>> Frequently Asked Questions
What is this site? [TOC]
Why is the temperature reported on the
LLNL Weather page different from the temperature seen on a local
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
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.
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
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
Where is the Site 300 meteorological
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
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
which can cause problems for older browsers. For example, versions of
Netscape Navigator prior to v2.2 for the Macintosh behave poorly when
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
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
To adjust back to absolute atmospheric pressure, multiply the sea level
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
Suggestions for additions to this list should
be directed to Tony Wegrecki.