Even though Bob Dylan sang "You don't need a weatherman to know which way the wind blows" (Subterranean Homesick Blues) it might be advantageous to use a meteorologist to help find and analyze weather information that is crucial to your claim or case.
Like many things, obtaining weather information is considerably more difficult than it seems. I can’t remember the number of times that I have been told by a client, adjuster or attorney that “I already got the weather from the Internet (or the newspaper in the old days)”. I then have to explain (gently) that what they obtained was 1) not from the nearest location, 2) was not a representative location or 3) was not really what they really needed in the first place to substantiate their case. And this applies to all types of meteorological data including temperatures, rain, wind, snow, ice, lightning, hurricanes, tornadoes and floods.
For today let’s look at just temperature data to see how and where it is measured, by whom and what it really represents.
First and foremost, temperature measurements are taken in the shade NOT in direct sunlight. You want to measure the air temperature not the direct energy from the sun! And it can make a huge difference. On a recent project I took a temperature reading in the shade and it was 82 degrees (F). At the same time, a second thermometer that I had set on a bench in direct sunlight was reading 135 degrees! (This is why I yell at the football announcers that show a thermometer sitting in the sun on an Astroturf field and say the temperature “down on the field in 120 degrees”)
Second, temperature measurements should be taken over a natural surface and free of external heat sources. It’s amazing how much “global warming” there was at a site where the parking lot was extended to within five feet of the weather station. The combination of black asphalt and very hot engine made for some exceptionally warm days.
And finally, thermometers should be in a well ventilated area. We all know how stifling the heat can be in an enclosed area. Many higher quality weather measuring systems actually have small fans that draw fresh air past the sensor to keep it ventilated. Temperature enclosures are usually painted white to reflect some of the heat and keep it from building up. Conversely, the local bank thermometer is often in a dark colored non-ventilated box. No wonder they say it’s at least 99 degrees so often!
There are dozens of sources of weather records, especially temperature and precipitation. The National Weather Service (NWS), Federal Aviation Administration (FAA) and Department of Defense (DOD) takes detailed observations at nearly 1000 airports and another 8000 weather temperature and rainfall observations are taken by NWS Cooperative Observers. In addition many local agencies such as water districts, air pollution districts and agricultural networks gather weather data. And finally there are thousands of weather stations at schools and private establishments.
The NWS, FAA and DOD data is quality controlled and archived at the National Climatic Data Center in Asheville, NC. This is the “gold standard” of data and I have never had a problem getting it admitted into court. However, some of these other datasets bear close scrutiny when they are being used and may not be “admittable”. But they can be used to further verify or “fill-in” information from some of the official sources.
Probably the biggest problem that I have repeatedly run into is in regards to how far above the ground that temperatures are measured and how is that representative of the temperature near the ground. The standard measuring height is 5.5 feet (1.5 meters) above the ground but this seldom corresponds to ground level readings.
To understand this we need a little Meteorology 101. The air is only very slightly warmed directly by the sun’s rays. Instead the ground is warmed by the sun and it in turn warms the lower levels of the air by a combination of conduction (i.e., direct contact and transfer of heat) and convection (i.e, warm air rising). Conversely at night the ground radiates its heat off into space and cools rapidly and in turn cools the lowest layers of the atmosphere first.
What this means as far as temperature measurements is that during the day the ground level air temperature is warmer than at 5.5 feet and at night the ground temperature is cooler. And the differences can be significant. A fairly typical lapse rate when there is little wind is on the order of a degree per foot. For example, with an afternoon high temperature of 80 degrees measured at 5.5 feet the air temperature right next to the ground would be approximately 85 degrees.
Even more critical in many insurance and litigation matters involving ice is this same lapse rate of about 5 degrees. Consequently when the weather data that you get from the local airport says it was 36 degrees it’s still very possible that the temperature at ground level is only 31 degrees and there could be ice. This is exacerbated on elevated structures like overpasses and bridges which loss their heat in all directions and can be even colder than a 5 degrees lapse rate would indicate. (This is why you will often see highway signs to the effect that “bridges freeze first”).I have had more than one case resolve itself very quickly when these temperature variations were pointed out to the parties involved and they realized there initial assumptions about the air temperature were flawed.
The bottom line is that there is lots of weather information available but a basic understanding of what it really means will allow you to more ably assist your clients.
Questions or comments? Contact Jan Null, jnull@ggweather.com or 510-657-2246 or visit Golden Gate Weather Services on the web at http://ggweather.com .
