What are the trade winds?

Posted on August 7, 2023 by WeatherGuys Editor

On the morning of August 3, 1492, Christopher Columbus set sail on his first voyage to the Americas. He sailed south along the west coast of Africa, searching for the easterly “trade winds,” discovered earlier by the Portuguese, that would propel his fleet westward toward what he thought was India.

The trade winds are created by a cycle of warm, moist air rising near the equator. The air eventually cools and sinks a bit further north in the tropics. This phenomenon is called the Hadley cell. (Image credit: NASA/JPL-Caltech)

The trade winds are a component of a global-scale tropical circulation feature known as the Hadley cell. This cell is driven by the fact that the equator is heated more intensely than latitudes to its north and south, resulting in the production of collections of thunderstorms girdling the globe roughly along the equator.

The rising air that fuels these thunderstorms spreads out at the tropopause (about nine miles high) and heads both north and south. Cooling at the cloud tops of this poleward-directed air forces it to sink at about 25 degrees latitude on either side of the equator. The sinking air that reaches the surface in the Northern Hemisphere heads southward toward the equator in a huge vertical circulation cell. The rotation of the Earth, and the resulting Coriolis force, turns this southward moving air to its right, creating the near surface trade winds.

Though known by mariners for centuries, it was not until 1735 that George Hadley proposed this explanation for the trade winds. It was not until the 1950s, amazingly, that observational evidence provided by balloon soundings of the atmosphere finally confirmed the veracity of Hadley’s conjecture.

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, Tropical

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Why are cities hotter than the surrounding rural areas?

Posted on July 31, 2023 by WeatherGuys Editor

On average, the city is warmer than the countryside. This difference in temperature is referred to as the urban heat island effect. A number of factors contribute to the relative warmth of cities, such as heat from industrial activity, the thermal properties of buildings and the evaporation of water.

The architecture of cities intensifies Urban Heat Island effect and the absence of vegetation reduces evaporative cooling. (Image credit: CIMSS, UW-Madison)

For example, the heat produced by heating and cooling city buildings, and running planes, trains, buses and automobiles contributes to the warmer city temperatures. Heat generated by these objects eventually makes its way into the atmosphere, adding as much as one-third of the heat received from solar energy.

The thermal properties of buildings and roads are also important in defining the urban heat island. Asphalt, brick and concrete retain heat better than do natural surfaces. Buildings, roads and other structures add heat to the air throughout the night and, thus, reduce the nighttime cooling of the air so that the maximum temperature difference between the city and surroundings occurs during the night. The canyon shape of the tall buildings and the narrow space between them magnifies the longwave energy gains. During the day, solar energy is trapped by multiple reflections off the many closely spaced, tall buildings, reducing heat losses by longwave radiation.

The urban heat effect of Chicago and other smaller cities is readily apparent in this enhanced IR satellite image. (Credit: CIMSS)

Evaporation of water may also play a role in defining the magnitude of the urban heat island. During the day, the solar energy absorbed near the ground in rural areas evaporates water from the vegetation and soil. Thus, although there is a net energy gain from the sun, heating is reduced to some degree by evaporative cooling during evapotranspiration. In cities, where there is less vegetation, the buildings, streets and sidewalks absorb the majority of solar energy input and warm up rapidly.

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, Meteorology, Seasons

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Can ChatGPT forecast the weather?

Posted on July 24, 2023 by WeatherGuys Editor

Yes, ChatGPT can create weather forecasts, but the real question will be their accuracy.

Hurricane specialists at NOAA study satellite imagery and computer models to make their forecasts. (Photo credit: Mike Mascaro, NOAA)

People have started exploring how to use ChatGPT and other artificial intelligence methods to experiment with forecasts. This is not surprising as we have always needed weather forecasts and humans have used many techniques over the decades. Some of the earliest forms of forecasts were short rhymes, and, although memorable, were often of uneven quality.

Forecasts improved as weather observations became more available and consistent. A statistical forecast uses historical data to project what future weather could be based on what happened in the past with similar conditions. A trend forecast assumes that weather will change based on the movement and approach of current weather features. A trend forecast assumes that although the weather features are moving, they are not otherwise changing. Numerical weather prediction is a basis for today’s modern forecasts, which use mathematical models to predict the weather based on current weather conditions.

ChatGPT, or any AI approach, can forecast weather patterns that are statistically plausible given previous events. AI methods are being developed and applied to radar observations for short-term forecasting of precipitation. Observations of high-quality weather radar are freely available across the U.S. An AI adoption of a trend forecast to radar observations has the potential to account for changing speed, size, intensity, and direction of movement of the storm.

Given the capability of AI to handle large data volumes, upcoming AI weather forecasts are likely to combine numerical weather predictions with current and recent weather observations to make a forecast.

There are risks in relying on AI forecasts. All AI models require data training sets. Those data records might not include extreme events. AI systems can be unpredictable when the existing conditions have never, or rarely, been encountered.

AI weather forecasting requires the same constraints as all forecasts — accurate and consistent observations.

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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Winter in July?

Posted on July 17, 2023 by WeatherGuys Editor

On more than one occasion in this column we have commented on the areal extent of air colder than 23 degrees about 1 mile above the ground as a measure of the extremity of winter.

In the middle of January, about 68 million square kilometers of the Northern Hemisphere are covered by air that cold at that level. Between about July 5 and July 20 that area shrinks to zero, and the complete absence of such air lasts only a very few days.

We are just past the point in this summer when that area is beginning to increase again. Thus, despite the fact that we have been through a couple of really hot and humid weeks this July, the return of winter has already begun in terms of this measure.

This is largely because the number of daylight hours has already begun to decrease — noticeably here in Madison. At every latitude north of 66.5N, however, the sun was out for 24 hours long on the day of the Summer Solstice, June 21. Ever since, the spectre of nighttime has been creeping poleward with the daylight decreasing ever so slightly just above the Arctic Circle. The associated nighttime cooling is greater during a longer night, resulting in the gradual increase in the amount of cold air covering the Northern Hemisphere.

So, if these past couple of weeks have been a bit too much for you, perhaps some solace can be found in the fact that the tide is already, however imperceptibly, beginning to turn.

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, Seasons

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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

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