How is the Dew Point Measured?

Posted on August 1, 2011 by Weather Guys Editor

One way to measure the dew point is based on the observation that a bathroom mirror will “fog up” when the temperature equals the dew point during a bath or shower.

A dew point hygrometer consists of a small mirror, a narrow light beam and a light detector. The light beam is reflected off the mirror into the detector, which measures the brightness of the reflected light. When the mirror surface cools to the dew point temperature, dew forms on the mirror.

The water droplets on the chilled mirror block light from reaching the detector, causing the reflected light beam to look less bright. The reduction in the measured reflected light indicates the mirror is at the dew point temperature.

A psychrometer is an alternative instrument for determining the dew point. A psychrometer consists of two thermometers, one of which has a wet wick around its bulb and is called the wet-bulb thermometer.

Evaporation of water off the wick removes heat from the thermometer. The temperature of the wet-bulb thermometer drops according to the rate of evaporation, which depends on the dew point.

The thermometers have to be ventilated by either whirling the instrument around or using a fan.

After a few minutes, the temperature of the wet-bulb stabilizes at a particular temperature.

The temperature difference between the two thermometers is converted into a dew point temperature using a chart.

Category: Meteorology
Tags , , , , ,
Comments Off on How is the Dew Point Measured?

What is a Heat Wave?

Posted on July 24, 2011 by Weather Guys Editor

Because our reaction to weather is relative to the climate we live in, there are a few definitions of a heat wave. All of the definitions indicate it as a period of abnormally and uncomfortably hot and usually humid weather.

The World Meteorological Organization is specific in its definition by stating that a heat wave is when the daily maximum temperature for more than five consecutive days exceeds the average maximum temperature by 9 degrees.

Heat waves are caused by very hot, stagnant air masses. Regions that suffer under intense hot spells are usually dominated by a surface high-pressure system with a mid-tropospheric ridge aloft. Dew points are also high, and to compound matters, wind speeds are often low.

Clear or partly cloudy skies allow intense solar energy to further heat the ground and the air mass. During the heat wave of last week, Wisconsin experienced heat index readings that were over 100 degrees.

High humidity and stagnant air reduce the body’s ability to cool down through sweating. Lives are endangered when these conditions persist day and night for several days. Each summer in the United States, approximately 175 to 200 deaths are attributable to heat waves. Most of these deaths occur in cities, particularly northern cities.

Heat waves also have a strong economic impact. A prolonged heat wave can cause the widespread use of air conditioning, leading to increased demands for power that stress gas and electric utilities. Transportation can be stymied when highway surfaces and railways buckle and warp in the heat. All types of outdoor work, such as landscaping and construction, experience reduced productivity. Agriculture is especially vulnerable as heat waves stunt crops and kill livestock.

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

What are Straight-line Winds?

Posted on July 17, 2011 by Weather Guys Editor

The term “straight-line winds” is used to describe ground-level winds that come out of a thunderstorm and do not have rotation. If these winds travel at speeds that exceed 57 mph, then the storm is classified as a severe thunderstorm. Storms with severe straight-line winds can also have hail and tornadoes.

Straight-line winds can cause considerable damage because these winds often do not let up. Straight-line winds will push objects over, all in the same direction as the wind is blowing.

The lack of rotation, or spin, in these straight-line winds allows meteorologists to differentiate damage from tornadic winds. Tornadoes scatter objects all over because they rotate so quickly.

Straight-line winds can be hazardous as they can push over objects that land on top of people, causing injury and death. On July 1, straight-line winds blew down trees in forested areas of northwest Wisconsin. Many areas had wind speeds of greater than 60 mph, with some wind speeds greater than 100 mph.

Thunderstorms have upward air motions, called updrafts. These supply warm moist air to the storm and help to form the precipitation. There are also downdrafts, or sinking air in a storm. Such downdrafts carry air from high elevations in the atmosphere rapidly to the ground. Since wind speed is nearly always much larger at high elevations, the downdrafts carry very high momentum air to the surface creating the straight-line winds.

Downdrafts also carry liquid water with them. When these downdrafts hit a region of dry air, such as below the cloud base, the drops evaporate. This cools the air in the downdraft, making it denser and thus causing the air in the downdraft to fall to the ground faster. You can sometimes notice this blast of cool air at the surface, often before it rains.

Category: Severe Weather
Tags , , ,
Comments Off on What are Straight-line Winds?

What is the Midwest’s hottest summer on record?

Posted on July 10, 2011 by Weather Guys Editor

Though it has been hot and humid lately in southern Wisconsin, this summer has been relatively mild compared to previous summers.

Since June 1 in Madison, the daily average temperature has been 1.8 degrees above normal and we have had four days with high temperatures hotter than 90 degrees (and three more at 89 degrees).

In 1988, Madison had 35 days with high temperatures over 90 degrees (but only 11 before July 8 ) with one stretch of nine straight days over 90 degrees from late July through the first week of August.

The summer of 1955 has the record of 40 days with highs over 90 degrees. For comparison, the average number of such days in Madison over the last 15 years has been 5.6 per year, with zero occurring in 2004.

The summer of 1936 easily stands as the hottest summer in the modern history of North America. First, in Madison, the hottest day of all time occurred on July 13, 1936, when the temperature reached 107 degrees (the all-time state record of 114 degrees was set the same day at Wisconsin Dells). That second week of July 1936 was so unbearably hot that residents of Madison elected to sleep outside on the Capitol Square seeking relief from the heat of their homes.

Across the Midwest it was even worse. In Steele, N.D., the all-time state record of 121 degrees was set July 6. Chicago’s Midway Airport recorded high temperatures of 100 degrees or higher on 12 consecutive days from July 6-17. Later that summer a string of 18 consecutive days at or above 100 degrees was recorded in Mount Vernon, Ill., from Aug. 12 to Aug. 29. Finally, Lincoln, Neb., recorded a daily minimum temperature of 91 degrees on July 25.

So despite the recent heat, it could be much worse.

Category: Climate
Tags , , ,
Comments Off on What is the Midwest’s hottest summer on record?

Do we have a new ‘normal temperature’?

Posted on July 3, 2011 by Weather Guys Editor

Yes, the entire nation now has a new ‘normal temperature.’

These climatological temperatures, and other weather parameters, are computed by averaging all temperatures over a 30-year period. These averages are called normal temperatures. These averages serve as a reference point and are used to help us interpret average climate conditions at a particular location. A comparison of today’s temperature with the normal temperature helps us determine if today is an atypical weather day.

Private industry also uses these temperatures in planning. For example, energy companies use the normal temperature for long-term planning of energy usage. Agriculture uses this as they monitor a particular growing season.

The National Ocean and Atmosphere Administration’s National Climatic Data Center (or NCDC) calculates the normal weather conditions over a 30-year period for more than 7,500 locations in the United States.

Since this time period is a reference point, we have to define the 30-year period. As of July 1, this averaging period is 1981-2010. Prior to that date, the averaging period was 1971-2000. So, what does this new period tell us?

The normal temperature for the entire US is about 0.5 degrees warmer now than it was during the 1971-2000 time period. The normal low temperature for WI is about 0.8 degrees warmer now than it was in the 1971-2000 period; and Wisconsin’s normal high temperature is about 0.6 degrees warmer. According to Assistant State Climatologist, Dr. E. Hopkins, the new normal high and low temperatures for Madison are 56.6 and 36.7, which are 0.2 and 1.2 degrees higher than the previous 30-year period.

Category: Climate
Tags , ,
Comments Off on Do we have a new ‘normal temperature’?