Weather's Effect on Failure

Weather can play an important role in failures and accidents. Some effects are quite well known, such as:

• Ice and snow leading to increases in traffic accidents.
• High winds leading to structural failures.
• Low temperatures leading to freezing and bursting pipes.
• High temperatures leading to pavement buckles.
• Persistent rains leading to floods.

Other effects are not as well known. For example:

• Increasing the water content of some soils leads to a decrease in shear strength. As a result, heavy rains can lead to sudden soil subsidence and damage to overlying structures.
• Some clays swell dramatically when moistened, leading to foundation and roadway damage.
• The impact strength and fracture toughness of many steels undergoes a sharp decline at low temperature. This can lead to a brittle fracture in cold weather under loads that could otherwise be supported without failure.
• Bituminous materials (pitch, tar, asphalt) can soften in hot weather and, under load, can slowly flow and deform out of their original shape.

These effects, and others, often make it necessary for the failure investigator to determine the weather conditions at the time of the accident or for some period of time before the accident. The archives of local newspapers can be a good source for th is data, but a convenient and authoritative source is the National Climatic Data Center. The NCDC collects and maintains weather records from hundreds of stations across the country and will send you, for a small fee , data for the stations and dates you request. The available data varies from station to station, but typically includes an hourly reading of the temperature, wind speed and direction, relative humidity, rainfall and snowfall totals, and cloud conditions.

If you don't need hourly data, but do need daily summaries, the NCDC has set up an ftp site for downloading what they refer to as First Order Daily Summaries. These typically include the high and low temperature for the day, the peak wind speed and direction, the average wind speed and direction, the total snowfall, and the total precipitation in equivalent inches of water. Other information such as total minutes of sunlight and depth of snow on the ground may be included, depending on the station.

The data file for each station is given a name of the form

fsod_ascii.nnnnn

where nnnnn is the five-digit code that identifies the station. The files are in ASCII (plain text) format and, once downloaded, can be read by any word processor. To determine where the stations are and what their codes are, you must first download the station list file. This will give you a list of all the station names, their locations, their codes, and the dates for which data is available. Once you've downloaded the data for the station you're interested in, you can decode the information using the instructions given by the NCDC.

To demonstrate the first order daily summary data, we've created a form you can use to gather temperature, wind gust, and precipitation data for the years 1990-1994 for the twenty-five largest metropolitan areas in the U.S., as detemined by the 1990 census. Choose the city and date from the popup menus and click on the "Submit Query" button. A new page will appear with the results of the search. You can return to this page by using the "back" button on your browser.

Note: The data for Cincinnati does not include wind gust information, so you will get "???" for gust speed and direction.