of pressure differences. Wind flows outward from high pressure areas to low pressure areas. The primary cause of all changes in the earth’s weather is the variation of solar energy received by different areas on the earth. Solar heat is most concentrated where the sun’s rays strike the earth almost perpendicular to the surface. Since the earth’s axis is tilted 23 1/2°, the most intense areas change from the Tropic of Cancer on June 21 to the Tropic of Capricorn on Dec 21.

Since this unequal heating of the earth’s surface modifies the air density, different pressure readings are observed by Meteorologists. These pressure readings from weather reporting stations, are plotted on charts, usually in millibar or hectopascal, with connecting points of equal pressure with lines called isobars. When isobars are spread widely apart, the gradient is considered to be weak and when they are closely spaced, this represents a strong gradient.

Isobaric charts are used to identify pressure systems, which are classified as highs, lows, ridges, troughs, and cols (See Fig. 2). A high is a center of high pressure surrounded on all sides by lower pressure. A low is an area of low pressure surrounded by higher pressure. A ridge is an elongated area of high pressure and a trough is an elongated area of low pressure. A col (a region of slightly elevated pressure between two anticyclones) can be either a neutral axis between two highs or two lows, or the interaction of a ridge or trough. Low pressure is characterized by areas of rising air which can produce bad weather, while high pressure is areas of descending air which can produce good weather.

Fig. 2 Isobaric Chart with Wind Direction and Magnitude

Wind, is caused by airflow from cool, dense high pressure areas into warm, less dense, low pressure areas.  The speed of the wind is a result of the pressure gradient forces.  The stronger the gradient force, the stronger the wind.  As the earth rotates beneath this airflow, the Coriolis Force (an effect whereby a mass moving in a rotating system experiences a force perpendicular to the direction of motion and to the axis of rotation – named after the French engineer Gaspard Coriolis) counterbalances the pressure gradient force and deflects airflow to the right as it flows out of a high pressure area in the northern hemisphere.  This results in a clockwise circulation leaving a high pressure area and counterclockwise, or cyclonic, circulation leaving a low pressure area (See Fig 3).

Fig 3 Graphical Representation of the Coriolis Force Effect

For this reason, winds aloft tend to parallel isobars. However, due to the earth’s surface frictional affect, pressure gradient forces tend to cause surface winds to cross the isobars an angle causing them to shift when you descend within 2000 feet of the surface (AGL).

3. Moisture, Precipitation, and Stability

Water vapor is added to the atmosphere through a process of evaporation or sublimation. Evaporation occurs when heat is added to liquid water, which changes it to a gas. Sublimation is the changing of ice directly to water vapor, thus bypassing the liquid the liquid state. Water vapor is thus removed from the atmosphere by condensation and deposition. Condensation occurs when the air becomes saturated, and water vapor in the air becomes liquid. Deposition is when water vapor freezes directly to ice. Therefore, moisture is added to a volume of air by evaporation and sublimation.

The amount of water vapor the air can hold decreases with the air’s temperature. When the air is cooled to the dew point temperature, it contains all the moisture it can hold at that temperature, and it is defined as being saturated. The relative humidity increases as the temperature/dew point spread decreases. When the air is saturated, the relative humidity is 100 %. At 100 % humidity water vapor condenses, forming clouds, fog, or dew. One can anticipate the formation of fog or very low clouds when the temperature/dew point spread is decreasing below 2 °C or 4 °F. Likewise, frost forms when the temperature of the collecting surface is below the dew point and the dew point is below freezing.

Precipitation is caused by condensed water droplets that grow to a size where the atmosphere can no longer support their weight, these water droplets fall as participation. Water droplets that fall and remain liquid are in the form of rain or drizzle. With low relative humidity, rain may evaporate before it reaches the ground. When this occurs, it is called virga. Virga is best described as streamers of participation trailing beneath clouds which evaporate before reaching the ground. Another form of precipitation is ice. When water droplets remain liquid, even though they are cooled below freezing, and strike an airplane in flight or the earth’s surface, they immediately turn to ice or freezing rain. In contrasts, ice pellets, freeze as they fall through cold air and are more likely to bounce off an aircraft rather than freezing to it. The presents of ice pellets normally indicates freezing rain at higher temperatures. Snow forms through the process of deposition. If the temperature of the air remains below freezing, the snow falls to the ground as snow; otherwise, it melts and turns to rain. The presence of wet snow indicates the temperature is above freezing at your flight altitude.

Stability is the atmosphere’s resistance to vertical motion. The stability of a volume of air determines whether it rises or sinks in relation to the air around it. Stable air resists vertical movement, where as unstable air has a tendency to rise. The combined effects of temperature and moisture determine the stability of the air and, to a large extent, the type of weather produced. The greatest instability occurs when the air is both warm and wet. Air that is both cool and dry resists vertical movement and is very stable.

The dry adiabatic lapse rate (DALR) is 3 °C (5.4 °F) per 1000 feet that a volume of unsaturated air is lifted. Thus when unsaturated air is forced to ascend a mountain slope, it cools at this rate. When condensation occurs in a volume of rising air, the adiabatic cooling is partially offset by warming due to the release of latent heat. A saturated volume of air continues to cool as it rises, but at a slower rate than if it is dry. The rate of cooling of a rising, saturated volume of air is called the moist or saturated adiabatic lapse rate (SALR). Although DALR is a constant of 3 °C per 1000 feet, SALR is variable. Air is unstable when the adiabatic lapse rate is less than the ambient air lapse rate (See Fig 4). The standard temperature at sea level is 15 °C (59 °F) and decreases at an average rate of 2 °C (3.5 °F) per 1000 feet as a rule. Therefore, one could project the temperature at 10000 feet is -5 °C (15 °C – (2 °C per 1000 feet x 10000 feet = - 20 °C) = -5 °C). In conclusion, the ambient lapse rate allows you to determine atmospheric stability.

Fig. 4 Temperature versus Height Representation of SALR & DALR

The condensation level is the level at which the temperature and dew point converge, and a cloud forms in rising air. To estimate the bases of cumulus clouds, in thousands of feet, divide the temperature/dew point spread at the surface by 2.5 °C (4.4 °F), or for an approximate method use 4 °F (2.2 °C).
By knowing the stability of the air mass, one can predict its characteristics. Stable air is associated with stratus clouds, poorer visibility, and a lack of turbulence. Unstable air supports cumulus clouds, good visibility outside of the clouds, and generally more extreme weather such as icing, heavy rain, hail, and turbulence.

4. Clouds

As air cools to its saturation point, condensation changes invisible water vapor to a visible state. The most commonly visible state is in the form of clouds or fog. Clouds are composed of very small water droplets, or the temperature is low enough, ice crystals.

The four families of clouds are high, middle, low and those with extensive vertical development. Cloud names are based on the terms, cumulus (heap), status (layer), nimbus (rain), and cirrus (ringlet). The prefixes alto and cirro denote cumulus and stratus clouds from the middle and high families, respectively. The prefixes nimbo and the suffix nimbus denote clouds that promote rain.

Low clouds extend from the near surface to about 6,500 AGL. Low clouds usually consist of almost entirely of water, but sometimes may contain supercooled water which can create icing hazard for aircraft. Types of low clouds include stratus, stratocumulus, and nimbostratus.

Nimbostratus

Middle clouds have bases that range from about 6,500 to 20,000 feet AGL. The can contain water, ice crystals, or supercooled water, and may contain moderate turbulence and potentially severe icing. Altostratus and altocumulus are classified as middle clouds.

Altocumulus

High clouds have bases beginning above 20,000 feet AGL. They are general white to light gray in color and form in stable air. They are composed mainly of ice crystals and seldom pose a serious turbulence or icing hazard. The three basis types of high clouds are called cirrus, cirrostratus, and cirrocumulus.

Cirrus

Clouds with extensive vertical development are present when lifting and instability are present. Cumulus clouds may build vertically into towering cumulus or cumulonimbus clouds. The bases are typically 1,000 to 10,000 feet MSL and their tops sometimes exceed 60,000 feet MSL. Towering cumulus clouds indicate a fairly deep layer of unstable air and contain moderate to heavy convective turbulence with icing. Often these develop into thunderstorms. Cumulonimbus, or thunderstorms, are large vertically developed clouds that form in moist, unstable air. They contain large amounts of moisture, turbulence, icing, and lightening. The primary aviation hazards from thunderstorms are not from the lightening, but from icing and turbulence.

Cumulonimbus

5. Air mass

An air mass is a large body of air (See Fig. 5) with fairly uniform temperature and moisture content. When a cold air mass moves over, or is heated by a warm surface, the result is cumuliform clouds, turbulence, and good visibility. When the air is moist and unstable, the updrafts are particularly strong, resulting in cumulonimbus clouds. Cooling from below increases the stability of an air mass and warming from below decreases it.

Figure 5 Air mass Movements from Different Areas

6. Fronts

When an air mass moves out of its source region and comes in contact with other air masses that have different moisture and temperature, the boundary between these two is called a front. Fronts often contain hazardous weather. A change in the wind direction is always associated with the passage of a frontal system.

A cold front separates an advancing mass of cold, dense, and stable air from an area of warm, lighter, and unstable air (See Fig. 6). Because of its greater density, the cold air moves along the surface and forces the less dense, warm air upward.

Fig 6 Advancing Cold Front

Fast moving cold fronts are pushed by intense high pressure systems located well behind the front. These types of fronts rapidly force warmer air to rise, which can cause widespread vertical cloud development along a narrow frontal zone. If sufficient moisture is present, an area of severe weather forms well ahead of the front.

Slow moving cold fronts produce clouds far behind the surface front. When this type of front comes in contact with stable air; a broad area of stratus clouds form behind it.

7. High Altitude Weather

The tropopause, which is the boundary between the troposphere and the stratosphere, is between 24,000 feet MSL near the poles, and 50,000 feet MSL near the equator. For the ISA conditions, the height is defined as 36,000 feet MSL. From this altitude up to 66,000 feet, the temperature remains constant at -57 °C.

Jet streams are often embedded in the zone of strong westerly winds that break into to the tropopause. A jet stream is a narrow band of high speed winds that reaches its greatest speed near the tropopause. Typically jet stream speeds range between 50 knots to 240 knots. Jet streams normally are several thousand miles long, several hundred miles wide, and a few miles thick (See Fig. 7). The strength and location of the jet stream is normally weaker and farther north in the summer. During the winter months in the higher latitudes, the jet stream shifts toward the south and increases in speed.

Fig 7 Graphical Representation of a Jet Stream

Fig 8 Prediction of Jet Stream Turbulence Areas

Although jet streams can provide beneficial winds when flying west to east, they can be associated with strong turbulence (See Fig 8). Proper flight planning using jet streams can reduce flight times and decrease fuel consumption.

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