Why is May 6 such a special day in weather science history?

Immediately after World War II, it became fashionable to imagine technologies that might allow human beings to control the weather. In fact, one goal advanced by influential scientists was actually to explode nuclear bombs in the right locations and in the right quantity so as to alter the weather in favorable ways.

Such an enterprise would require accurate forecasts of the weather thought possible by using the brand new computer technology to make the millions of requisite calculations.

The drive to use computer models for weather forecasting was initiated at a secret meeting at the U.S. Weather Bureau headquarters in Washington, D.C., on the rainy morning of Jan. 6, 1946. After a series of successes and setbacks that mostly discouraged the broad meteorological community, the first operational computer generated forecasts were issued on the afternoon of May 6, 1955.

Thus, in less than 10 years the notion of computer-based forecasts went from dream to reality. In the intervening 69 years the combination of increased theoretical understanding both of meteorology and computational science, increased observational capacity (a good deal of which stems from satellite data), and sheer hard work on the part of a legion of dedicated scientists has resulted in our current forecasting capability.

The fact that our ubiquitous smart phones give everyone access to quite reasonable forecasts several days in advance is the end result of what might be considered the greatest scientific advance of the second half of the 20th century. So, as you consult your phone for the forecast, remember that one of the first baby steps in the march toward the modern miracle of numerical weather prediction were taken 69 years ago today!

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: History, Meteorology

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How does hail get its shape?

Hail is precipitation in the form of balls or chunks of ice.

Hailstones begin as small ice particles that grow primarily by accretion; to grow large, they require abundant water droplets. As the hailstone moves up and down through a storm, it collides with water droplets and ice crystals, growing larger with each collision. Hailstones can be smaller than peas or as large as oranges and grapefruits. The small hailstones are roughly spherical in shape, while large ones can take on jagged shapes.

When a hailstone is cut in half, you can see rings of ice. Some rings are milky white; others are clear. This ringed structure indicates that hailstones grow by two different processes: wet growth, represented by the clear layers, and dry growth, which forms the milky white layers. Counting the layers of clear and milky white ice gives an indication of how many times the hailstone traveled through the storm.

This hail stone from Madison Wisconsin exhibits both dry and wet growth. (Credit: M. Mooney)

Dry growth of hailstones occurs when the air temperature is well below freezing. In these conditions a water droplet freezes immediately as it collides with the hailstone. This quick freezing leads to air bubbles “frozen” in place, leaving cloudy ice.

In wet growth, the hailstone is in a region of the storm where the air temperature is below freezing but not very cold. When the hailstone collides with a drop of water, the water does not freeze on the ice immediately. Instead, the liquid water spreads over the hailstones and slowly freezes. Because the water freezes slowly, air bubbles can escape, resulting in a layer of clear ice.

With wet hail growth, a thin layer of liquid water can remain on the surface of a hailstone. This thin layer helps hailstones that collide to freeze to each other, forming jagged shapes.

Steve Ackerman and Jonathan Martin, professors in the UWM Adison 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@ssecwisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Severe Weather

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Are ‘chem trails’ a real thing?

For years we have fielded questions on our monthly radio show on WHA regarding the nature of condensation trails left in the wake of jet airliners.

Widespread contrails over the southeast U.S. in January 2004. Credit: NASA MODIS

These contrails are composed of ice crystals that develop from the exhaust of jet engines in portions of the atmosphere that contain sufficient water vapor. Sometimes these condensation trails can persist for a very long time because the environmental conditions are moist enough that sublimation (the direct transformation from solid ice to invisible water vapor) is easily resisted by the aircraft-produced ice particles.

Sometimes there is not enough water vapor available along a portion of a flight route for the formation of a trail. The variability of upper tropospheric water vapor is such that sometimes the same aircraft can create a condensation trail along a segment of its track and nothing along an immediately adjacent segment.

With the increase in air travel over the past half century, these interesting, thoroughly explainable and naturally occurring results of air travel have become the font of an enduring conspiracy theory which suggests the condensation trails are actually “chem trails” — short for “chemical trails.” The idea is that some governmental agency is responsible for producing these “chem trails” for any of a number of malevolent purposes, including altering precipitation patterns so as to create drought, altering the chemical composition of the atmosphere so as to promote global cooling, etc.

None of these theories is in any way valid — and all of them disregard the enormous scale of the clandestine enterprise that would need to be taking place in order for such schemes to have any discernible impact on the atmosphere. It has taken over a century of continuing, unchecked increase in carbon dioxide from the burning of fossil fuels to have produced the actual climate change that many of the same people warning about “chem trails” deny has even begun. And yet, just last week, the state of Tennessee succumbed to this nonsense and passed a bill that forbids “intentional injection, release, or dispersion” of chemicals into the air — code for eliminating “chem trails.”

We live in a strange time.

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: Phenomena

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Did the total solar eclipse impact the weather?

If you were in the path of the total solar eclipse last week, you may have observed a change in your environment. The more sunlight that was blocked, the more dramatic the changes.

Skiers and hikers on Saddleback Mountain in Maine during the April 8 total solar eclipse. (Photo credit: R. Bukaty, Associated Press)

A range of surface and near-surface meteorological observations can occur during a total solar eclipse. If it was a cloud-free day, or mostly cloudy day, you probably felt a drop in temperature. As the moon crossed in front of sun, it cast a shadow blocking solar energy from reaching your location. While it may have lasted only a few minutes, the reduction in solar radiation would result in a drop in temperature. In some locations, the temperature dropped by as much as 10 degrees. As the sun reappeared, the temperature increased.

A drop in temperature would include a corresponding increase in the relative humidity. So, it might have felt more humid. You may have also observed a change in the wind speed.

The planetary boundary layer, also referred to as the atmospheric boundary layer, is the lowest part of our atmosphere. It is about 0.6 miles thick and is where the atmosphere exchanges energy directly with the ground. The exchanges are primarily mechanical (e.g. wind and turbulence) and thermal. The thermal contact between the boundary layer and the ground surface is a result of the amount of solar radiation.

You may have already observed, in the absence of a storm, that after sundown the wind calms. When the sun sets, the radiative cooling of the ground increases the stability of the lower atmosphere. This reduces the energy exchanges between the atmosphere and the surface. This can result in the wind dying down. The drop in temperature you experienced during the eclipse would similarly cool the ground.

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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What are good weather apps for smart phones?

Good weather apps for smart phones provide easy access to current weather and forecasts. Many apps tell you about the high temperature for the day and can provide an hour-by-hour breakdown of temperatures, chance and type of precipitation, air quality and other weather information.

Your device likely comes with an installed weather app, but consider exploring other apps. Apps that include live weather radar and any severe weather alerts for your area are valuable. They are useful for identifying precipitation and storm location and movement. Many include lightning flash locaters, too. Apps with current weather radar data provide useful information when you need to be outside and precipitation is in the area.

Weather apps are pulling data that is freely provided by National Oceanic and Atmospheric Administration The NOAA and National Weather Service provide weather observations, output from numerical weather prediction models and professional analysis thereof. Weather apps translate this data into easily accessible information using a sunny icon, a rainy cloud, or a lightning flash. Some also include analysis and interpretation of the NWS data by their own meteorologists.

There are apps that allow you to upload your precipitation observations, such as mPING and CoCoRaHS. This data is then visible to others for their use.

As professional meteorologists, we have access to weather information from various resources. However, we also have weather apps on our phones. Apps that include wireless emergency alerts that are activated during severe weather are particularly valuable.

A potential downside to some free apps is that they include ads that can be distracting. Some apps may collect, use, and share your data. That is why it’s important to understand their privacy policies, too.

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

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