When is the summer solstice?

Posted on June 18, 2014 by WeatherGuys Editor

The summer solstice in the Northern Hemisphere is the day when the sun is farthest north. In 2014, this occurs on June 21 at 5:51 a.m.

As Earth orbits the sun, its axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of Earth’s axis to the sun is always changing as Earth orbits around the sun.

As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude. The summer solstice is the day of the year with the most daylight in our hemisphere.

On the Northern Hemisphere’s summer solstice, the northern spin axis is tilted toward the sun and latitudes north of the Arctic Circle (66.5 degrees N) have 24 hours of daylight.

The length of daylight is related to how high the sun gets in the sky. At the summer solstice, the sun reaches its highest point in the sky and so daylight is longest.

Our earliest sunrise, however, occurred Saturday, while our latest sunset occurs about a week later than the summer solstice. So, while the summer solstice has the longest daylight hours, that day does not correspond to the earliest sunrise or the latest sunset. The reason that the earliest sunrise and latest sunset do not occur on the summer solstice is a combined effect of the tilt of Earth’s axis and the elliptical path of Earth around the sun.

Category: Seasons

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Did weather play a role in D-Day?

Posted on June 13, 2014 by WeatherGuys Editor

Friday marked the 70th anniversary of the Allied invasion of Europe and the beginning of the end of Nazi tyranny and murder in World War II. A perfect combination of full moon (for nighttime aerial navigation purposes), low tide (so that German mines in Brittany would be exposed) and light winds were necessary to give the invasion any reasonable chance of success.

Thus, days on which the tides and phase of the moon were optimal were known some weeks in advance. As late May arrived, it became increasingly clear that choosing among those optimal days for invasion would depend crucially upon the weather and the ability to make an accurate forecast of weather conditions.

In 1944 there were no satellites and no weather radar. In addition, there was no such thing as numerical weather prediction by high-speed computers. Thus, a one-day forecast for western Europe, with the Atlantic Ocean stretching endlessly to the west, was informed only by careful analysis of surface observations and a few upper atmospheric observations near Iceland and Greenland.

The Allies had a rather unorganized approach to the forecasting enterprise, employing a British and an American team of forecasters. These two teams used decidedly different techniques to make their forecasts, and the differences came to a head on June 4 when, in the face of very different forecasts, Group Capt. James Martin Stagg, Gen. Dwight D. Eisenhower’s chief meteorologist, advised the general to delay the invasion one day, from June 5 to June 6.

The British forecast team bet that there would be a sufficiently calm period between two of the endless collection of storms that battered western Europe in June 1944 to allow the invasion to proceed on June 6. They turned out to be right.

While riding to the Capitol on his inauguration day in 1961, President John F. Kennedy asked President Eisenhower why the Normandy invasion had been so successful. Eisenhower answered, “Because we had better meteorologists than the Germans.”

Category: Meteorology, Severe Weather, Weather Dangers

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What causes urban flooding?

Posted on June 4, 2014 by WeatherGuys Editor

Urban flooding occurs when water flows into an urban region faster than it can be absorbed into the soil or moved to and stored in a lake or reservoir. It can be caused by flash flooding, coastal flooding, river floods or rapid snow melt.

On Tuesday, the Madison area experienced urban flooding caused by an intense rainfall. On that day, a daily record rainfall of 1.46 inches fell at Dane County Regional Airport, breaking the city’s previous May 27 record of 1.27 inches. The city sewage system and draining canals did not have the necessary capacity to drain away the large amounts of rain that fell in a short period of time.

Flash floods occur suddenly, and the water can flow quickly. Urban flooding is most often an inconvenience, particularly when it occurs during commuting time as Tuesday’s did, but it can also be dangerous.

Never drive your car across a flooded road. The economic cost of urban flooding can also be high, as water damage to homes and buildings can be very expensive to repair.

Urbanization of the United States and other countries is increasing. Common consequences of urban development include the removal of vegetation and an increase in impervious surfaces and drainage networks, all of which increase precipitation runoff into streams and rivers. This may result in more flooding.

Urban planning explores design strategies that include storm water conveyance systems and other methods to prevent flooding. This long-term planning must include forecasts of expected weather. Current research indicates a trend to more intense precipitation, which could result in more urban flooding if it’s not properly planned for.

Category: Climate, Meteorology, Phenomena

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Do wind farms modify climate?

Posted on May 27, 2014 by WeatherGuys Editor

The turbines on wind farms convert the kinetic energy of the wind into electricity. So, we might expect to see some changes in the region due to this energy conversion. The impact will be a function of how big the wind farm is and how long the turbines operate.

One way to estimate changes is through observations. The best method is to compare the observation of a region before and after wind farms are installed. Unfortunately, there are rarely enough observations before the wind farm installation to make strong conclusions about any observed relationships.

So, another approach is to measure weather conditions around the turbines with a similar area without turbines. This measurement approach has indicated that the soil around the turbines is drier and warmer than the surrounding region. This soil temperature and moisture is a very local effect, impacting the wind farm area, not the global temperatures.

In particular, nighttime temperatures are warmer, on the order of half a degree. This is because without the turbine the ground cools down faster than the air above. The turbine mixes up the atmosphere and adds energy to the ground from the warmer air above. Fruit growers use this mixing theory to prevent early autumn frosts from damaging plants.

Since wind turbines extract kinetic energy from the air that flows through them, we should expect a change in the local wind. This will be a local, not a global impact. The wind slows down and is more turbulent behind the wind turbine. That way turbines are not lined up one behind another. You might predict this from your experiences with wind blowing around an obstacle like a building. The wind downwind of the building is gustier then the air in a nearby open field.

Category: Climate, Meteorology

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When is the last frost?

Posted on May 19, 2014 by WeatherGuys Editor

The growing season is often defined as the number of days between the last time the temperature was 32 degrees in spring and the first time the temperature falls to 32 in fall.

The latest frost in spring is important to gardeners as they seek to plant after a long winter, and for most plants it is wise to do so when the temperatures are likely to remain warmer than 32 degrees.

For our region, based on temperature observations between 1981 and 2010, the median date of the last frost is between May 1 and May 10. That is why many gardeners consider Mother’s Day a good day to plant.

One reason for the variation in the last frost date is how the first frost date is defined.

Giving the median date of last frost means that there is still a 50 percent chance that a frost will occur after this date. Some definitions require that there is only a 10 percent chance of a 32 or lower temperature.

Some sites use a different 30-year period, for example, between 1951 to 1980. As it turns out, our nighttime minimum temperatures have been getting warmer, so the last frost date has been moving earlier in the spring.

The change is not much for Dane County, but in northwestern Wisconsin, the last frost date now occurs about two weeks earlier than it did in 1950.

According to the Wisconsin State Climatology Office, for Madison, based again on the 1981-2010 time period, the median date for a temperature of 32 is May 2, and it isn’t until May 20 that the probability of an occurrence of 32 has dropped to 10 percent. On April 18, there’s a 90 percent chance of a later occurrence of 32 degrees.

So, if you have not yet done your planting, now would be a good time.

Category: Climate, Seasons

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