When does the Overnight Low Temperature Occur on a Windless Day?

Posted on August 21, 2011 by Weather Guys Editor

As the summer begins to fade we have begun to have the first cool nights of the season. When the sky is clear, the winds are calm, and the ground is dry as the sun sets, the conditions are perfect for an interesting phenomena to occur the next morning.

If you pay careful attention to the temperatures in the hour surrounding sunrise the next morning, you will most likely observe that the lowest temperature will be recorded some minutes after the sun rises. This repeatable observation requires explanation.

All objects both absorb and emit energy in the form of radiation. The surface of the Earth is one such object and it absorbs a lot of solar energy from sunlight during the daylight hours. We all know that this causes the surface temperature to increase. But, while the ground is absorbing energy from the sun, it is also emitting radiant energy (infra-red energy).

As long as the amount emitted is less than the amount absorbed, the temperature of the surface increases. After sunset, absorption ceases but emission continues. Consequently, since the emission is greater than the non-existent solar absorption, the surface temperature drops. This condition persists throughout the night.

When the sun finally rises and sends the first, low-intensity beams of radiation to the surface, it is still true (for a short time) that the meager amount of solar absorption is smaller than the emission from the surface.

Until the absorption is equal to the emission, the temperature continues to fall, even if only slightly.

Hence, the lowest temperature on such a morning occurs after sunrise.

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How Uncomfortable has this Summer Been?

Posted on August 14, 2011 by Weather Guys Editor

After what seems like forever to most of us, this past week finally brought some relief from the almost uninterrupted heat and humidity of July and early August.

One way to measure the degree of discomfort of such a spell is to examine how many overnight low temperatures dropped below 60 degrees. In order for the overnight low to get below 60 degrees, the dewpoint temperature (which we have discussed as a measure of human comfort in this column before) must also get below 60 degrees.

Our sense is that most people begin to notice the humidity when the dewpoint is 60 degrees, are somewhat uncomfortable when it is 65 degrees, and are downright miserable when it is at or over 70 degrees. Perhaps not surprisingly, no one remembers June 2011 as particularly muggy, the month had 18 days on which the overnight low dropped below 60 degrees.

July, on the other hand, had only five such nights and, through Aug. 10, only one such night has occurred this month. Thus, beginning June 30, we have had only six out of the last 42 days with dewpoints low enough to drop the overnight low temperature to below 60 degrees. That is pretty darn oppressive!

During that same stretch, we have been 4.97 degrees above normal in temperature and 2.5 inches below normal in precipitation. That further testifies to the heat and the fact that it has not been broken up by many cooling thunderstorm events here in Madison.

Over the next 10 days or so it appears that we will continue to enjoy a relief from the hot July so the worst may be behind us.

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Is High Pressure Always Warm Air?

Posted on August 7, 2011 by Weather Guys Editor

High pressure systems can be cold or warm, humid or dry. The origin of a high-pressure region determines its weather characteristics.

If a high-pressure system moves into Wisconsin from the south during the summer, the weather is usually warm and clear. If the high pressure originates from the north, it will generally bring cold or cooler weather.

When high pressures form, they adopt the characteristics of the source regions over which they form. Cold, high-pressure air masses form in polar regions, and are called polar air masses. Warm air masses are of subtropical or tropical origin; both are referred to as tropical air masses.

Air masses that form over water are referred to as maritime, whereas those generated over continents are referred to as continental. Maritime air masses are usually moister than continental air masses formed at the same latitude.

Atmospheric pressure is greater in a high-pressure system than the surrounding areas. High-pressure areas at ground level are normally caused by air above that is moving downward. This sinking air, called subsidence, is bad for cloud formation, so high pressure systems often have few clouds.

You may have also noticed that the winds are light when we are under the influence of a high pressure. The lack of wind causes pollutants to build up in the high, leading to reduced air quality. Few clouds also mean good sunshine that, along with the warm temperatures in the summer, is good for ozone formation at the ground.

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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.

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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.

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