Cumulus Wiki - User contributions [en]

Pressure Measurement

2010-10-13T02:23:13Z

TideMan:

Atmospheric pressure (also known as barometric pressure) is the pressure exerted by the weight of the air above the Earth's surface. It is measured using a [http://en.wikipedia.org/wiki/Barometer barometer]. Atmospheric pressure and its rate of change are important parameters for weather and the ocean.

==Units==
The units used for pressure are hPa (hecto Pascals) or the equivalent mb (millibars), but in the US inHg (inches of mercury) are still used by some.

==Datum==
Pressure decreases exponentially with altitude, so for consistency it is reduced to the common datum of mean sea level (MSL).

==Setting the Datum==
In Cumulus, the pressure datum is set by going to Configuration/Calibration and entering a number into the Pressure slot in Offsets.

The best way to get MSL pressure at your location is to go to a site like [http://www.wunderground.com/ Weather Underground] and enter your location. It will find a weather station nearby and give you the current MSL pressure. It's a good idea to choose an airport site because you can be sure that it has been accurately reduced to MSL pressure.

On the other hand, most airport sites only give pressure to the nearest hPa, so you can be 0.5 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision.

The best time to set your datum is when the pressure is steady; in other words when Trend shown at the bottom left in the Cumulus display is zero hPa/h.

To figure out what to put into the Cumulus box, use the following equation, where <math>P_c</math> is the pressure on your console and <math>P_s</math> is the pressure from a nearby station:
<math>Correction = P_s - P_c</math>

==Locating the Barometer==
For most personal weather stations, the barometer is in the console, usually attached to the printed circuit board. These barometers are very cheap devices that simply record the load on a diaphragm. Unlike the barometer in an automatic weather station, they have no temperature compensation. Therefore, for best pressure readings, you should locate the console in a place where the temperature is reasonably constant, not in direct sunlight or near a heater.

==How Accurate are PWS Barometers?==
The figure below shows a comparison between the pressure measured by a Fine Offset PWS and a nearby AWS. You can see that the comparison is excellent.
[[file:KyleVsBrockworthPress.png|500 px]]

[[Category:Terminology]]

Pressure Measurement

2010-10-12T18:59:45Z

TideMan: Created page with 'Atmospheric pressure (also known as barometric pressure) is the pressure exerted by the weight of the air above the Earth's surface. It is measured using a [http://en.wikipedia.o…'

Atmospheric pressure (also known as barometric pressure) is the pressure exerted by the weight of the air above the Earth's surface. It is measured using a [http://en.wikipedia.org/wiki/Barometer barometer]. Atmospheric pressure and its rate of change are important parameters for weather and the ocean.

==Units==
The units used for pressure are hPa (hecto Pascals) or the equivalent mb (millibars), but in the US inHg (inches of mercury) are still used by some.

==Datum==
Pressure decreases exponentially with altitude, so for consistency it is reduced to the common datum of mean sea level (MSL).

==Setting the Datum==
In Cumulus, the pressure datum is set by going to Configuration/Calibration and entering a number into the Pressure slot in Offsets.

The best way to get MSL pressure at your location is to go to a site like [http://www.wunderground.com/ Weather Underground] and enter your location. It will find a weather station nearby and give you the current MSL pressure. It's a good idea to choose an airport site because you can be sure that it has been accurately reduced to MSL pressure.

On the other hand, most airport sites only give pressure to the nearest hPa, so you can be 0.5 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision.

The best time to set your datum is when the pressure is steady; in other words when Trend shown at the bottom left in the Cumulus display is zero hPa/h.

==Locating the Barometer==
For most personal weather stations, the barometer is in the console, usually attached to the printed circuit board. These barometers are very cheap devices that simply record the load on a diaphragm. Unlike the barometer in an automatic weather station, they have no temperature compensation. Therefore, for best pressure readings, you should locate the console in a place where the temperature is reasonably constant, not in direct sunlight or near a heater.

==How Accurate are PWS Barometers?==
The figure below shows a comparison between the pressure measured by a Fine Offset PWS and a nearby AWS. You can see that the comparison is excellent.
[[file:KyleVsBrockworthPress1.png]]

File:KyleVsBrockworthPress1.png

2010-10-12T18:58:58Z

TideMan: Comparison between pressure measured by a Fine Offset barometer and an AWS nearby.

Comparison between pressure measured by a Fine Offset barometer and an AWS nearby.

File:KyleVsBrockworthPress.png

2010-10-12T03:41:33Z

TideMan: Comparison of pressure measured by a Fine Offset PWS and a nearby AWS.

Comparison of pressure measured by a Fine Offset PWS and a nearby AWS.

Wind measurement

2010-10-11T22:19:26Z

TideMan:

Wind observations are measured in two ways .... wind speed and wind direction. Where possible measuring devices should be in a large open area away from possible interference such as building, trees etc. It is generally accepted that measurements are based on readings at 10 metres (33 feet) above ground and the distance between the anemometer and any obstruction is at least ten times the height of the obstruction!

=Wind Speed=

Wind speed is measured using an [http://en.wikipedia.org/wiki/Anemometer anemometer]. It is typically made up of cups attached to spindle. The stronger the wind the faster the cups rotate and the higher the reading. Several readings should be taken over a couple of minutes and an average taken. The measurement is usually made in [http://en.wikipedia.org/wiki/Knot_%28unit%29 knots] or miles per hour but may be converted to metres per second or kilometres per hour.

==Compensation==

Raising an anemometer to a height of 10 metres is not always possible, especially for weather enthusiasts, therefore one must build in a compensation figure (or multiplier) in order to take into account the height factor.

It is generally accepted that the following formula will provide a multiplier to compensate for lower height anemometer.

= 1/(0.233 + 0.656*log10(h+4.75))

Where <tt>h</tt> is the height of the anemometer in metres above ground level

Examples:

{| class="wikitable"
!|Height (metres) || Multiplier
|-
|0.5 ||1.42
|-
|1.0 || 1.37
|-
|1.5 || 1.32
|-
|2.0 || 1.29
|-
|2.5 || 1.25
|-
|3.0 || 1.22
|-
|3.5 || 1.20
|-
|4.0 || 1.18
|-
|4.5 || 1.15
|-
|5.0 || 1.13
|-
|5.5 || 1.12
|-
|6.0 || 1.10
|-
|6.5 || 1.08
|-
|7.0 || 1.07
|-
|7.5 || 1.06
|-
|8.0 || 1.04
|-
|8.5 || 1.03
|-
|9.0 || 1.02
|-
|9.5 || 1.01
|-
|10 || 1
|-
|}

==Wind Multiplier in Cumulus==

Within Cumulus it is possible to set a multiplier which will be used to upscale your actual reading from your anemometer to take into account its lower height. Use the formula above (or the table of examples) to locate your multiplier. Remember you are measuring the height of your anemometer from the ground in meters.

Within Cumulus click the 'Configuration' menu then 'Calibration'. Locate the 'Windspeed multiplier' and change this from its default of 1 to the value from above. Note Cumulus only accepts two decimal places if you have calculated using the formula rather than the table.

In addition to a Wind Speed multiplier, Cumulus allows for a Wind Gust multiplier. However, it is generally felt that wind gusts should not take into account their height from ground level therefore a multiplier of 1 is acceptable.

==Beaufort Scale==

The [http://en.wikipedia.org/wiki/Beaufort_scale Beaufort Scale] was developed as a common way to measure and describe wind speed prior to any accurate devices. Initially it was based on the condition of the sea however the modern Beaufort scale incorporates land observations too.

Using the table below you could asses if your Calibration within Cumulus is roughly accurate based on observations.

{| class="wikitable"
! rowspan="2" | Beaufort number !! rowspan="2" | Description !! colspan="4" | Wind speed !! rowspan="2" | Land conditions
|-
!width="70"|km/h !!width="70"|mph!!width="70"|kts!!width="70"|m/s
|-
| 0
| Calm || < 1 || < 1 || < 1 || < 0.3 || Calm. Smoke rises vertically.
|-
| 1
| Light air || 1 – 5 || 1 – 3 || 1 – 2 || 0.3 – 1.5 || Wind motion visible in smoke.
|-
| 2
| Light breeze || 6 – 11 || 3 – 7 || 3 – 6 || 1.5 – 3.3 || Wind felt on exposed skin. Leaves rustle.
|-
| 3
| Gentle breeze || 12 – 19 || 8 – 12 || 7 – 10 || 3.3 – 5.5 || Leaves and smaller twigs in constant motion.
|-
| 4
| Moderate breeze || 20 – 28 || 13 – 17 || 11 – 15 || 5.5 – 8.0 || Dust and loose paper raised. Small branches begin to move.
|-
| 5
| Fresh breeze || 29 – 38 || 18 – 24 || 16 – 20 || 8.0 – 11 || Branches of a moderate size move. Small trees begin to sway.
|-
| 6
| Strong breeze || 39 – 49 || 25 – 30 || 21 – 26 || 11 – 14 || Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic garbage cans tip over.
|-
| 7
| High wind, Moderate gale, Near gale || 50 – 61 || 31 – 38 || 27 – 33 || 14 – 17 || Whole trees in motion. Effort needed to walk against the wind. Swaying of skyscrapers may be felt, especially by people on upper floors.
|-
| 8
| Gale, Fresh gale || 62 – 74 || 39 – 46 || 34 – 40 || 17 – 20 || Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded.
|-
| 9
| Strong gale || 75 – 88 || 47 – 54 || 41 – 47 || 21 – 24 || Some branches break off trees, and some small trees blow over. Construction/temporary signs and barricades blow over. Damage to circus tents and canopies.
|-
| 10
| Storm, Whole gale || 89 – 102 || 55 – 63 || 48 – 55 || 25 – 28 || Trees are broken off or uprooted, saplings bent and deformed. Poorly attached asphalt shingles and shingles in poor condition peel off roofs.
|-
| 11
| Violent storm || 103 – 117 || 64 – 72 || 56 – 63 || 29 – 32 || Widespread damage to vegetation. Many roofing surfaces are damaged; asphalt tiles that have curled up and/or fractured due to age may break away completely.
|-
| 12
| Hurricane || ≥ 118 || ≥ 73 || ≥ 64 || ≥ 33 || Very widespread damage to vegetation. Some windows may break; mobile homes and poorly constructed sheds and barns are damaged. Debris may be hurled about.
|}

=Wind Direction=

Wind direction, like speed, should ideally be measured in a large open space away from buildings etc which could provide shelter or help funnel winds.

Wind direction is measured using a wind vane. This is ideally mounted on a roof top or mast 10 metres above ground. The wind vane is made up of an arrow mounted on top of a spindle, allowing the arrow to move with the wind. The arrow always points in the direction '''from''' which the wind is blowing and similarly wind direction is reported as '''from''' where the wind is blowing. Example, a Southerly wind is wind coming from the South

Wind direction is usually described in [http://en.wikipedia.org/wiki/Cardinal_direction Cardinal points]

[[Category:Terminology]]