What is the Heat Index?

Posted on June 19, 2011 by Weather Guys Editor

The heat index, or apparent temperature index, indicates how hot it feels. It is expressed as a function of air temperature and the relative humidity.

When our bodies get hot we cool down by sweating. It is not the sweating that cools our bodies; it is the evaporation of the sweat. If the air has a high humidity, then the rate of evaporation is reduced. This hampers the body’s ability to maintain a nearly constant internal body temperature. This is why we are uncomfortable on hot, muggy days.

When the temperature is high but the relative humidity is low, the heat index is less than the actual temperature. This is because cooling by evaporation of sweat is very efficient in these situations. However, high relative humidities prevent evaporation and make it seem hotter than it really is. In these cases, the heat index is greater than the actual temperature.

Summer heat waves are often associated with high heat indices and can be dangerous. For example, a heat wave in the Midwest in late July 1999 resulted in more than 200 deaths in the Midwest. The heat wave that affected portions of the northeastern U.S., central Europe, and Russia in the summer of 2010 contributed to the deaths of as many as 15,000 people in Moscow.

The National Weather Service will issue a heat advisory when the heat index is predicted to be 100 degrees. You are then advised to limit vigorous outdoor activity and drink plenty of fluids.

Category: Meteorology
Tags , , , , ,
Comments Off on What is the Heat Index?

What is a Heat Burst?

Posted on June 12, 2011 by Weather Guys Editor

Heat bursts are sudden, usually dramatic increases in the surface temperature, often accompanied by strong winds, that sometimes occur in the vicinity of a thunderstorm. Though they are not an everyday occurrence, they are not exceptionally unusual either.

A recent example from Wichita, Kan., summarizes the characteristics of these interesting phenomena. At 12:22 a.m. on Thursday, the temperature at Wichita’s Mid-Continent Airport was 85 degrees with a dewpoint of 60 degrees. At 12:42 a.m., just 20 minutes later, the temperature was 102 degrees, the dewpoint dropped to 30 degrees and the winds were gusting in the 40 to 50 mph range.

A similar phenomena occurred just west of Milwaukee on Wednesday morning when Milton, Whitewater, Fort Atkinson, and the National Weather Service Forecast Office at Sullivan reported temperature increases of between 7 and 16 degrees, dewpoint decreases of between 8 and 19 degrees, and gusty winds in the 35-55 mph range.

Milton’s temperature went from 76 degrees to 92 degrees while its dewpoint dropped from 65 degrees to 46 degrees while winds gusted to 42 mph.

A line of rain showers with cloud bases at about 12,000 feet moved through the area on Wednesday morning. The air beneath the cloud base had a very low relative humidity and supported rapid evaporation of the falling raindrops. The evaporation led to cooling of the air at cloudbase which rendered it heavier than its surroundings and it accelerated toward the surface.

As the air sinks to lower elevation (higher pressure) it is compressed and warms up — in cases such as this one, the warm-up is extreme and led to the heat burst.

Category: Phenomena
Tags , , ,
Comments Off on What is a Heat Burst?

What are the Light Beams Coming from Clouds Called?

Posted on June 5, 2011 by Weather Guys Editor

Brilliant beams of light extending from clouds are often seen during the early morning or early evening when the sun is shining. This phenomenon is known as crepuscular rays, as it occurs during crepuscular hours — those around dusk and dawn.

Rays of light can change direction when they encounter small particles suspended in the atmosphere. This changing of directions is called scattering. A cloud between you and the sun can block some but not all of the sun’s light. The darker regions are the shadowed area of the cloud. Where the light peeks through the object, scattering illuminates its path from the sun to your eyes. This creates beams in the sky.

These beams appear to converge toward the sun but this is an illusion, similar to the impression that the rails on train tracks appear to come together in the distance.

Mountains can also generate crepuscular rays when they shadow the sun. You also can see this effect in buildings with tall ceilings when the sunlight shines directly through the windows, provided there are enough haze or dust particles in the building so that the sunlight can be scattered towards you. When crepuscular rays spread down to the ground, they are called Jacob’s Ladder, a “ladder to heaven.”

You can occasionally see beams of light converging on the opposite side of the sun, or the anitsolar point. These rays are called anticrepuscular rays and are similar to crepuscular rays.

Category: Phenomena
Tags ,
Comments Off on What are the Light Beams Coming from Clouds Called?

What’s behind the Nationwide Outbreaks of Deadly Tornadoes?

Posted on May 29, 2011 by Weather Guys Editor

The enormous loss of life associated with the 2011 tornado season thus far is truly staggering.

After the Joplin, Mo., tornado May 22 and the recent outbreak in the southern Plains near the end of last week, the number of fatalities has risen to 504.

Over the last 31 years, only two times have tornado deaths risen above 100 for the full season: 130 in 1998 and 125 in 2008.

The unusually high death toll from this season’s storms is likely a result of the fact that some of the most intense storms have made direct hits on large populated areas (Tuscaloosa, Ala.; St. Louis, and Joplin, Mo., are standouts). It is also true that with 1,200 storms already reported, we may be seeing a record season for tornadoes. The current record is 1,819 storms in 2004 with the 31-year average being 1,105.

Currently we are investigating the hypothesis that tropical weather systems many days and thousands of miles upstream of the Midwest can play a central role in shaping the jet stream – a major ingredient necessary for tornado outbreaks. To the extent that this connection exists, then a warmer globe may well increase the likelihood of such outbreaks in the future as stronger springtime jet streams may result from a warming of the sea surface in the western tropical Pacific Ocean.

The atmosphere is a delicately balanced machine and, as hard as it may be to believe at times, slight perturbations to the global temperature may have such notable consequences.

Category: Severe Weather
Tags , , , , , ,
Comments Off on What’s behind the Nationwide Outbreaks of Deadly Tornadoes?

What’s the Outlook for Hurricanes this Year?

Posted on May 22, 2011 by Weather Guys Editor

Hurricane season starts June 1 and runs through November. The National Oceanic and Atmospheric Administration is forecasting an above-average season.

We usually expect the hurricane season to have 11 named storms over the Atlantic Ocean; this year’s forecast is for 12 to 18. Of those, 6 to 10 might become hurricanes with winds of 74 mph or more. NOAA also is forecasting that as many as six of these storms could become major storms with wind of at least 111 mph.

There are a couple of conditions that contribute to this expected above-average season. One is the observation that the average ocean temperatures where hurricanes tend to form and travel is two degrees above normal. Sea-surface temperatures of at least 80 degrees is a required condition for hurricane formation. So, a warmer-than-average ocean means there is extra energy to help form and intensify hurricanes.

A second factor is that the La Nina conditions in the Pacific Ocean, which helped to contribute to our cold winter, are weakening. These changing La Nina conditions result in more favorable conditions for hurricane development over the Atlantic Ocean by promoting environments in which the difference between the surface and upper level winds is smaller than usual.

Hurricanes are named to better communicate forecasts to the general public as names can reduce confusion about which storm is being described. A list of names is generated by the National Hurricane Center. The first four Atlantic hurricanes this year will be named Arlene, Bret, Cindy and Don.

Category: Tropical
Tags , , , ,
Comments Off on What’s the Outlook for Hurricanes this Year?