Frequently Asked Questions

What is this site?
Why is the temperature reported on the LLNL Weather page different from the temperature seen on a local thermometer?
Where is the LLNL meteorological tower?
Where is the Site 300 meteorological tower?
Why is the "Current Conditions" weather page displaying old data?
Why does my browser hang or crash when I try to view Metdat pages?
What is sigma theta?
What is solar radiation?
What is stability class?
Is wind direction relative to true north or magnetic north?
Why is reported solar radiation sometimes greater on a partly cloudy day than on a clear day?
Is the reported barometric pressure adjusted to sea level?
Where do I send suggestions for additions to this FAQ?

Answers

What is this site? [TOC]

The "LLNL Weather" web site provides access to meteorological data collected at Lawrence Livermore National Laboratory's (LLNL) Livermore Site (located in Livermore, CA), its Site 300 (located in the foothills approximately 17 miles east of Livermore, CA and 8 miles southwest of Tracy, CA) and Sandia National Laboratory (located across the street south of LLNL's Livermore Site).

Current conditions, archived data reports, recent data plots since yesterday, information about LLNL weather stations and related links of potential interest can be accessed by clicking on the links located at the top of each web page.

Why is the temperature reported on the LLNL Weather page different from the temperature seen on a local thermometer? [TOC]

There are many reasons the temperature reported by the LLNL met tower may be different from that of other thermometers around the lab.

The temperature reported on the current conditions page is taken at 2 meters above the ground. Other thermometers may take readings at other heights. Other thermometers are often near the source of heat - the ground. We also have thermistors at 10, 23, and 52 meters. The difference between the 2m and 10m sensors can be over 4° F on a clear day or night.

Our thermistors are independently audited every year and have to be accurate to within ±0.9 F of a reference thermometer calibrated to traceable NIST standards. In fact, our thermistors are actually more accurate than ±0.9 F. One of the most common styles of consumer thermometers is the bi-metal strip thermometer. These are notorious for being out of calibration and out of linearity and are typically not as accurate as our thermistors.

Our thermistors are installed in white ventilated shields to minimize effects of radiant heating (direct sunlight) or cooling. This way we get a more representative reading of true air temperature, not temperature elevated by buildup of heat. Other thermometers are usually not ventilated and are often between buildings that slow down the air.

Even though a thermometer (other than ours) may be in the shade, it is often attached to a wall. The wall may conduct heat from portions located in the sun.

Where is the LLNL meteorological tower? [TOC]

The LLNL Livermore Site meteorological tower is located in the west buffer zone near the corner of Vasco and Patterson Pass Roads. The base of the tower is about 570 feet above mean sea level. For more information about the Livermore tower and its instrumentation visit the about page.

Where is the Site 300 meteorological tower? [TOC]

The LLNL Site 300 meteorological tower is located on a ridge in the north-central portion of LLNL's Site 300. The base of the tower is about 1270 feet above mean sea level. For more information about the Site 300 tower and its instrumentation visit the about page.

Why is the "Current Conditions" weather page displaying old data? [TOC]

The "Current Conditions" weather page displays the most recent 15 minute averaged meteorological data and is updated on the hour and at 15, 30 and 45 minutes past each hour. Occasionally, for the reasons listed below, the data are not updated as scheduled and a message "NOTE: DATA IS XX MINUTES OLD" is displayed next to the data/time stamp of the data being displayed. The reasons for the data not being updated as scheduled could be:

The met tower may be offline. This can occur due to scheduled maintainence, emergency maintainence and hardware failure.

The computers serving the data may not be able to communicate. This could be the result of hardware, software or network problems.

There can be latency between the software that collects the data from Sandia National Laboratory and the software that serves it to the web.

Why does my browser hang or crash when I try to view Metdat pages? [TOC]

Your browser may hang or crash your machine if you are using a really old version of your browser. Most of the Metdat pages use JavaScript, which can cause problems for older browsers. For example, versions of Netscape Navigator prior to v2.2 for the Macintosh behave poorly when they encounter JavaScript.

What is sigma theta? [TOC]

Sigma theta is a measure of horizontal wind direction fluctuations. Mathematically, it is the standard deviation of the horizontal wind direction. The wind direction is measured by our wind vane every second. The sigma theta displayed on our website is a 15-minute average value based on the 900 wind direction readings in the 15-minute period and is calculated by the data logger.

Sigma theta can be used to estimate the potential for the atmosphere to spread a plume. The EPA provides guidance to calculate a common plume dispersion index called the stability class from measurements of wind speed and sigma theta.

What is solar radiation? [TOC]

Solar radiation is the electromagnetic emitted from the sun. It is measured by a simple photocell pointed directly up. The voltage from this sensor is directly proportional to the sun's energy striking the horizontal surface. The photocell we use is sensitive to a broad band of wavelengths.

What is stability class? [TOC]

The stability class is a characterization of the stability (turbulence) of the atmosphere. Stability class is used to estimate how much a plume will spread as it is carried by the wind away from its source. The amount of plume spread (both vertically and horizontally) and wind speed are used in dispersion modeling to calculate pollutant concentrations in a plume emitted from its source.

There are six stability classes used to characterize the stability (turbulence) of the atmosphere: A through F. Stability classes A, B, and C represent an unstable, fairly turbulent atmosphere and only occur during the daytime. Class A is very unstable and occurs on hot, calm days and leads to the greatest amount of dispersion. A plume of effluent is broken up and spread wide with A stability. Stability classes E and F represent a stable, fairly non-turbulent atmosphere and only occur during the nighttime. Class F is very stable. A plume experiencing E or F stability will feature very little dispersion. Stability class D represents a neutrally stable atmosphere and can occur during the daytime or nighttime. Class D is the most frequently occurring stability class.

LLNL uses the solar radiation/delta-T (SRDT) method to calculate stability class. It is one of several methods that the USEPA allows for calculating stability class for air dispersion modeling. The SDRT method uses the incoming solar radiation (daytime only), the vertical temperature gradient between 2 meters and 10 meters, and wind speed to calculated stability class. Here is a link to a short on-line tutorial on many aspects of air quality. For a direct link to the part about atmospheric dispersion and stability, click here.

Is wind direction relative to true north or magnetic north? [TOC]

The wind direction values presented are all relative to true north. We do use a compass to orient the wind vane, but we do take into account the local declination angle.

Why is reported solar radiation sometimes greater on a partly cloudy day than on a clear day? [TOC]

We use a Kipp & Zonen CNR-1 net radiometer to measure solar radiation. This sensor is sensitive to short wave radiation. Clouds reflect these short waves towards the sensor. So, when the geometry is just right, white clouds will add to direct sunlight.

Is the reported barometric pressure adjusted to sea level? [TOC]

The pressure presented on our web pages is adjusted to sea level. This is common practice and makes it possible to compare data between stations.

To adjust back to absolute atmospheric pressure, multiply the sea level pressure by: =((288-0.0065*ElevFeet/3.28)/288)^5.256 where ElevFeet is the elevation in feet. You can see that ElevFeet/3.28 is the elevation in meters.

The sensor is usually accurate to within 0.5 mb.

You can get barometric pressure data from our Site 300 or from the Sandia barometer from our web site. Also, you can get barometric pressure data from the Livermore Airport (KLVK), Oakland (KOAK), and Stockton (KSCK) here.

Suggestions for additions to this list should be directed to Tony Wegrecki.