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.  A person standing at sea level also experiences the pressure  of the atmosphere, but the pressure is a force of pressure  over the entire surface of the skin. The actual pressure at a  given place and time will differ with altitude, temperature,  and density of the air. These conditions also affect balloon  performance, especially with regard to useful load and burner  performance.  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.  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] 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. 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.  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  4-3 $_{Figure 4-2.}$  Comparison of Fahrenheit and Celsius temperature  scales. Temperature Temperature Scales Measurement of Atmospheric Pressure Water boils Water freezes Dry ice (solid CO$_{2}$) Liquid air Absolute zero Fahrenheit Celsius 212 °F 98.6 °F 32 °F –40 °F –108 °F –312 °F –459 °F 100 °C 50 °C 0 °C –40 °C –78 °C –200 °C –273 °C