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?
The current weather page loads but is missing values.
My browser hangs or crashes 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 are there discrepencies between the temperature reported by these web pages and the web page at http://www.llnl.gov/llnl-bin/weather?
Why is reported solar radiation sometimes greater on a partly cloudy day than on a clear day?
Why do the reports seem to be an hour behind the current time?
Is the reported barometric pressure adjusted to sea level?
Suggestions for additions to this list.

Answers

What is this site? [TOC]

The Lawrence Livermore National Laboratory's Weather Pages provide access to local meteorological information in a variety of formats, and links to other weather-related sites.

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 at LLNL" 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 a thermistor at 10 meters. The difference between the 2m and 10m sensors can be over 2 F on a sunny day.

Our thermistors are assessed every three months and have to be within 0.5 F of a standard. They're usually closer than that. 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.

Our thermistors are ventilated by a fan. This way we get a 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 may be in the shade, it is often attached to a wall. The wall may conduct heat from places in the sun.

Where is the LLNL meteorological tower? [TOC]

The LLNL 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.

Where is the Site 300 meteorological tower? [TOC]

The Site 300 meteorological tower is located on a ridge near the center of the site. The base of the tower is about 1200 feet above mean sea level.

The current weather page loads but is missing values. [TOC]

There are a couple of reasons why the current weather page might load with missing values.

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 and the software that serves it to the web. The met data is scheduled to be collected on the quarter hour. The software that serves this data to the web tries to access the most recent data. Depending on the load on the machine, the data collection software may not run until sometime after scheduled. In these cases, the server software tries to access data that does not yet exist. In most cases, reloading the page a few minutes later resolves the problem.

My browser hangs or crashes 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 badly when they encounter javascript.

What is Sigma Theta? [TOC]

Sigma Theta is measured by our wind vane and is related to horizontal turbulence. Mathematically it is the standard deviation of the vane position. This calculation is tricky because the vane can swing in a circle (it uses polar coordinates).
At LLNL we take a reading of the wind vane every second. Every 15 minutes the data logger takes the 900 readings to calculate an average wind direction and sigma theta. The dynamics of our Met One 020 wind vanes are well known and within specifications set down by EPA for damping ratio, starting threshold, and distance constant.
We use Sigma Theta to estimate the potential for the atmosphere to spread a plume. The EPA provides guidance to calculate a common plume dispersion index (Pasquill Stability Class) from measurements of wind speed and sigma theta.

What is Solar Radiation? [TOC]

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

What is Stability Class? [TOC]

Stability Class is used to determine the spread of a plume. There are accepted equations for the width and height of a plume given the distance from the source and the Stability class. Surprisingly, wind speed is not included in these functions. To estimate stability class we use a method suggested by the EPA. The wind speed and sigma theta are combined in this method. There are other methods for estimating stability class. An old one uses wind speed and percent cloud cover. Another method uses solar radiation and wind speed in the daytime, and vertical temperature difference and wind speed at night. There are six Pasquill Stability categories: A through F. The Unstable classes (A, B, and C) occur during the daytime. The Stable classes (E and F) only occur at night. The Neutral class can occur during the day or night. Class A is the Very Unstable and corresponds to hot, calm days and leads to the greatest amount of dispersion. A plume of effluent is broken up and spread wide with A Stability. Class D is neutral. It corresponds to windy days or the transition times of dawn and dusk. This is the most frequently occurring stability class. Class F is Very Stable and corresponds to nights with low winds. A plume experiencing F Stability will feature very little dispersion. Here is a link to a short on-line tutorial on many aspects of Air Quality: http://www.shodor.org/metweb/ or more directly to the part about atmospheric dispersion and stability: http://www.shodor.org/metweb/session6/stable.html

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 are there discrepencies between the temperature reported by these web pages and other pages using the same data? [TOC]

The web page at http://ptavv.es.net/, for example, uses the instruments on the met tower located at 2m above the ground. Our web page https://www-metdat.llnl.gov/cgi-pub/reports/current.pl uses the instruments located at 10m above the ground, which is the regulatory standard. By using our weather reporting facility ( https://www-metdat.llnl.gov/cgi-pub/reports/report.pl ) you can view the temperature readings from the 2m package. However, there is still a discrepency between our sites of at most 1 °. This reported difference is due to rounding. Our site truncates the fractional portion of temperatures reported in the farenheit scale. The other site uses rounding.

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

We use a Licor photometer to measure solar radiation. This sensor is sensitive to the 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.

Why do the reports seem to be an hour behind the current time? [TOC]

During daylight savings time, the data will appear to be an hour off. All reports use PST as the time basis to ensure reports are consistent even when the reporting period includes one of the time change dates.

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 3 mb. If you need more accuracy than this, you'll have to get a mercury barometer and take your own measurements.

You can get data from our Site 300 barometer from our web site. Also, you can get barometric data from the Livermore Airport (KLVK), Oakland (KOAK), and Stockton (KSCK) here. You might have to start at the University of Wyoming home page and click on Weather and Surface Observations.

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