An environmental lapse rate refers to the rate by which temperature decreases with altitude. It is often calculated so as to predict the behaviour of air parcels in the lowest layer of the atmosphere, the troposphere, which accounts for 80 per cent of the atmosphere's mass and provides the stage for most of its active weather patterns. Temperature changes throughout the entirety of the atmosphere are used to divide it into its standard layers: For example, temperature tends to increase with ascent through the stratosphere, which lies above the troposphere, then decrease in the next highest layer, the mesosphere--and then increase again in the topmost layer, the thermosphere.
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Equip a helium or hydrogen balloon with a thermometer. Meteorologists, such as those with the National Oceanic and Atmospheric Administration's National Weather Service, typically embed these thermometers with other equipment in an instrument parcel called a radiosonde.
Include a radio transmitter in the radiosonde that is coordinated with ground-based receivers. This allows temperature readings--as well as other data, such as wind speed, atmospheric pressure, and relative humidity--to be sent in real time from the aloft weather-balloon instruments to meteorologists.
Collect the radiosonde's temperature readings during its flight. National Weather Service (NWS) balloons ascend to heights of better than 115,000 feet and travel more than 100 miles from their release point; the flights end when the balloons burst at a certain diameter (usually 20 to 25 feet, according to the NWS). The radiosonde floats to the ground with the aid of a parachute.
Calculate the average rate of change of temperature in ascent by dividing the change in temperature by the change in altitude. Normally, the temperature decreases at a particular rate with increasing altitude. The average lapse rate in the troposphere is about -15.78 degrees Celsius per 1,000 feet.
Compare the observed environmental lapse rate with the changes in temperature of a rising parcel of air, referred to as the adiabatic lapse rate. This rate differs for dry and moist air parcels and impacts the stability or instability of the atmosphere. For example, an air parcel with a dry adiabatic lapse rate less than the present environmental lapse rate will cool down more slowly than the surrounding air mass if shunted into ascent. Being thus warmer than its surroundings, the parcel will be less dense and will continue rising.
Tips and warnings
- Identify such variations from the norm as temperature inversions by recognising deviations from the average in the rate of change. A temperature inversion occurs when temperature rises with increasing altitude, as may happen when cold air becomes trapped in a steep-sided valley bottom.
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