Science and Weather Quiz | Reading Forecasts, Weather Maps and Observations, Set 3

An isobar joins points of equal atmospheric pressure on a weather chart. It is different from a contour line, which joins equal elevations, and an isotherm, which joins equal temperatures. Surface weather charts normally use sea-level pressure so that different locations can be compared.

A high-pressure system is an area where pressure at the same altitude is higher than its surroundings and is enclosed by closed isobars. There is no fixed hPa threshold, so A can be a high even at 1012 hPa. B cannot be classified from its number alone, especially when its pressure is below that of its surroundings.

Station pressure decreases as elevation increases, so comparing raw values can mistake terrain effects for a pressure system. Surface weather charts therefore normally use pressure reduced to sea level. A's 900 hPa station pressure alone does not prove that it is the center of a low-pressure system.

Satellites provide crucial observations of cloud distribution and movement, but those observations alone do not determine the future atmosphere. The JMA feeds observations from around the world, including surface and upper-air data, into numerical prediction and calculates changes from physical laws. Observation and prediction have different roles.

Probability of precipitation is the chance that any given point in the forecast area will receive a total of at least 1 mm during the specified period. It does not state what proportion of the time it will rain, what proportion of the area will be covered, or how much rain will fall. A 70% value alone therefore cannot tell you whether rain will be brief or continuous.

Humidity can vary locally with indoor or outdoor conditions, nearby grass or ponds, wind direction, and other surroundings, so a single forecast value is difficult to apply broadly. The JMA observes current humidity but does not issue a general numerical humidity forecast; it uses dry air advisories when dry conditions are expected to persist. This does not mean humidity cannot be measured.

AMeDAS is the Automated Meteorological Data Acquisition System, whose ground stations automatically observe precipitation, wind direction and speed, temperature, humidity, and related variables. Not every station measures every variable. Weather radar, by contrast, uses radio waves to detect the spatial distribution of rain and snow.

Radar uses the round-trip travel time of a radio wave to find the distance to rain or snow and the strength of the reflected wave to estimate precipitation intensity. Doppler radar can also use a frequency shift to observe movement in a precipitation area. The key is not to reverse the information used for distance and intensity.

Anomalous propagation and reflections from terrain or structures can create radar echoes where no precipitation exists. Quality control removes many such echoes but cannot remove every one, so an unusually fixed echo should be compared with ground observations and nearby motion. This limitation does not make radar data as a whole useless.

For atmospheric pressure, 1 mb and 1 hPa are numerically equal, so 1013 mb corresponds to 1013 hPa. As part of the transition to SI units, the JMA changed its pressure unit from mb to hPa on December 1, 1992. The unit name changed, but no factor of ten or one hundred is needed here.