Frequently Asked Questions
Some answers to regularly received questions.
Some of the archived data seem to have errornous values. Why don't you fix these up or remove them?
Relative humidity values are sometimes over 100% - even as high as 105 or 110%. Is this right?
I don't understand the time axis and does the time change when we start daylight time?
How do I convert from kPa to hPa and how do these relate to millibars and inches of Hg?
How do I convert from degrees Celsius to Fahrenheit?
The precipitation units in the 15 minute data are *4 mm/hr. What does this mean?
- Some of the archived data seem to have errornous values. Why don't you fix these up or remove them?
- The data are used for teaching. A valuable lesson for students (and others) to learn is that observed data can contain errors and must be checked.
- Relative humidity values are sometimes over 100% - even as high as 105 or 110%. Is this right?
- These very high values indicate an instrumentation problem. Relative humidity is derived from the dry and wet bulb temperatures. The high values occur because of a problem with the wet bulb temperature.
If not wet enough will read too high and so RH too great. But how can it go greater than the dry bulb? Does the wick keep it warm? Or is it a calibration problem?
- I don't understand the time axis and does the time change when we start daylight time?
- The time is the local standard time in Sydney. Although, according to Precision
Timing Word list and Meteorological
Glossary this should simply be called standard time. The local is added here to emphasis that it is the standard time for Sydney, Australia.
Do not confuse it with local time.
Standard time remains constant with respect to a fixed meridian - it is not Daylight Savings Time or Summer Time. For Sydney, standard time is Universal Time Coordinated (UTC) plus 10 hours. UTC was originally known as Greenwich Mean Time (GMT) and is sometimes referred to a Zulu (Z) time.
For some more information on time visit EXPLORES! Basic Skills: Time Conventions. To find the time somewhere try using Local Times Around the World or World Time Zones. For North Americans North American Time Zones and GMT.
- How do I convert from kPa to hPa and how do these relate to millibars and inches of Hg?
-
The Systems
International unit for pressure is the the Pascal (Pa) (which is
one Newton per square metre). The pressure at mean sea level is about 100000
Pa. To make atmospheric pressure into a number that is more convenient
to work with, it is usually quoted in 100s of Pascal. The prefix h stands
for hecta, or 100. Thus 1000 hPa is 1000 times 100 Pa or 100000 Pa.
The prefix k stands for kilo, or 1000. Other prefixes are shown in this table. Therefore, 1 kPa equals 1000 Pa, or 10 times 100 Pa - which equals 10 times 1 hPa. Thus 1 kPa equals 10 hPa. To convert kPa to hPa multiply by 10.
One millibar equals one hPa. In a standard atmosphere mean sea level pressure is 1013.25 hPa or 29.92 inches of mercury. Therefore one inch of mercury is about 33.87 hPa. A useful summary of units, prefixes, conversion factors, constants, etc. can be found at http://nwoca7.nwoca.ohio.gov:70/0gopher_root%3a[_reference]_isou.
- How do I convert from degrees Celsius to Fahrenheit?
- Temperature Unit Conversion from the National Weather Service EL Paso web site may be useful for
the celsiusly challenged.
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The precipitation units in the 15 minute data are *4 mm/hr. What does this mean?
-
Precipitation is measured using a tipping bucket rain gauge. Each tip is
0.25 mm. In the fifteen minute data, the data give the number of tips in
each fifteen minute period multiplied by 0.25 mm. Thus to convert the rainfall
rate for each fifteen minute period to an equivalent rainfall rate for
an hour you need to multiply by 4. For example, 4 tips in fifteen minutes
is 1 mm in that fifteen minutes which would equal 4 mm in an hour if the
rate of rainfall was the same as the fifteen minute interval for a whole
hour. To find the actual rainfall in some hour you need to sum the values
for the four fifteen minute periods making up that hour.
Useful Links