I've often said that observations are at the heart of weather forecasting. That's why the National Weather Service maintains a network of about 1000 weather observing sites across the country. Collectively, this group is known as the "first-order" network, and most sites are at airports or military bases. There are several first-order observing sites in our area - they're indicated with dots on this map.

Each site is assigned a three-character code - these are the same city codes airlines use to route your luggage. For example, ACY is the Atlantic City Airport, while PHL is Philadelphia International. Wilmington is ILG - the actual observation site is at the New Castle County Airport. Other local first-order sites include the Northeast Philadelphia Airport, Dover Air Force Base, and the Mercer County Airport in Trenton.

First-order observing sites report weather conditions on the hour, and sometimes more frequently if the weather is changing rapidly. Typically, temperature, humidity, pressure, wind, cloud cover, visibility, and rainfall are observed. In most cases, the measurements are made by an automated set of instrumentation known as ASOS, for Automated Surface Observing System. But ASOS can't measure some things, such as snowfall, and that still has to be done by human observers.