Read A Barometer

How a Barometer Works

Enjoying the use of your analog barometer – with the needle!

Your barometer is just as accurate a weather forecaster as the TV meteorologist you watch with the news. Under most conditions it forecasts weather for the 12 to 24 hours ahead. Its mission is to measure air or atmospheric pressure and indicate its rising or falling.

The pressure is shown on your barometer’s dial, usually expressed in “inches” referring to “inches of mercury” (inch Hg). The early barometers measured the pressure by the rise or fall of a column of mercury. This mercury barometer was invented back in 1643 by a pupil of Galileo named Evangelista Toricelli.

Meteorologists use “millibars” in charting atmospheric pressure and your barometer has a second scale or ring which reads in millibars (mb), or “hectopascals” (hPa). The latter is used in some countries as a unit of pressure. Standard air pressure at standard elevation (sea level) at 15°C and 45° latitude is 1013 hPa or 29.92 inches of mercury.

Most dial type barometers employ an air pressure sensor with a limited range of sensitivity. Generally the working altitude range covers only about 3000 to 4000 feet. So be sure to choose a barometer that has a sensor range factory set for use at your elevation (e.g. 0-3000 ft., 3000-7000 ft. or 7000 ft and above).

Looking at setting your barometer…

When you receive your barometer it must be adjusted before being put to use and a local weather broadcast will give barometric pressure for your approximate location. The adjusting screw is located in a hole in the back of the barometer and can be reached with a small screw driver to adjust your instrument to this reading.

Turn the adjusting screw while observing the face of the dial so the indicating hand moves clockwise to match the broadcast reading for your elevation. Tap the display lightly and further adjust the hand if necessary to the desired setting.

Barometer BackBarometer Front

The center knob on the barometer’s front moves the set hand. Use it to mark or place over the location of the black inside hand at the time you set the barometer. This outside set hand will then serve as a reference marker so you can easily see if pressure is rising, falling or steady.

Your barometer’s reading may not exactly coincide with a broadcast due to difference in location, time of reading and other factors.

What about correcting to sea level?

Note that when using a weather barometer (usually with a scale range between 28 and 31 inches Hg) at a location above sea level, the reading must be corrected back to sea level. This is automatically accomplished when you initially match your barometer’s reading to that reported by local TV or radio weather forecast. These reported readings have already been “corrected” to sea level, thus eliminating any pressure differences due to elevation.

Weather forecasting today – a scientific activity.

The chart below illustrates how to interpret your barometer’s inches of mercury reading. This provides a reasonably accurate forecast for the next 12 to 24 hours.

Barometric Reading

– Forecast

Over 30.20″
Rising or steady
– Continued fair
Slowing falling
– Fair
Rapidly falling
– Cloudy, Warmer
29.80″ to 30.20″
Rising or steady
– Same as present
Slowing falling
– Little change
Rapidly falling
– Precipitation likely
Under 29.80″
Rising or steady
– Clearing, cooler
Slowing falling
– Precipitation
Rapid falling
– Storm

 

Note:

  • The words, “stormy”, “rain”, “change”, “fair”, “dry”, etc. are traditional carry overs from the days when weather forecasting was less of a science, and should not be used in predicting weather changes.
  • Do not expect large changes in the position of the indicating hand. Normal daily changes may be in a range of .02 to .10 of an inch on the barometer scale.
  • Changes in pressure readings will be of greater magnitude in winter months than in summer and will also depend on location and altitude.

Metric

bar
0.03386
kilopascal (kPa)
3.386
hectopascal (hPa)
33.86
megapascal (MPa)
0.003386
millibar
33.86
pascal (Pa)
3,386
kilogram per square centimeter (kgf/cm2)
0.03453
kilogram per square meter (kgf/m2)
345.3
newton per square meter (N/m2)
3,386
kilonewton per square meter (kN/m2)
3.386
meganewton per square meter (MN/m2)
0.003386
newton per square centimeter (N/cm2)
0.3386
newton per square millimeter (N/mm2)
0.003386

Download the chart here!

 

Air Pressure:
Air pressure, also known as atmospheric pressure, is the weight of air above a given point. Air pressure is usually measured with an instrument called a barometer (see Barometer).

Changes in the air pressure or barometric pressure mean a change in weather. Barometers are used to predict the weather depending on whether the air pressure is rising or falling. Higher pressures usually mean clear weather because air is sinking toward the earth. Lower pressures usually mean bad weather because air is rising.

Barometer:
A barometer is an instrument used to measure air pressure. Air pressure is usually expressed in units of millibars (mb) or inches of mercury (in. Hg.). At sea level, standard air pressure is 1013.25 mb. or 29.92 in. Hg. Changes in the air pressure or barometric pressure mean a change in weather. Barometers are used to predict the weather depending on whether the air pressure is rising or falling. Higher pressures usually mean clear weather because air is sinking toward the earth. Lower pressures usually mean bad weather because air is rising.
Cloud Formation : 
Clouds form as air rises, expands and cools. This expansion and cooling causes condensation to occur. Condensation is simply the process by which water vapor turns to water. If there is dust, pollution or small particles in the air, the water vapor will condense around these small particles and form clouds.
Conduction:   
Conduction is the transfer of heat from an area of warmer temperature to an area of colder temperature. Heat is transferred from a substance or through a substance by molecular activity. A hot parking lot or a warm ocean can conduct heat to the air above it.
Convection:  
Convection is the motion of air upward, caused by heating. For example a hot parking lot can heat the air above it and this heating causes the air to rise.
Fronts:   
A front is the transition zone between two different air masses with different temperatures and humidity levels. There are several different types of fronts:1) Warm Front – an area in which warm, moist air from tropical areas is replacing colder, dryer air from the poles. Warm fronts are drawn on a weather map as a solid red line with half circles indicating the direction the front is moving. Warm fronts typically move at about half the speed of cold fronts.2) Cold Front – an area in which cold, dry air from the poles is replacing warm, moist tropical air. A cold front is shown on a weather map as a solid blue line with triangles showing the direction the front is moving. Cold fronts typically move twice as fast as warm fronts.

3) Stationary Front – a front that has no movement, with cold air on one side and warm air on the other. This front is indicated on a weather map by an alternating red and blue line.

4) Occluded front – when a cold front overtakes a warm front. This type of front is shown on the weather map as alternating cold-front triangles and warm-front half circles.

Humidity:
Humidity is the amount of water vapor in the air. Relative humidity is the amount of water vapor that is currently in the air compared to how much the air can hold at a specific temperature. Relative humidity is measured in percents, with 100% being the highest amount of water vapor the air can hold at a particular temperature. Fifty percent humidity would be half the amount of water vapor the air can hold at the same temperature. Humidity is measured using a hygrometer. Humidity can greatly contribute to your discomfort on a hot, muggy day. Have you ever heard the phrase, “It’s not the heat, it’s the humidity”? Humidity can make a hot day seem hotter since humidity prevents sweat from evaporating from out bodies as fast as it would in low humidity. When sweat evaporates, it cools us. So a very humid day can feel a lot hotter than a day of the same temperature with low humidity.
Precipitation:
Precipitation is any form of water that comes out of the atmosphere and hits the ground. Precipitation could be drizzle, rain, snow, sleet or hail. The form the precipitation takes is determined by the temperature of the air near the ground and up in the atmosphere where the clouds are. Did you know that much of the precipitation the world over begins as snow but thaws before it reaches the ground as rain? If the air below such clouds weren’t warm, imagine how many snow days you might take off school! Precipitation is often measured using a rain gauge.
Radiation:
Radiation is also known as radiant energy. On earth, most radiant energy comes from the sun and is responsible for warming the air and the land. The majority of the sun’s energy is reflected back into the atmosphere, but some is absorbed by land, water, air and clouds.
Temperature:  
Temperature is the amount of heat (hotness or coldness) of an object or substance such as, you guessed it, air! Temperature can also be thought of as a measure of the speed of atoms and molecules. As energy is added, molecules and atoms move faster and the temperature increases. As energy is released, atoms and molecules move more slowly and the temperature decreases. This is an important idea to remember when it comes to weather! It explains a lot of what happens when weather changes and why it changes the way it does. The sun is the source of the energy that warms the air, oceans and land surfaces on earth. Temperature is measured using a thermometer (see Thermometer).
Thermometer:
Temperature is measured using a thermometer and the units of temperature are usually given as degrees Fahrenheit or degrees Celsius, also known as Centigrade (although that term is not used much anymore). Most scientists use Celsius units, as do meteorologists in other countries. People in the United States seem to prefer Fahrenheit, so most TV weather people use that scale.
Wind:   
Wind is air moving over the earth’s surface. Most winds are caused by either topography or the movement of high and low pressure systems, or fronts. If two fronts come together that are not very different in humidity and temperature, gentle winds will result. In areas where fronts of very different temperature and humidty come together, high winds will result. Winds on the ground can be very different from winds high up in the atmosphere. Winds that are high up are affected by the rotation of the earth more than by high and low pressure systems.

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