Tornadoes, Cyclones, Air Masses & PAGASA

• Tornadoes and tropical cyclones are among nature's most destructive forces
• On average, 20 tropical cyclones enter the Philippine Area of Responsibility (PAR) per year
• About 8 or 9 of those that enter the PAR cross the Philippines
• The peak of the typhoon season is from July through October
• Nearly 70% of all typhoons develop during the peak season
• PAGASA closely monitors typhoon events

Air Masses

• An air mass is a large body of air with relatively similar temperature and humidity properties in any horizontal direction at a given altitude
• A single air mass may cover more than a million square kilometers
• Regions where air masses originate are known as source regions
• For a huge mass of air to develop uniform characteristics, the source region should be generally flat and of uniform composition with light surface winds
• The longer the air remains stagnant over its source region, the more likely it will acquire properties of the surface below
• Air masses are classified according to their source region
• Polar (P) air masses originate in high latitudes and are composed of cool or cold air
• Tropical (T) air masses originate in low latitudes and are warm
• If its source region is over land, an air mass tends to be dry and is called continental (c)
• If an air mass forms over the ocean, it is more humid and is classified as maritime (m)
• Four basic types of air masses emerge: continental polar (cP), continental tropical (CT), maritime polar (mP), and maritime tropical (mT)

Continental Air Masses

• Hot, dry air masses are continental tropical (cT) air masses
• cT air masses occur in the Desert Southwest of North America, the Sahara and Arabian Deserts, Tibet, and central Australia
• In areas under the influence of cT air masses, conditions can be oppressively hot and dry for many months during the year
• Arctic or Antarctic (A) air masses are formed near the North and South Poles
• They are extremely cold and very dry because the water-vapor capacity is low in such cold air
• Cold, dense air has a tendency to sink and therefore, these air masses reinforce the region of polar high pressure (polar high) that resides over each pole
• Surface winds near the pole move east-to-west (easterlies), so the surface A air masses tend to be steered westward as they drift away from the poles
• Continental polar (cP) air masses are located farther away from the poles than A air masses
• They are cold and dry, but less so than A air masses
• cP air masses form over cold, inland areas, like northern Canada, Siberia, Mongolia, and north-central Europe
• The southern edge of cP air masses is the polar front
• In the winter, cP air masses can move significant distances equatorward, bringing cold air into the U.S. and southern Eurasia

Maritime Air Masses

• Due to their proximity to the equator, equatorial (E) air masses are very warm and moist
• The high temperature and moisture support instability to great heights in the atmosphere, resulting in tropical clouds and storms
• Storm development is aided by the general circulation of the atmosphere which in equatorial regions promotes rising of the air, especially along the ITCZ (Intertropical Convergence Zone)
• Maritime tropical (mT) air masses form over warm, oceanic regions, and are warm and moist
• mT air masses have influence over much of the Earth, particularly in the low latitudes
• mT air tends to make most of the Atlantic Ocean and adjacent eastern U.S. warm and humid in summer and are commonly near the subtropics
• Maritime polar (mP) air masses are cool and humid
• They are not as cold as cP air masses because oceanic regions are not as cold as continental areas at the same latitude, particularly in winter
• mP air masses are a cold, damp influence in places like Seattle and most of western Europe, as the westerlies push the cool, damp air eastward from their oceanic source region

Fronts

• The narrow zone or the transition zone between two different air masses (different densities) is called a front
• This is often the site of rising atmospheric motion
• Whenever different air masses meet along a front, the less dense, the warmer air mass is pushed up over the denser, colder one
• If the rising air cools to its dew-point temperature, cloud formation will begin, perhaps followed by precipitation
• There are several types of fronts, depending on how one air mass displaces the other and whether the front is moving
• Fronts are associated with many of the storms most regions experience
• Blue lines represent cold fronts, the leading edges of cold air masses
• Blue triangles along a cold front indicate the direction that the cold air mass is pushing
• Red lines represent warm fronts, the leading edges of warm air masses
• Red semicircles along a warm front indicate the direction in which the warm air mass is moving
• Stationary Fronts are Shown With Blue Triangular Points on One Side of the Front and Red Semicircles on the Other

Warm Fronts

• When the surface position of a front moves, warm air occupies territory formerly covered by cooler air
• This is called a warm front
• The less-dense, warm air has a hard time displacing denser cold air because the friction with the ground slows as cold air retreats
• For this reason, the boundary separating these air masses acquires a very gradual slope
• As warm air ascends the retreating wedge of cold air, it expands and cools adiabatically to produce clouds and, frequently, precipitation
• A gradual increase in temperature occurs with the passage of a warm front which is most noticeable when there is a large temperature difference between the adjacent air masses
• The moisture content and stability of the encroaching warm air mass largely determine when clear skies will return
• During the summer, cumulus and occasionally cumulonimbus clouds may be embedded in the warm, unstable air mass that follows the front
• Precipitation from these clouds can be heavy but is usually scattered and of short duration

Cold Fronts

• A boundary is called a cold front when dense cold air is actively advancing into a region occupied by warmer air
• As with warm fronts, friction tends to slow the surface position of a cold front more so than its position aloft
• Because of the adjacent air masses' relative positions, the cold front advance at speeds and steepens as it moves
• These two differences—the rate of movement and steepness of slope—largely account for the more violent nature of cold-front weather than the weather generally accompanying a warm front
• As a cold front approaches, generally from the west or northwest, towering clouds can often be seen in the distance
• Near the front, a dark band of ominous clouds foretells the ensuing weather
• The forceful lifting of air along a cold front is often so rapid that the latent heat released when water vapor condenses appreciably increases the air's buoyancy
• Heavy downpours and vigorous wind gusts associated with mature cumulonimbus clouds frequently result
• No matter which air mass is advancing, it is always the warmer (less dense) air that is forced aloft, whereas the cooler (denser) air acts as the wedge on which lifting takes place
• When air is forced aloft, adiabatic cooling and cloud formation are to be expected

Occluded Fronts and Stationary Front

• Occluded Front: If a cold front catches up to and overtakes a warm front, the frontal boundary between the two air masses
• There are two main types of occlusions, cold-type occlusions, and warm-type occlusions with the warm-type occlusion is by far the most commonly observed
• The primary difference between the warm- and cold- type occluded fronts is the upper-level front location
• In a cold-type occlusion, there is a warm upper-level front that follows the occluded surface front
• In a warm occlusion, there is an upper-level cold front that precedes the upper-level cold front precedes the surface occluded front
• Stationary Front: Condition called a stationary front occurs, when the flow on both sides of a front is neither toward the cold air mass nor toward the warm air mass but almost parallel to the line of the front, thus the surface position of the front does not move
• On a weather map, stationary fronts are shown with blue triangular points on one side of the front and red semicircles on the other
• At times, some overrunning occurs along a stationary front, most likely causing gentle to moderate precipitation

Tropical Cyclone

• Tropical cyclones, hurricanes, and typhoons are characterized by low atmospheric pressure (form over tropical oceans), intense convective (thunderstorm) activity, strong cyclonic circulation, large areas of strongly rotating winds, and locally elevated sea levels near the storm

Characteristics of a Tropical Cyclone

• Tropical cyclones are huge, circulating masses of clouds and warm, moist air
• They are zones of low atmospheric pressure that cause air to rise and condense, creating locally intense rainfall
• Warm tropical ocean waters fuel tropical cyclone formation where water vapor evaporated from the sea surface mixes with the air, rises, cools, and produces clouds, releasing energy during condensation, which warms the air around it
• This energy further enhances the upward motion, drawing more surface moisture to replace the rising air: In this way, tropical cyclones, once formed, provide their fuel (energy)
• If the tropical cyclone encounters additional warm water in its path, extra energy is input to the storm, winds increase, and the storm grows in strength and size
• A tropical cyclone dissipates when it passes over land (tropical cyclone reaches land is called landfall) or cold water, or when it mixes with much drier air
• Differences in atmospheric pressure between the surrounding environment and the tropical cyclone center cause sea level to bow up by several centimeters
• The strongest winds and most severe thunderstorms are usually found within the eyewall, which is the area immediately surrounding the eye
• The relatively clear area at the center is the eye
• Within the eye, dry air flows down the center of the storm where it compresses and evaporates clouds, forming a cylinder of relatively clear, calm air

Tropical Cyclone Formation and Dissipation

• The Right Environment: Tropical cyclones form over tropical waters where the winds are light, and the humidity is high in a deep layer extending up through the troposphere
• The surface water temperature is warm, typically 26.5°C or greater, over a vast area
• Tropical cyclones do not form spontaneously—they require a trigger” to start the air converging
• Surface winds converge along the intertropical convergence zone (ITCZ) which occasionally, when waveforms along the ITCZ, an area of low pressure develops, convection becomes organized, and the system grows into a tropical cyclone
• The Developing Storm: A tropical cyclone's energy comes from the direct transfer of sensible heat and latent heat from the warm ocean surface
• A cluster of thunderstorms must become organized around a central area of surface low pressure to create a tropical cyclone
• Heat is taken near the warm ocean surface, converted to kinetic energy (energy of motion or wind), and lost at its top through radiational cooling
• The maximum strength a tropical cyclones can achieve is proportional to the air temperature difference between the tropopause and the surface and the potential for evaporation from the sea surface
• As a consequence, the warmer the ocean surface, the lower the minimum pressure of the storm, and the higher its winds
• The Storm Dies Out: If a tropical cyclone remains over warm water, it may survive for a long time
• Tropical cyclones weaken rapidly when they travel over colder water and lose their heat source
• Tropical cyclones also dissipate rapidly when they move over a large landmass, where they not only lose their energy source, but friction with the land surface causes surface winds to decrease and blow more directly into the storm, an effect that causes the tropical cyclone's central pressure to rise
• A tropical cyclones will rapidly dissipate should it move into a region of strong vertical wind shear
• In summary, the development of a tropical cyclone occurs when there is the proper combination of circulation, divergence, and convergence, which is maintained over a considerable period on an accurate scale

Conditions that Strengthen or Weaken a Tropical Cyclone

• Changes in Water Temperature*
• If the cyclone encounters even warmer sea temperatures, it may strengthen through added heat and additional moisture
• If the cyclone encounters cooler water, it loses thermal energy to the water and the cooler water will not contribute much moisture, reducing intensity
• Encounters with Other Air Masses*
• If a cyclone advances into, or incorporates, a wetter air mass, the additional moisture can strengthen the cyclone
• More commonly, a cyclone advances into, or incorporates, drier air. Cloud development ceases and thins, diminishing latent heat from the storm
• Encounters with Land*
• Once it travels to land, the cyclone no longer has an unlimited supply of empowering moisture. The land can be dry and cold, which will remove moisture and energy from the storm
• Upper-Level Wind and Pressure*
• Vertical wind shear is the difference in wind speed or direction between higher and lower levels in the atmosphere. it generally tears apart or dissipates tropical cyclones
• Upper-level pressures can act to strengthen or weaken a tropical cyclone

Classification of Tropical Cyclones and Average Frequencies

• The classification of tropical cyclones is according to the strength of the winds

Thunderstorm

• A thunderstorm is a storm that generates lightning and thunder
• Thunderstorms frequently produce gusty winds, heavy rain, and hail
• A thunderstorm may be produced by a single cumulonimbus cloud that influences only a small area, or it may be associated with cumulonimbus clouds covering a large area
• Thunderstorms form when warm, humid air rises in an unstable environment
• Approximately 2000 thunderstorms are in progress on Earth at any given time, with the most significant number occurring in the tropics
• Mechanisms that produce thunderstorms include the unequal heating of Earth's surface and the lifting of warm air

Single-Cell Thunderstorms

• The first stage is known as the cumulus stage, or growth stage
• It occurs when the surface heats more rapidly than the atmosphere above it
• This causes the heated near-surface air to rise relative to adjacent air, forming an updraft
• A parcel of warm, humid air rises, cools, and condenses into a single cumulus cloud
• Usually, such conditions begin to appear in the mid-to-late morning, particularly in the summertime
• No rain is falling during this stage—the cloud is still growing, as indicated by the puffy, cauliflower-shaped top
• Updrafts keep water droplets and ice crystals suspended within the cloud, colliding particles with one another to grow larger and heavier
• The beginning of rainfall signals the onset of the next stage - the mature stage
• At this stage, the updrafts that allowed the cloud to grow in the cumulus stage are accompanied by downdrafts induced by falling precipitation with most lightning occurs during this stage
• During its mature stage, the thunderstorm is most intense, reaching a stable region of the atmosphere (as high as the stratosphere)
• The top of the cloud begins to take on the familiar anvil shape as upper-level winds spread the cloud's ice crystals horizontally
• the dissipating stage occurs after the storm enters the mature stage, and it begins to dissipate in about 15 to 30 minutes
• Downdrafts tend to dominate throughout the cloud until the moisture rained out of the cloud. At this point, the cloud begins to disappear
• The entire life cycle of an individual single-cell thunderstorm is on the order of 1 to 2 hours
• Single-cell thunderstorms are far more likely to occur in summer and more likely to occur between late morning and late afternoon

Cloud Mergers

• A single-cell thunderstorm can form or grow when two cumuliform clouds are forced together by winds
• Adjacent fair-weather cumuliform clouds are supported by uplift associated with an unstable atmosphere, but no cloud has enough moisture and uplift to be dangerous by itself
• The instability and associated uplift beneath the collision point dictate that both clouds' moisture must move upward, which causes very rapid vertical development

Multicell Thunderstorms

• Thunderstorms can also occur in clusters, called multicell thunderstorms, containing several single-cell thunderstorms, each in a different stage of development
• Wind speed increases rapidly with height, producing strong wind-speed shear
• This type of shearing causes the cell to tilt so that the updraft rides up and over the downdraft with this rising updraft is capable of generating new cells
• Precipitation inside the storm does not fall into the updraft (as it does in the single-cell thunderstorm), so the storm's fuel supply is maintained, and the storm complex can survive for a long time

Supercell Thunderstorms

• An intense, long-lasting thunderstorm with a single violently rotating updraft is called a supercell thunderstorm
• Supercell thunderstorms commonly have a flat-topped, anvil shape where the long point of the anvil generally indicates the direction the storm moves, and indicates the presence of vertical wind shear
• The rotating vortex is interpreted to have started horizontally, where it was formed by horizontal shearing of the wind against the surface
• Subsequent updrafts can spiral into the mesocyclone, strengthening it by tightening its rotation and ultimately leading to a tornado's formation
• About one-third of mesocyclones form tornadoes
• In a supercell case, the system is so strong that much of the available latent energy released in condensation and deposition is consumed in large and long-lasting supercells

Tornado

• A tornado is a naturally occurring, rapidly spinning vortex of air and debris that extends from the base of a cumulonimbus cloud to the ground
• Tornadoes typically have diameters of tens to hundreds of meters
• Most individual tornadoes remain on the ground for minutes to tens of minutes and have paths on the ground of less than a few kilometers
• Pressures within some tornadoes have been estimated to be as much as 10 percent lower than immediately outside the storm

Tornado Formation and Cycling

• Tornado formation generally begins with horizontal winds moving across the surface
• Wind speeds are usually higher aloft and decrease downward due to friction with the surface, therefore wind shear can result in rotating, subhorizontal vortex
• In an unstable atmosphere, rising air (an updraft) can perturb the rotating tube of turbulent air, eventually, a segment of mechanical turbulence may become vertical, at which point it merges with the original and unstable updraft
• Once it spans the distance from the cloud to the ground, it is a tornado, however, If a funnel-shaped cloud does not reach the ground, it is a funnel cloud

Essential Aspects to Form a Tornado

The main ingredient is a well-developed cumulonimbus storm cloud, especially one associated with a supercell thunderstorm Another necessary element is wind shear near the surface, caused by higher-speed winds aloft and slower winds near the sur The formation of a tornado and its host thunderstorm requires an unstable atmosphere A storm must have strong updrafts to lift the horizontally spinning tube of air and the host storm must also possess strong internal rotation

Tornado Life Cycle

Major tornadoes usually evolve through a series of stages:

• dust-whirl stage
• organizing stage
• mature stage
• shrinking stage
• decay stage