When is hurricane season?

Hurricane Arthur off the east coast of the United States on May 18, 2020. (Photo credit: NOAA)

The peak of hurricane activity in the tropical waters south and southeast of the United States is typically in early to mid-September.

But hurricanes can occur any month of the year; the calendar isn’t the important thing — the ocean temperatures and the lack of wind shear are.

Hurricanes are large, whirling storms that obtain their energy from warm ocean waters and originate in the Atlantic basin, which includes the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and the eastern North Pacific Ocean.

All tropical storms 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.

In general, hurricanes will not form unless the water temperature is at least 80 degrees.

Hurricane season carefully follows the seasonal cycle of ocean temperature. The Atlantic hurricane season officially starts June 1 and lasts until Nov. 30, which is the period when about 97% of hurricanes occur.

Hurricane Alex was declared a hurricane on Jan. 14, 2016, and maintained hurricane-force winds until Jan. 15. The Northern Atlantic Ocean is typically not 80 degrees in January.

Alex formed over waters whose temperatures were about 68 degrees, far below the norm. But the air high above was very cold, at minus 76 degrees. The 144-degree difference between the air and water temperature led to strong evaporation, helping to give the storm enough energy to become a hurricane.

An average hurricane season produces 12 named storms, of which six become hurricanes, including three major hurricanes.

An above-normal Atlantic hurricane season is expected this year. The 2020 seasonal forecast expects 13 to 19 named storms (winds of 39 mph or higher), of which six to 10 could become hurricanes (winds of 74 mph or higher), including three to six major hurricanes (with winds of 111 mph or higher).

The first hurricane of the 2020 season, Hurricane Arthur, took place May 16-19.

Category: Seasons, Severe Weather, Tropical

Comments Off on When is hurricane season?

Why is Memorial Day weather so fickle?

Joe and Heather Shilts, of Madison, wear headgear to shield themselves from the oppressive heat of the sun with temperatures in the 90s during the World’s Largest Brat Fest over the 2018 Memorial Day weekend. Due to the COVID-19 pandemic, A Virtual Brat Fest is taking place this year at www.bratfest.com. (Amber Arnold, State Journal archives)

Memorial Day weekend weather can be absolutely glorious in Wisconsin or it can be rainy and cold.

Perhaps no other major holiday suffers from such a Jekyll-and-Hyde split in our expectations, and there are really good scientific reasons that underlie this duality.

By the end of May the Northern Hemisphere is just about over the prior winter and the cold air that characterized it is almost completely left to very high latitudes where the longer days act quickly to erode what is left even near the Pole.

The process of “shedding” the cold air from winter sometimes involves the excursion southward of regional cold air vortices in the mid-troposphere, which meteorologists refer to as “cut off” low pressure systems.

If such a cut off low parks itself over the Great Lakes states, the weather is often persistently showery and chilly. This is because the air at about 3 miles above the ground is colder than normal in such a circumstance and, coupled with the relatively warm surface temperatures of late May, the atmosphere is made less stable and the threat of showery weather increases substantially.

These cut off lows can persist for several days in a row. It turns out that the seasonal maximum for such cut off lows over our area runs from mid-May to mid-June. Consequently, Memorial Day weekend can be plagued by the presence of one.

Category: Climate, Meteorology, Seasons

Comments Off on Why is Memorial Day weather so fickle?

What makes the weather?

Pierre Seguin, of Madison, encounters morning fog in Yahara Place Park along Lake Monona on a winter morning, an example of the fundamental cause of weather: the effect of the sun on Earth. (Photo credit: John Hart, WI State Journal archives)

The term “weather” refers to the temporary conditions of the atmosphere, the layer of air that surrounds Earth and is referenced to a particular location and moment.

The fundamental cause of weather is the effect of the sun on Earth. At any time, only half of Earth is warmed by the sun, while Earth’s other side is shadowed. This causes uneven heating of Earth’s surface by the sun every day, with some regions warmer than others.

These temperature differences cause weather: winds, clouds and precipitation. Seasonal weather patterns result from variations in temperature caused by Earth’s tilt toward the sun in summer and away from the sun in winter. The distribution of water and land, and the topography of the land, also contribute to the shaping of Earth’s weather patterns.

Six main variables describe weather: temperature, atmospheric pressure, wind, humidity, precipitation and cloudiness. Knowledge of how these variables change, through the action of atmospheric processes governed by the laws of physics, help forecasters to predict weather.

In the modern era, meteorologists also use computer-generated forecasts as a guide, and interpret that guidance using their knowledge of weather processes.

Weather at one location is often related to weather at other locations. So, an important part of studying and understanding the weather is seeing how observations of the atmosphere relate to each other geographically. A list of numbers doesn’t help much; we need a map.

Meteorologists represent weather at a particular location and time using the station model. This model is a clever method of representing many weather variables, including cloud cover and type, wind speed and direction, temperature, dew point temperature, visibility, precipitation type and intensity, atmospheric pressure, and the change in pressure over the last three hours in one visual. A weather map is filled with symbols indicating different types of weather systems.

Category: Meteorology, Seasons

Comments Off on What makes the weather?

What is a freeze and a hard freeze warning?

The National Weather Service issues frost and freeze warnings during the growing season, as cold temperatures can damage plants and impact agriculture, nurseries and home gardening. (Photo credit: Racine Journal Times archives)

A freeze warning is issued when the nighttime low temperatures across the whole county or forecast zone are predicted to be at or below 32 degrees.

A freeze warning is issued when low temperatures are expected to be 29 to 32 degrees, and a hard freeze warning is issued when temperatures are expected to be 28 degrees or less.

A frost, freeze or hard freeze watch may be issued a few days ahead of time if the potential exists for temperatures to fall into the appropriate thresholds.

The National Weather Service, or NWS, officially issues all weather warnings and watches. Those include frost, freeze and hard freeze watches and warnings. The NWS issues these types of watches and warnings after the beginning and before the end of the growing season. A watch or warning can be extended beyond that time if necessary.

The NWS issues these warnings as cold temperatures can damage plants and impact agriculture, nurseries and home gardening. The beginning of the growing season is set by the average date of the last freeze in spring. The end of the growing season is the average date of the first freeze in autumn.

The Midwest Regional Climate Center has maps that show the first and last days of the year for a freeze at go.madison.com/freeze.

Category: Meteorology, Seasons, Severe Weather

Comments Off on What is a freeze and a hard freeze warning?

When did computer-based weather forecasts begin?

Numerous computers and monitors enable National Weather Service operations.
(Photo Credit: NWS Green Bay)

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

Such an enterprise would require accurate forecasts of the weather thought possible by using 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 U.S. Weather Bureau headquarters 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 forecasting went from dream to reality.

In the intervening 65 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 a 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 numerical weather prediction was taken 65 years ago Wednesday.

Category: Meteorology

Comments Off on When did computer-based weather forecasts begin?