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Measurement of Atmospheric Pressure
Constant pressure charts and hurricane pressure reports are written using millibars (mb). Since weather stations are located around the globe, all local barometric pressure readings are converted to a sea level pressure to provide a standard for records and reports. To achieve this, each station converts its barometric pressure by adding approximately 1 "Hg for every 1,000 feet of elevation gain. For example, a station at 5,000 feet above sea level, with a reading of 24.92 "Hg, reports a sea level pressure reading of 29.92 "Hg. Using common sea level pressure readings helps ensure aircraft altimeters are set correctly, based on the current pressure readings. In order to compensate for pressure variations due to different station elevations, all observations are mathematically corrected to mean sea level (MSL). Altimeter settings are obtained by mathematically reducing station pressure to MSL. This enables the pilot to read MSL altitudes on the altimeter.
When charting atmospheric pressures over various areas of the Earth, the meteorologist is primarily interested in the pressure difference per unit of distance—the pressure gradient.
The MSL pressure is plotted in mb at each reporting station on a surface weather map. The isobars outline pressure areas in somewhat the same manner that contour lines outline terrain features on contour maps. Standard procedure on surface weather maps in North America is to draw isobars spaced at four mb of pressure apart, with intermediate two mb spacing when appropriate. Although the isobar patterns are never the same on any two weather maps, they do show patterns of similarity.
By tracking barometric pressure trends across a large area, weather forecasters can more accurately predict movement of pressure systems and the associated weather. For example, tracking a pattern of rising pressure at a single weather station generally indicates the approach of fair weather. Conversely, decreasing or rapidly falling pressure usually indicates approaching bad weather and possibly severe storms.Temperature
Temperature is a measurement of the amount of heat and expresses a degree of molecular activity. Since different substances have different molecular structures, equal amounts of heat applied to equal volumes of two different substances will result in unequal heating. Every substance has its own unique specific heat. For example, a land surface becomes hotter than a water surface when equal amounts of heat are added to each. The degree of “hotness” or “coldness” of a substance is known as its temperature, and is measured with a thermometer.
The Earth’s surface is heated during the day by the sun. This incoming solar radiation is called insolation, while heat radiated from the Earth by outgoing radiation is called terrestrial radiation. The cooling that occurs at night is terrestrial radiation. Temperature Scales
Two temperature scales are important to the balloon pilot: Fahrenheit (F) and Celsius (C). On the Fahrenheit scale, the freezing point is 32° and the boiling point is 212°, a difference of 180°. On the Celsius scale, the freezing point is 0° and the boiling point is 100°. For many years, the Celsius scale was the choice for technicians and those countries and organizations utilizing the metric system. In recent years, the United States has transitioned to almost exclusive use of the Celsius scale in weather reports, primarily because of the International Civil Aviation Organization (ICAO) convention agreements. [Figure 4-2]
A quick and easy way to convert Fahrenheit to Celsius is to subtract 30, and divide the number by two. To convert Celsius to Fahrenheit, double the number, and add 30. These formulas give a good approximation for most calculations in ballooning. Conversion charts are also available on the Internet.
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Temperature Variations
The amount of solar radiation (insolation) received by any region varies with the time of day, with seasons, and with latitude. These differences in insolation create temperature variations. Temperatures also vary with differences in topographical surface and with altitude. These temperature variations create forces that drive the atmosphere in its motion. Simply stated, heat and, therefore, temperature differences cause weather.
Diurnal variation is the change in temperature from day to night brought about by the daily rotation of the Earth. The Earth receives heat during the day by insolation, but continually loses heat by terrestrial radiation. Warming and cooling depend on an imbalance of insolation and terrestrial radiation. During the day, insolation exceeds terrestrial radiation and the surface becomes warmer. At night, insolation ceases, but terrestrial radiation continues and cools the surface. Cooling continues after sunrise until insolation again exceeds terrestrial radiation. Minimum temperature usually occurs after sunrise, sometimes as much as one hour after. The continued cooling after sunrise is one reason that fog sometimes forms shortly after the sun is above the horizon.
　
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