Can we modify hurricanes?

Posted on September 9, 2019 by WeatherGuys Editor

There have been a few theories for how humans might be able to dissipate a hurricane, though they all have downsides. (Photo credit: NOAA)

There have been several suggestions on how humans might modify the intensity or path of a hurricane.

One method suggested was to change the “energy budget” of the environment around the storm. It was suggested that this could be done by dispersing, from aircraft, carbon soot. That soot would absorb solar energy and warm the atmosphere, which would enhance the evaporation of ocean water and promote the formation of thunderstorms.

Those thunderstorms, if they formed in the correct place, could weaken a cyclone’s eye wall. You can imagine why that is not a favorable approach.

For one, the sediment, which would fall out of the atmosphere and onto the ocean quickly, would be messy.

Another is the cost of sending planes out to do this and the amount of carbon needed to affect the environment. How would we be assured that the thunderstorms formed in the correct place? Moreover, even if this was practical, it would only work during the day.

Another approach, first suggested in the late 1950s, was the use of nuclear bombs.

Hurricanes get their energy from evaporation of warm ocean water. Exploding a nuclear bomb in the ocean would bring cooler water to the surface, reducing the fuel for the hurricane. If dropped in the eye of the hurricane, it was theorized that a nuclear explosion could push warm air up and out of the storm’s eye, cooling the storm center and causing it to dissipate.

While nuclear bombs are terribly destructive, the energy in a nuclear bomb pales in comparison to nature’s tropical storms. The heat released by a hurricane is equivalent to a 10-megaton nuclear bomb exploding every 20 minutes. So, nuking a hurricane very likely wouldn’t do much to diminish it, although it might make it radioactive — another downside.

Other methods proposed include using mechanical pumps to bring cooler ocean water to the surface and seeding the storm with silver iodide to accelerate cloud formation and dry out the storm precipitation.

Rather than trying to modify the path or intensity, it is probably wiser to be prepared for its arrival.

Category: Meteorology, Severe Weather, Tropical

Comments Off on Can we modify hurricanes?

What is fulgurite?

Posted on August 26, 2019 by WeatherGuys Editor

Lightning is an atmospheric discharge of electricity that is five times hotter than the surface of the sun. When lightning strikes the ground, it is hot enough to fuse silica sand and clay together into fulgurites: shafts of glass produced by lightning. The word fulgurite comes from fulgur, the Latin word for thunderbolt.

Fulgurite: hollow, glass-lined tubes with a sandy outside, aka “petrified lightning”. 
Source: Discover Magazine

Glass can be made by heating sand, which is mostly silicon dioxide, or quartz. The lightning bolt vaporizes the sand it encounters, generating a hollow tube. As the heat moves outward from the entry hole, the sand grains melt and form a smooth inner surface. As the heat moves further outward, it fuses grains of sand.

The outside is rough, looking like petrified lightning when the fulgurite has branching assemblages. But the inside is hollow and smooth. The composition of the sand determines the color of the fulgurite.

Fulgurites are very fragile, as they are hollow and have lots of air spaces. They can get longer than 10 feet, although most are a few inches. Since they trap air bubbles, old fulgurites can be used to study the composition of air in ancient times. Additionally, the number of fulgurites found along with the date formed can provide information on the frequency of lightning.

The Libyan Desert is pure white sand composed of quartz. Fulgurites have been found in that desert. Also discovered were pieces of fused quartz with the clarity of clear glass. Such a piece adorns the mummified body of Tutankhamen. The piece is estimated to have formed 26 million years ago. To form such a piece of glass requires very hot temperatures, hotter than lightning. The most reasonable theory is that this was formed by a high-energy impact of a meteor.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison Department of Atmospheric and Oceanic Sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.
Category: Phenomena

Comments Off on What is fulgurite?

Do storms impact Arctic sea ice?

Posted on August 19, 2019 by WeatherGuys Editor

Researchers look out from the Finnish icebreaker MSV Nordica as the sun sets over sea ice in the Victoria Strait along the Northwest Passage in the Canadian Arctic Archipelago. The occasional extreme late-summer storm can be quite damaging to the ice. (Photo credit: David Goldman, Associated Press Archives)

As we head into the second half of August the days are noticeably shorter. That change is even more dramatic in the polar regions where the summer ice melt season is nearing its end.

This year’s melt has been particularly dramatic, with the Arctic sea-ice extent likely heading to its lowest level since satellite measurements began in 1979. The previous low extent record was set in 2012.

The sea-ice is subject to a number of different processes that can force its melting in summer. A good deal of those processes are tied to the progression of late-summer storms over the ice.

Though most such storms are too weak to do excessive damage to the ice, the occasional extreme late-summer storm can be quite damaging for two reasons. First, the extreme storms have very strong winds associated with them and those winds can break up the ice edge through the action of large waves. The resulting small rafts of ice easily melt away in the surrounding water.

Second, the storms have extensive cloud canopies associated with them. The composition of these clouds — whether they are mostly liquid water droplets or ice crystals -– has a large impact on the amount of radiation they emit toward the ice. Liquid water clouds emit more radiation toward the ice than ice crystal clouds and therefore contribute to more melting.

Whether a cloud canopy is mostly liquid water or ice crystals may be directly related to the way in which the storm itself is generated. At UW-Madison we are getting involved in a project that seeks to take measurements in the Arctic in the summer of 2021 to learn more about these interesting and impactful storms.

Category: Meteorology, Seasons

Comments Off on Do storms impact Arctic sea ice?

What is warm rain?

Posted on August 12, 2019 by WeatherGuys Editor

Warm rain is more common in places such as the Iao Needle, west of Wailuku, which is part of a moist, narrow valley in the center of West Maui, than in Madison. (Photo credit: Max Wanger, Hawaii Tourism Authority)

Warm rain results from the joining together of a cloud’s liquid water droplets. For the rain to be warm, temperatures throughout the cloud must be above freezing, so ice particles are absent.

Rainmaking is not easy. A single, small raindrop is a collection of about 1 million cloud droplets. A typical cloud droplet is usually 10 times smaller than the periods in this article.

One process to produce a large drop quickly is to combine many smaller particles. To form rain, the cloud droplets have to bump into each other and merge together through a process called collision and coalescence.

The process of combining cloud droplets through collision-coalescence is an important mechanism for forming precipitation in clouds composed solely of liquid water droplets.

Here’s how the process works. Water droplets of different sizes move at different speeds as gravity and vertical motions within the cloud act on them. The difference in speed increases the chance of collisions, as does any turbulent motions in the cloud.

Almost all precipitation particles that fall in Madison begin as ice particles, even in summer. The frozen particles completely melt, reaching the ground as raindrops, which means that rain is usually cold. In contrast, rain in Hawaii is typically warm rain, as the cloud top temperatures are typically below the freezing level.

Lightning requires frozen ice particles along with liquid droplets. Because most rain in Hawaii is warm rain, you rarely hear thunder in that state.

Category: Meteorology, Tropical

Comments Off on What is warm rain?

Why are there so few hurricanes every year?

Posted on August 5, 2019 by WeatherGuys Editor

This satellite image from the National Oceanic and Atmospheric Atmospheric Administration shows Hurricane Irma obscuring most of Florida in 2017. Even in a particularly active year, not many hurricanes actually develop. (Photo credit: NOAA)

We are about five weeks away from the climatological peak of the hurricane season, which stretches from early June to November.

During that period, even in a particularly active year, not many hurricanes actually develop. Forming over tropical oceans ensures that warm sea-surface temperature (SST) provides a mature hurricane with a means to warm and moisten the air that flows toward the important eye-wall convection. Thus, it is not surprising that hurricanes struggle to develop if the SST is not 79.7 degrees or warmer.

Tropical cyclones also require environments in which the wind speed and direction changes very little with increasing height, or where the vertical wind shear is small.

Certain vast stretches of the tropical ocean have SSTs above the threshold value of 79.7 degrees and thus qualify as locations where the development of tropical cyclones is favored. However, within such areas, it is only when the vertical shear is very low — from the surface to about 8 miles above the surface — that hurricanes can form and grow to maturity. In a given location in the tropics, it is much more likely that the shear condition, not the SST, will vary from one day to the next.

There are a number of physical factors that can conspire to render the vertical shear too extreme to allow for hurricane development. One such factor is the presence of the so-called subtropical jet stream, which is located between 20 degrees and 30 degrees latitude and about 8 miles above the ground in both hemispheres.

The subtropical jet stream is an ever-present feature of the general circulation of the tropics and has wind speeds routinely in excess of 130 mph. Such strong winds well above the surface are more than sufficient to provide a toxic amount of vertical shear to a nascent tropical cyclone. The small number of hurricanes every year testifies to the hostility of the environment to their development.

Category: Severe Weather, Tropical

Comments Off on Why are there so few hurricanes every year?