Do hurricanes heat the ocean?

Posted on July 10, 2023 by WeatherGuys Editor

Hurricanes form over warm waters. The evaporation of the warm ocean waters condenses to form clouds and precipitation releasing latent heat energy that helps to maintain the storm.

A general rule of thumb is that hurricanes will not form unless the water temperature is at least 80 degrees. Photographs of hurricanes over the ocean clearly show that hurricanes churn water at the surface, mixing it with cooler waters below.

Hurricanes draw their energy from warm sea surface waters. As lower strata of the ocean warm up along with the rest of the planet, deeper waters once cool enough to weaken hurricanes at the surface, are now becoming warm enough to strengthen them. (Illustration: Natalie Renier, Woods Hole Oceanographic Institution)

Measurements also show that hurricanes can push heat deep below the sea surface. The mixing of heat into the ocean by hurricanes occurs by the strong winds and also by underwater waves produced by the storm. Ocean measurements before and after hurricanes show that these waves transport heat to depths where the heat is stored far below the surface. The depth is such that the heat cannot by quickly released into the atmosphere. In fact, the deep ocean currents can transport that heat thousands of miles away from the storm.

One recent study found that a hurricane traveling across the western Pacific Ocean could supply warm water to the coast of Ecuador years later. The heat transported deep in the ocean by the storm doesn’t resurface in the vicinity of the hurricane.

The impacts of hurricanes on climate will depend on the depth to which the storms deposits the heat. Mixing of cooler waters below the ocean surface by the hurricane winds cools the sea surface temperature. The heat transported deep in the ocean from hurricanes may reside there for decades without returning to the surface. Hurricanes can help to slow down global warming by transporting heat deep in the ocean where it is stored.

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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Tropical

Comments Off on Do hurricanes heat the ocean?

Were May and June unusually dry in Madison?

Posted on July 3, 2023 by WeatherGuys Editor

While we all contended with the remarkable and dangerous smoke in the sky this past week, we also wrapped up a record dry spell in Madison’s history.

The 61 days of May and June 2023 were the driest May and June ever, with a paltry 2.01 inches of total precipitation falling. The next closest rival on this ignominious list occurred in May and June 1992, when only 2.65 inches of rain fell during the two months. Individually, May and June were the sixth-driest May and June in Madison’s history, suggesting how rare it is for both of them to be so void of precipitation.

The consequences of this extended dry period are everywhere — lawns are brown and looking like it’s mid-August, the ground is hard and cracking in some spots, and leaves are falling from some trees well before their time. Over the course of these two dry months, we have fallen 7.37 inches behind in precipitation — May/June averages 9.38 inches.

Since the annual average for the year is 36.9 inches, even if the rest of the year finds its way back to normal, we will end up just shy of 20% below our normal annual precipitation.

As we have said in prior columns, this local dryness is part of a larger regional dryness that has sparked an early initiation to eastern Canada’s wildfire season, which has polluted our skies intermittently during the past month. Perhaps the only good thing about this record dryness is that it seems to have kept the mosquitoes at bay for the time being.

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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, History, Meteorology, Seasons

Comments Off on Were May and June unusually dry in Madison?

What holds the clouds up in the air and what makes some clouds appear to be fluffy on top but flat on the bottom?

Posted on June 26, 2023 by WeatherGuys Editor
Buoyant air causes the tiny liquid water droplets that form clouds to bubble to the top, where they eventually cool and form larger drops that fall to earth. (Photo credit: Steven A. Ackerman)

One of our readers awoke to some beautiful clouds in the summer sky recently and two excellent questions popped into her mind: What holds the clouds up in the air and what makes some clouds appear to be fluffy on top but flat on the bottom?

Clouds are composed of tiny liquid water droplets — whose diameters are about the width of a human hair — and tiny shards of ice in a variety of shapes, or habits. Whether a cloud is mostly liquid water droplets or ice particles depends, as you might guess, on the ambient temperature of the air in the cloud.

Tiny cloud liquid water droplets can remain in the liquid state to temperatures as low as about 14 degrees and when they do they are known as supercooled liquid water droplets. These droplets feel the downward directed force of gravity just like a baseball or a watermelon would. Because the droplets are so small, and therefore have small masses, the gravitational force can easily be balanced by an upward directed friction force resulting from the interaction of the droplets with the air molecules around them and so the droplets remain suspended — this is what holds clouds up in the air.

When these droplets grow, by a variety of interesting processes, they gain water mass, and eventually the gravitational force overwhelms the friction force, and the now larger droplets fall to the surface.

The fluffy appearance of the tops of some clouds are evidence of convection — that is, the air parcels within the cloud are buoyant and literally bubble to the top. As the air rises, it cools by expansion as it encounters environments with lower and lower pressure the higher it goes. This cooling increases the relative humidity of the air, and once the relative humidity gets to 100% condensation of the invisible water, vapor begins to produce the cloud liquid water droplets.

The bottom of clouds often appears flat because the first level at which rising air parcels begin to condense is usually rather uniform over a given region. This level is known as the lifted condensation level — that is, the level at which lifted air parcels first begin to experience condensation.

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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Uncategorized

Comments Off on What holds the clouds up in the air and what makes some clouds appear to be fluffy on top but flat on the bottom?

Can wildfires generate weather?

Posted on June 19, 2023 by WeatherGuys Editor

Fires require something to burn plus air to supply oxygen and a heat source to get the fuel to its ignition temperature.

Once a fire starts, weather is one factor of how it will spread and if it will grow. The important weather factors are temperature, wind and humidity. Warmer temperatures allow fuels to ignite quickly, and low humidity keeps the fuel dry and easy to burn. Wind brings oxygen to the fire and also can help to spread it.

A large fire can generate a wind pattern of its own that can help to spread the fire. Heated air near the ground is constantly and quickly rising in a large fire. As the air near the ground moves upwards, air from all around the fire rushes in towards the rising column of air. This movement creates an updraft.

A large, hot fire can form a pyrocumulonimbus, or a “fire storm cloud.” These are much like thunderstorms. Pyrocumulonimbus can produce lightning, which could set off new fires. They also generate stronger winds that fan the fire, making it hotter and helping it to spread.

Satellite imagery (Visible and IR) of a Pyrocumulus Cloud associated with the 2021 Bootleg Fire. Credit:CIMSS

Rotating winds can develop along the edge of a hot fire, a result of the contrast between the hot air associated with the fire edge and the cooler air over the adjacent, non-burning region.

There is a wide range in the properties of a fire tornado, but they are usually 30 to 200 feet tall and about 10 feet wide. The largest fire vortices are associated with wildfires.

Fire tornadoes can be generated when the vortices are tilted from the horizontal to a vertical direction. Fire tornadoes can be composed of flames or black smoke and can toss burning debris into the non-burning area, helping to spread the fire.

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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena, Severe Weather, Weather Dangers

Comments Off on Can wildfires generate weather?

Are we seeing more ozone advisories in southcentral Wisconsin?

Posted on June 12, 2023 by WeatherGuys Editor

Ozone (O3) is a molecule formed by three oxygen atoms.

Ozone that resides in the stratosphere absorbs ultraviolet rays of the sun, protecting life on Earth’s surface from these high-energy electromagnetic waves. O3 also can occur near the ground, where it is considered a pollutant, as it is a chemically reactive gas that can cause respiratory problems when breathed.

O3 near the ground is created when oxides of nitrogen chemically react with volatile organic compounds. Typically, ground level O3 has highest concentrations in mid-summer with hot weather, minimal amounts of cloud cover and light winds. In Wisconsin, O3 is highest near Lake Michigan due to transport from the Chicago area as well as chemistry occurring over the lake. But triggers such as the chemicals found in forest fire smoke can result in high concentrations of O3 outside that typical time period.

Because O3 can be a health hazard, there are air quality standards set to protect public health. Public warnings are issued whenever these thresholds are approached or exceeded. The Clean Air Act requires the Environment Protection Agency to periodically review all the National Ambient Air Quality Standards to ensure that they provide adequate health protection. The current standard for O3 is 70 parts per million.

Get Air Quality info via airnow.gov

The EPA uses an Air Quality Index (AQI) for reporting air quality so that risks can easily be interpreted. The AQI is divided into six categories corresponding to different levels of health concern. Each category also has a specific color to enable the public to quickly determine air quality in their community: green is good air quality, yellow moderate, orange unhealthy for sensitive groups, and red is unhealthy.

During the past three years, there have been more early-season ozone alerts in April and May, in part due to hotter temperatures earlier in the season, and in part this year due to the transport of wildfire smoke from Canada to Wisconsin.

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. Send them your questions at stevea@ssec.wisc. edu or jemarti1@wisc.edu.

Category: Climate, Phenomena, Weather Dangers

Comments Off on Are we seeing more ozone advisories in southcentral Wisconsin?