The Invisible Killer

There is a weather phenomenon known to pilots which has caused the loss of many lives. This weather phenomenon is called a microburst and it has been compared to tornadoes and hurricanes. Microbursts can significantly affect people or property on the ground, but it is considered to be more dangerous to pilots. The reason microbursts are said to be “invisible,” is because they are difficult to detect.

Microbursts are a form of wind shear. Wind shear is a sudden and drastic change in wind direction, speed, or both within a small area. Wind shear can be divided into two components: vertical wind shear, wind shear measured in the vertical direction and horizontal wind shear, wind shear measured in the horizontal direction. The effects of a microburst are more dangerous when encountered near the surface. When wind shear occurs at lower levels in the atmosphere near the surface it is known as Low-Level Wind Shear (LLWS).

LLWS can form in several ways, but most of the time thunderstorms are the cause of the most severe type of wind shear, if under the right conditions. Certainly changes in vertical motions constantly happen through the life cycle of a thunderstorm. During the dissipating stage, the final stage of the thunderstorm cycle, the thunderstorm dissipates due to the overcoming downdrafts (downward moving–negatively buoyant air) that can reach approximately 6000 feet per minute.

Thunderstorms create the anvil–like shapes during the dissipating stage. The anvil–like shape is due to the weakening of vertical motions near the top of the storm and downdrafts overcoming the updrafts and dissipating the cloud from the bottom up. Downdrafts are formed by hydrometeor drag and evaporative cooling.

Precipitation (water and hail) drags air downward contributing to the generating of the downdraft is known as hydrometeor drag.  However, the biggest contributor to the downdraft formation within a thunderstorm is evaporative cooling.

Evaporative cooling is evaporation of rain and cloud drops or sublimation of ice crystals, making the air parcels colder than their environment and thus negatively buoyant. As cloud droplets becomes heavy enough to overcome the updraft and fall, they travels into a warmer environment, evaporating (changing state from liquid to gas) or sublimating (changing state from solid to gas) by taking heat away from its surrounding and cooling the air around. Entrainment, mixing in of drier air occurring along cloud edges, is also known to enhance the downdraft strength because it leads to evaporative cooling.

A lot of the mixing of drier air that occurs aloft is due in part by the jet stream at higher altitudes carrying dry air and mixing it with the thunderstorm. This increases the evaporative cooling and strengthens the downdraft and acting as a rear inflow jet by carrying the cold, dense and negatively–buoyant air to the ground.

When the downdraft occurs at the bottom of the thunderstorms, reaches the ground and spreads out, it is called a downburst. Downbursts can be subdivided into two: macrobursts and microbursts. A macroburst is a downburst in which a region of more than 4 kilometers (km) is affected; consequently, a microburst is a downburst affecting a region of 4 km or less.

Macrobursts can last up to 30 minutes with wind speeds of up to 116 knots. Microbursts however, are difficult to detect because they occur over a very small area. Some microbursts have been known to occur in a space of less than one mile horizontally and within 1000 feet vertically. The lifespan of a typical microburst is of approximately 15 minutes and although they are smaller than macrobusts, their wind speeds can be much higher, as high as 145 knots, the same as category 5 hurricane winds.

Dry microbursts also exist. They are harder to identify and are more commonly found in western U.S. in areas where there are dry conditions at lower altitudes and an increase in moisture with altitude. Dry microbursts often occur with less than 0.01 inch of precipitation as opposed to wet microbursts of 0.01 inch or more of precipitation. Virga, rain that evaporates before reaching the ground, is associated with dry microbursts and is present most of the time. Dry microbursts are as dangerous as wet microbursts.

But is this “killer” so “invisible”?—to an extent. Even though there are common systems already being used such as the LLWS Alert System and the Doppler Radar, there are other much simpler ways for pilots to detect and warn themselves of possible LLWS and/or microbursts as a last resort.

Taking a closer look at microbursts, there are some features that can be used to identify them. These features include some of the types of clouds generated by a thunderstorm as well as the use of a sounding.

A shelf cloud or arcus cloud is a cloud that forms as a result of the warm air lifted by the gust front of a thunderstorm and can be an indication of a microburst in the region. The downdrafts within a thunderstorm that reaches the surface as downbursts (microburst) transports the cold, dense, negatively–buoyant air and high wind speeds from higher altitudes to the surface from the rear inflow jet. Areas, sometimes called “pools” of cold air at the surface are known as the thunderstorm’s outflow. The storm’s outflow can be identified as small scale cold front, known as a gust front, decreasing temperatures at the surface and lifting warmer air.

Behaving as small scale cold fronts, under the right conditions, gust fronts can start the formation of other thunderstorms. Pilots inadvertently flying in thunderstorms can avoid entering a downburst situation if they spot a shelf cloud indicating the existence of a downburst.

Another indication of possible downbursts is the peculiarly–shaped Mammatus clouds. Mammatus are basically "blobs" of cooled air that sink under the anvil as a result of strong evaporational cooling by the interaction with dry air below the anvil. If flying into a thunderstorm inadvertently, pilots can predict the possibility of microburst situation through observing Mammatus clouds as an indication of strong evaporational cooling. However, this is still highly improbable since they form in a very turbulent environment. 

Soundings can also be used not only to predict the presence and strength of thunderstorms and thus the possibility microbursts, but also the type of microburst. “Inverted–V” soundings indicate dry microbursts are possible. This type of sounding indicates dry air at lower levels and moist air at higher levels, the opposite of the conditions expected with wet microbursts.

Quote:

“Success is not measured by what you accomplish, but by the opposition you have encountered, and the courage with which you have maintained the struggle against overwhelming odds.” Orison Swett Marden

The Ingredients

For endless reasons, weather has always been a major issue in the aviation industry. It's hard to predict, it's hard to understand, it's continuously underestimated, and it constantly poses a threat to anybody. Whether you are on the ground or in the air, weather will be there and it will affect you. According to the National Weather Service (NWS), weather has been responsible for an average 543 deaths in the United States each year for the past 10 years; about half are aviation related.

Normally, teachings in flight schools about weather, go only as far as the basics. Pilots learn how to interpret charts, read the undecoded, and understand the theory to some extent. What many pilots don’t learn is the details of how the weather phenomenon forms and comes to be as it is. Knowing what specific factors or “ingredients” are needed for the formation of an affecting weather phenomenon, will not only help pilots have a greater understanding of the theory, but also to have a better overall picture and know what to expect.

Thunderstorms often present some of the most hazardous conditions. They are responsible for damaging hail, heavy rain, icing, tornadoes, lightning, and turbulence in many forms, including downbursts. Normally, there a 2 categories of thunderstorms: Air-Mass Thunderstorms and Severe Thunderstorms.

Air-mass thunderstorms are isolated thunderstorm cells showing little organization that form in a maritime tropical air mass (warm and moist air mass). They are the most common type of thunderstorm and usually form as a result of daytime surface heating in areas of very little to no vertical wind shear. Vertical wind shear is the change in speed and direction of wind with increasing altitude.

Severe thunderstorms are the strongest type of thunderstorm and form in areas of strong vertical wind shear. These are most commonly found along or ahead of cold fronts and often show more organization. Unlike air-mass thunderstorms, severe thunderstorms are longer lasting, sometimes forming as a result of contrasting air masses, can spawn tornadoes under the right conditions, and present more hazardous conditions to both aircrafts and people on the ground.

Seven various ingredients are needed in creating a severe thunderstorm; these are:

1.   Wind shear

The foundation of a severe thunderstorm is dependent upon wind shear. With vertical wind shear, wind speed increases abruptly and changes direction with height; this gives a severe thunderstorm a tilt that helps to fuel the storm thus increasing its life span. The tilt creates updraft (rising air) on a section of the thunderstorm and downdrafts (sinking air) on sections where precipitation occurs. The updrafts and downdrafts side-by-side work together to increase the life span, generate hail and sometimes create tornadoes. Tornadoes occur when horizontal and vertical wind shears near the surface work together to spin and tilt columns of air.

Air-mass thunderstorms, on the other hand, form with little to no wind shear and therefore have a tower-like shape with no tilt and no chances of tornadoes. Without wind shear, a thunderstorms would be classified as air-mass thunderstorms.

2.   Contrasting air masses

Tornado Alley is an area of the United States known for its amount of tornadoes each year. Severe thunderstorms in that area are very common. One of the reasons why severe thunderstorms occur at this location is due to 2 contrasting air masses that interact in that very region causing strong fronts. Strong fronts are ideal lifting mechanisms for the formation of powerful long-lasting thunderstorms. Due to the differences in moisture levels, dry lines form causing the most hostile conditions.

3.   Low level moisture

In Tornado Alley, warm, moist air coming from the Gulf of Mexico to the south, lower in the atmosphere, brings the high levels of moisture. Moisture is needed for the formation of storm clouds since it is one of the things that fuels thunderstorms. The higher the moisture levels, the more powerful a thunderstorm will become.

4.   Cold, dry air above

Cold, dry air from the northwest also comes to Tornado Alley. Cold air is essential for strengthening a thunderstorm. The colder the air above than the air below, the more unstable the conditions become due to a higher decrease in temperature with altitude and strength of the cold front’s lifting mechanism.

5.   Low level jet

Low level jets are also needed for the formation of wind shear. A low level jet is a rapidly moving stream of air at lower levels in the atmosphere. In Tornado Alley the low level jet brings the warm, humid air from the gulf that helps fuel the storm.

6.   Upper level jet

An upper level jet bringing the cold, dry air from the west at high speeds is also necessary. The jet stream is a continuous rapidly moving stream of air. Pressure differentials at the surface sometimes accelerate a section of the jet stream, called jet streaks; as a result, jet streaks curve and speed up. This is a sign of strong upper level divergence and low level convergence in the area of low pressure at the surface. This means more unstable conditions and increase updraft strength to form severe thunderstorms.

7.   Upper level trough

A trough is an elongated area of low pressure. When there is an upper level trough, strong upper level divergence exists upstream (east side in the northern hemisphere) of the trough, a factor that contributes to strengthening the lifting mechanisms of the surface low pressure center and the updraft strength in a severe thunderstorms. Usually, one can expect stormy weather upstream of an upper level trough. 

Now that you know the basics of the ingredients, I encourage you to do a little more research. There are actually 3 ways in which severe thunderstorms organize themselves: Squall Lines, Multicell Thunderstorm Complexes, and Supercell Thunderstorms. The most common severe thunderstorm is the Squall Line and the rarest is the Supercell Thunderstorm (most powerful).




Quote:

"The more man meditates upon good thoughts, the better will be his world and the world at large." Confucious

A Line Separating Life from Death

Armstrong’s line is named after Harry George Armstrong, a man who made significant contributions in the field of aviation medicine. At around 63,000 feet or 12 miles high, Armstrong’s line is a point in our atmosphere where water changes from the liquid state to the gaseous state at human body temperature (98.6˚F). Upon reaching such an altitude, things like tears, sweat, saliva, and any other water content in the body will boil causing a slow and painful death—including blood?

The reason water boils at this altitude is mainly due to atmospheric pressure. At higher altitudes there is lower pressure than at lower altitudes. Being in a lower pressure environment (higher in the atmosphere), less vapor pressure is needed to boil water; therefore a lower temperature is needed to boil water.

In case you are wondering, according to americasblood.org, plasma accounts for 55% of the blood in a human body and 90% of plasma is made up of water. So concluding that water makes up about 50% of a person's blood, it is true that the water content in the blood will boil. In addition to boiling, gas bubbles inside the bloodstream can at some point block blood flow in an artery causing the person to die of a heart attack.

Not to worry, because thanks to Mr. Armstrong, fighter pilots today have pressurized suits keeping them alive at such high altitudes. In addition, pilots continuously breathe on oxygen masks to compensate for the decrease in oxygen partial pressure. Without oxygen masks at 60,000 feet, exposed pilots would become incapacitated due to hypoxia in a matter of seconds.

There are some humans who have been close to 12 miles high without pressurized suits; one example is passengers in a Concorde.

Quote:

“As a day well spent brings happy sleep, so life well used brings happy death.” Leonardo DaVinci