What sensors indicate the weather: part 2

With extreme weather conditions, people directly impacted by the pending situation need to know how severe it will be. For hurricanes, the Saffir-Simpson Hurricane Wind Scale provides a rating from 1 (very dangerous winds that will produce some damage) to 5 (catastrophic damage will occur) based on a hurricane’s maximum sustained wind speed. This approach has been used for decades to categorize hurricanes. However, it provides no indication about storm surge, inland flooding, tornadoes, storm size or storm duration over a community.

In a recent study, a hurricane season forecaster proposed that the amount of damage that a hurricane will cause is more appropriately related to a storm’s minimum sea-level pressure. A rising or falling surface pressure is commonly used to indicate if the weather is getting better or worse. Surface pressure is also used as a common test of strength for hurricanes and storm systems around the planet. Surface water temperatures are also used to determine the strength of an approaching hurricane.

For example, recent hurricane Helene gathered strength from the unusually warm surface water temperatures of the Gulf of Mexico that exceeded 84 °F (28 °C) and were nearly 1.8 °F (1 °C) warmer than the 2003 to 2014 mean. As it approached Florida, Helene was reclassified as a category 2 hurricane, then a category 3 hurricane, and finally a category 4 hurricane within only a few (<10) hours. During this time, its central surface pressure dropped from 960 millibars to 947 millibars and its maximum sustained winds increased from 100 miles (155 km) per hour to 130 miles (215 km) per hour.

Where and how are the measurements made?

Weather balloons and satellite imagery are two of today’s basic means of obtaining data for weather forecasting but there are other sources, too.

Worldwide, balloon observations are commonly made daily at 00Z and 12Z (6 a.m. and 6 p.m. EST) using radiosondes. In the United States, the National Weather Service (NWS) launches weather balloons twice every day, from nearly 100 locations. During these events, radiosonde sensors measure atmospheric pressure, temperature, humidity, wind speed, and direction.

A battery-powered radio transmitter sends the sensors’ data to a ground receiver. Tracking the position of the radiosonde in flight with the Global Positioning System (GPS) also provides measurements of wind speed and direction aloft.

In addition to these direct atmospheric measurements, the NWS also collects data by “remote sensing.” These measurements come from devices sensitive to electromagnetic energy such as light and heat (via weather satellites) and radio waves (via Doppler radar). This remote sensing provides observations of large regions.

Another important data source for numerical weather prediction (NWP) models is automated weather reports from commercial aircraft or Aircraft Meteorological Data Reports (AMDAR). This data includes static and total air pressure measured by an electronic barometer in the aircraft’s pitot static probe, air temperature measured by an immersion thermometer probe, wind speed calculated from the air speed (through the pitot static probe measurement), and ground speed (usually provided by an inertial navigation system).

An additional source of information comes from buoys located in coastal and offshore waters in different areas. Moored buoys measure and transmit barometric pressure, wind direction, wind speed and gusts, air temperatures, and sea temperatures as well as wave energy spectra to determine wave height, dominant wave period, and average wave period.

While many of the measurements are the same or similar, the actual means of obtaining them can vary greatly. When they are properly combined, weather forecasting improves significantly.