About LLNL Weather

This application presents essential weather data from the past 24 hours. By default, it displays plots featuring 24-hour measurements from Site 200 (LLNL), along with the latest readings from selected instruments at both Site 200 and Site 300.

Frequently Asked Questions

General Information

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

Data Reporting & Accuracy

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

  1. 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.
  2. 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.
  3. 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.
  4. 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.

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:

  1. The met tower may be offline. This can occur due to scheduled maintenance, emergency maintenance, and hardware failure.
  2. The computers serving the data may not be able to communicate. This could be the result of hardware, software, or network problems.

The site homepage displays both raw and adjusted barometric pressure. The raw pressure is the actual measurement taken at each tower. The adjusted pressure is adjusted down to sea level, which is common practice to allow for comparison between the stations. The Reports section displays raw pressure.

The sensor is usually accurate to within 0.5 mb.

Meteorological Terms & Measurements

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.

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.

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