What is the difference between mist and fog?

Posted on September 12, 2022 by WeatherGuys Editor
In 2021, the GOES-R Fog and Low Stratus satellite product became operational in National Weather Service offices across the country. Helping airlines and passengers avoid costly delays and warning smaller aircraft of potential danger. (Image credit: Corey Calvert, CIMSS)

Both a mist and a fog are water droplets suspended in the atmosphere in the vicinity the earth’s surface that affect visibility.

They both differ from a cloud only in that the base of a fog or a mist is at the earth’s surface, while a cloud’s is above the surface.

The difference between a mist and a fog is associated with the atmospheric visibility. A fog and a mist are both composed of microscopic water droplets or wet hygroscopic particles suspended in the air. Particles cause light to be refracted and reflected in many directions, reducing visibility.

By international definition, a fog reduces visibility to below 1 kilometer (5/8 of a mile), while a mist occurs when the visibility at the earth’s surface is greater than 1 kilometer. These visibility observations are made at ground or sea level.

A fog is denser and thicker than a mist. Consequently, it is more difficult to see through a fog than a mist. A mist dissipates more quickly than a fog. In addition, the term “mist” is used in weather reports when the corresponding relative humidity is between 95% to 100%.

Haze is a related but slightly different phenomenon. Haze is a suspension of extremely small particles in the air, reducing visibility by scattering light like a fog and a mist. Haze formation is caused by the presence of an abundance of condensation nuclei which may grow in size, due to a variety of causes, and become a mist, a fog or a cloud.

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, Weather Dangers

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Does lightning add nitrogen to the soil?

Posted on September 6, 2022 by WeatherGuys Editor

Our bodies need nitrogen to make proteins. The atmosphere’s composition is 78% nitrogen, but the nitrogen in the air is not available to our bodies.

GOES-East Geostationary Lightning Mapper (GLM) imagery over the southeast U.S. in April 2022

The two atoms in the airborne nitrogen molecule are held together very tightly. For our bodies to process that nitrogen, the two atoms must separate.

We obtain nitrogen from protein-containing foods in our diet. For example, plants absorb nitrates in the soil. When we eat plants, we get the nitrogen in a form that our bodies can use.

Plants also cannot make use of the nitrogen in the atmosphere. Fertilizers are one way to add nitrogen to the soil so that plants can use it.

Lightning is another way to get nitrogen into the soil.

Lightning is a huge electrical discharge that results from vigorous motions that occur in thunderstorms. Charges form in a storm composed of ice crystals and liquid water drops. Winds inside the storm cause particles to rub against one another, causing electrons to be stripped off, making the cloud particles either negatively or positively charged.

The charges get grouped in the cloud, often negative charges near the bottom of the cloud, and positive charges up high. This is an electric field, and because air is a good insulator, the electric fields become incredibly strong. Eventually a lightning bolt occurs to neutralize the electric field.

Bolts of lightning are powerful enough to break the strong bonds of the nitrogen molecule in the atmosphere. Once split, the two nitrogen atoms quickly bond to oxygen in the atmosphere, forming nitrogen dioxide. Nitrogen dioxide dissolves in water and forms rain droplets in the storm cloud, creating nitric acid, which forms nitrates.

The raindrops that carry the nitrates to the ground seep into the soil, where it can be absorbed by plants.

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

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What is energy’s relationship with rain?

Posted on August 31, 2022 by WeatherGuys Editor

Many locations across the country (and the world) have been experiencing unusually heavy rainfalls this summer.

Overshooting tops, gravity waves, lightning and outflow boundaries in this 2021 moonlit satellite image conveys some of the energy in a single summer thunderstorm complex.

Though in Madison we have had a relatively benign month of August in terms of temperature and humidity, we have still managed to find ourselves 2.09 inches above normal for rainfall for the month prior to Sunday’s rain.

That monthly surplus is almost entirely accounted for by the extremely heavy rainfall on Wednesday night/Thursday morning when 1.86 inches of rain fell at the Dane County Regional Airport — our largest daily total since July 5 and second largest of the summer.

Rain is the liquid form of water, the only substance that occurs naturally in all three phases — solid, liquid and invisible gas — in the Earth’s atmosphere. The 1.86 inches of rain Wednesday/Thursday began as an equivalent amount of water in the invisible vapor (gas) phase before transformed into liquid.

It may come as no surprise that energy (600 calories) is required to transform 1 gram of liquid water into 1 gram of water vapor, the familiar process of evaporation. An exactly similar amount of energy is released into the environment when 1 gram of invisible water vapor condenses into a puddle of liquid water.

The particular amounts of energy needed to accomplish these changes of phase are known as latent heats — the latent heat of evaporation for the first one and the latent heat of condensation for the second. Since we know the depth of accumulated liquid precipitation involved in our recent heavy rain event, the area of Dane County, and the latent heat of condensation, we can calculate how much energy was released to the atmosphere in the production of that much rain.

Without providing the details of the calculation, we can report that the amount of energy involved in just that one rain event could power the entire Madison metro area for approximately 3.9 years. Clearly, there are huge amounts of energy involved.

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, Severe Weather

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How is this year’s Atlantic hurricane season measuring up?

Posted on August 22, 2022 by WeatherGuys Editor

The Atlantic hurricane season is now more than two months old and has so far been fairly quiet with only three storms — all of them in the weakest category of tropical depression. This may well have been the case in the summer and fall of 1492 as well.

It is interesting to note that among the seemingly endless list of superstitions and fantastical falsehoods that surrounded voyages of exploration in the age of discovery — ranging from sea serpents of all kinds to boiling waters near the equator — there was no mention of hurricanes. This is obviously because no European had yet witnessed (and could not imagine) the frothing seas, with towering 100 foot waves, that these ferocious storms can create.

Columbus set sail on Aug. 3, 1492, from Palos de la Frontera and made the open ocean off the coast of Spain some days later. He reached the Bahamas in the first days of October, meaning that he sailed through the peak of the Atlantic hurricane season without incident. Twenty-seven years later, on Aug. 10, 1519, Ferdinand Magellan’s fleet of five ships left Seville on what would become the first circumnavigation of the globe. Magellan’s route also took him through the tropical Atlantic at the height of hurricane season and, again, without incident.

Thus, this year’s relatively inactive Atlantic hurricane season is not out of the ordinary and may end up being reminiscent of other inactive seasons from the past, in which one might argue the stakes were quite high.

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: Seasons, Tropical

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When does fog mostly occur in Madison?

Posted on August 16, 2022 by WeatherGuys Editor

Fog can occur anytime of the year, depending on the type of fog. In southern Wisconsin, fog is most frequent in the morning, between about 6 and 8 a.m.

Fog is a cloud in contact with the ground, and it forms as the dew point approaches the temperature of the air, increasing relative humidity. The air in contact with the ground can become very humid if it cools or when water from the surface evaporates into it. Either of these processes increases the relative humidity of the air. As the temperature of the air near the ground approaches the dew point, water vapor condenses on tiny particles suspended in the air to form a suspension of small water drops.

Fogs are named for the ways in which they form. Common types of fog in the Midwest include radiation fog, advection fog and evaporation fog.

Radiation fog in southern Wisconsin on August 11th as seen by satellite.

Radiation fogs form on clear, calm, long nights when the ground and the air in contact with it cool by radiation. As the temperature of the air drops, the relative humidity increases, and fog can form. Early-morning fogs are often radiation-type fogs.

When warm air is advected (blown horizontally) over a cold surface, the air near the ground cools because of energy exchanges with the surface. The relative humidity of the air increases with cooling and an advection fog may form.

Large unfrozen lakes are often shrouded in a fog in the fall and early winter. The lake water evaporates into the air above the lake surface. The air is cooled and moistened, causing the dew point to increase, forming an evaporation fog.

Each year about 700 fatalities occur in the United States because of traffic accidents during fog. A combination of high speed and low visibility is often to blame.

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

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