SL Helicopter Flying Handbook/Weather
SECTION 12. Weather
Weather is an important factor that affects many aspects of a flight. Weather is the overall state of the atmosphere at a place and time, incorporating factors such as temperature, humidity, visibility, cloud conditions, wind conditions, barometric pressure. This section begins with a discussion of basic weather theory (to the extent needed for SL), then goes on to describe various source of SL weather information. Shergood aircraft that support Weather 2.0, can be affected by the weather described in the section.
2 Weather Theory
This section describes the weather as generated by the Shergood SL Weather simulator. While intended to mimic many of the features of RL weather, it is a simplified model and does not include all the effects of RL weather. The simulator is based on conditions at mid-latitudes in the Northern Hemisphere of RL Earth. The results in Weather systems that generally move West to East, and winds that generally rotate counter-clockwise around low pressure systems.
What we feel as air pressure, is from the weight of the atmosphere around us. The atmosphere is a mix of gasses comprised of approximately 78% nitrogen, 21% oxygen, and 1% other trace gases that reaches up to approximately 350 miles above the surface. If we were to weight a 1 inch square column of air reaching for from the sea level to 350 miles above the surface it would weigh approximately 14.7 pounds. This results in a sea-level pressure of approximately 14.7 pounds/square inch. If one is above sea-level, either at an airport above sea-level or in flight, the weight of the air above that point is less. This is the principle upon which altimeters work.
In aviation, pressure is often measured in inches of mercury (in. Hg). Historically, this was measured by the height of a mercury column that the air pressure could support in a glass tube. Standard sea-level pressure is defined to be 29.92" Hg. In practice, typical sea-level pressure readings range 28 and 31" Hg.
2.1.1 Altitude Variation in Pressure
The actual pressure at a station also depends on the elevation. Pressure drops by about 1 in. Hg for every 1000 feet of altitude. This means that on a standard day, the pressure at a station at 5000 feet would be 24.92" Hg. When stations report pressure, they typically adjust it to sea level sea-level equivalent. For example if the actual recorded pressure at an airport at 5000 feet elevation was 25.25" Hg, this would get reported as 30.25" Hg. This allows pilots to have a consistent understanding of reported atmospheric pressure without needing to know the elevation of the station.
Differences in pressure drive much of what we experience as weather. These differences in pressure are caused by unequal heating of the surface resulting in what we experience as wind. Initially, air moves from high to low pressure areas, but as it does so, it will deflect to the right due to Coriolis effect from the rotation of the Earth. The result is the air will tend to flow clockwise around high pressure systems, and counter-clockwise around low pressure systems.
2.2 Moisture and Temperature
The atmosphere always contains moisture to some degree or the other. The temperature affects the amount of moisture the atmosphere can hold. Each 11C increase in temperature roughly doubles the amount of moisture the air can hold.
The amount of moisture in the atmosphere at a particular place and time is often described in terms of "relative humidity". Relative humidity is the actual amount of moisture in the air compared to the maximum amount that could be in the air for the particular temperature. For example, if the relative humidity is 40%, then the air is holding 40% of the moisture it could possibly hold. When the relative humidity reaches 100%, fog (if in on the ground) or clouds (if in the air) is the usual result.
Humidity is also sometimes expressed in terms of the temperature and dew point. The dew point is the temperature to which the air would be completely saturated if it were cooled to that temperature. The wider the temperature and dew point spread, the lower the humidity. When the temperature and dew point are equal, the air is completely saturated and the relative humidity is 100%.
2.2.1 Lapse Rate and Cloud Height
The lapse rate is the rate at which temperature and dew point decrease with altitude. An unsaturated parcel of air will decrease in temperature by 2C per 1000 feet of altitude. In RL, the dew point also decreases with altitude at a rate of 0.5C per 1000 feet. This results in the temperature and dew point converging at a rate of 1.5C per 1000 feet. This can be used to estimate the cloud height from the temperature/dew point spread. For example, if the temperature is 22C and the dew point is 19C, then the estimated cloud height would be (22-19)*(1000/1.5)=2000 feet.
However, using the RL lapse rates above result in cloud heights that are too high to be interesting for SL, since flights generally occur at much lower altitude in SL compared to RL. For this reason, we reduce the calculated cloud height by a factor of 10 for the purposes of SL.
3 Weather Services
Preflight preparations should always include a review of current and expected weather conditions along the route of flight. Shergood systems provides two methods for accessing SL weather information: 1) The Shergood Weather Services site and 2) Over the radio via ATIS broadcasts.
A METAR is an observation of current surface weather reported in a standard format. This discussion of METAR covers elements used in Second Life. METARs are issued on a regularly scheduled basis. The elements of a METER are listed in a standard order which we illustrate through the following example:
SLWS 151605Z AUTO 15012KT 10SM -RA OVC030 25/12 A3009 RMK AO2
The meaning of each of the elements are as follows:
- Station identifier— a four-letter code. In Second Life, airports have the prefix code "SL" while "HB" is the prefix for harbors. For example, White Star Airfield has the identifier "SLWS" with "SL" Being the prefix code and "WS" being the airport identifier.Station identifiers can be found on the Shergood Radar map.
- Date and time of report —depicted in a six-digit group (151605Z). The first two digits are the date, given as the day of the current month. In the axample, its the 15th. The last four digits are the time of the METAR, which is always given in coordinated universal time (UTC). A “Z” is appended to the end of the time to denote the time is given in Zulu time (UTC) as opposed to local time. Second life time is UTC minus 7 hours from approximately Mid March to Early November, and UTC minus 8 hours the remainder of the year. Times are given in a 24 hour format. In the example the time of observation is 16:05 which corresponded to 09:05 in second life, as the offset was minus 7 hours when the METAR was issued.
- Modifier—denotes that the METAR came from an automated source. If the notation “AUTO” is listed in the METAR the report came from an automated source. All METARS in Second life come from automated weather stations.
- Wind—reported with five digits (15012KT) The first three digits indicate the direction the true wind is blowing FROM. The last two digits will indicate the wind speed in Knots. In the example, the wind is blowing from 150, at a speed of 12 knots.
- Visibility—the prevailing visibility (10SM) is reported in statute miles as denoted by the letters “SM”, up to 10 miles, visibility greater than 10 miles will be reported as 10. One statute mile is equal to 5,280 feet or approximately 1.6 kilometers.
- Weather—can be broken down into two different categories: qualifiers and weather phenomenon (-RA). First, the qualifiers of intensity, proximity, and the descriptor of the weather are given. The intensity may be light (–), moderate ( ), or heavy (+). Weather phenomenon can be either rain (RA) or snow (SN). In the example, light rain is reported.
- Sky condition—always reported in the sequence of amount and height. (OVC030) The heights of the cloud bases are reported with a three-digit number in tens of feet AGL. The special height code /// is used when the cloud height is below the station level. Contractions are used to describe the amount of cloud coverage and obscuring phenomena. The amount of sky coverage is reported in eighths of the sky obscured. Coverage can be reported as Clear (CLR)- less than 1/8 coverage, Few (FEW) Clear to 2/8 coverage, Scattered (SCT) - 3/8 to 4/8 coverage, Broken (BKN) - 5/8 to 6/8 coverage, and Overcast (OVC) - 7/8 to full coverage. In the example, the sky is reported as overcast, with the base of the clouds at 300 feet AGL.
- Temperature and Dew point - The ambient air temperature and dew point, reported in degrees Celsius.(25/12) Temperatures below 0 are preceded my the letter "M". In the example, the temperature is 25 and the dew point is 12 degrees Celsius.
- Altimeter Setting - The altimeter setting, reported in inches of mercury. (A3009). The altimeter setting is a 4 digit number, always preceded by the letter "A". The decimal point is omitted from the METAR. In the example, the altimeter setting is 30.09.
- Remarks- This section contains additional information that does not fit into the other sections and may include any active notams for the station. Remark A02 indicates that the station is equipped with a precipitation sensor that can differentiate between rain and snow.
In full, the example METAR: SLWS 151605Z AUTO 15012KT 10SM -RA OVC030 25/12 A3009 RMK AO2 can be read as: METAR for White Star Airfield, for the 15th day of the month, 16:05 Zulu time, Automatic observation, Winds from 150 at 12 knots, Visibility 10 statute miles or greater, Light Rain, Clouds overcast at 300 feet AGL, Temperature 25, Dew point 12 degrees Celsius, Altimeter Setting 30.09, Station is equipped with a precipitation sensor that can differentiate between rain and snow.
3.1.1 Obtaining METARS
METARs for most airports can be found at the Shergood Weather Services site. On the METAR tab, enter the identifier codes for one or more airports and press "Get METAR Data". The current METARs for the requested locations will be shown below. METARs for a specific location can also be found on the Shergood Map. Click on an airport symbol to bring up its information box, the click on the "METAR>>" link to expand it into the current METAR for that location.
3.2 Graphical Weather Charts
Graphical weather products are a quick way to see a general overview of current weather conditions over a wide area. Shergood Weather Services provides the following graphical weather products:
- Synoposis - The synopsis chart shows pressure systems with lines at constant pressure labeled in millibars. Since wind generally blows parallel to the constant pressure lines, with winds being stronger the more tightly spaced the lines are, this chart can also give the pilot an idea of the winds to expect.
- Winds - This chart directly shows winds as arrows superimposed on the map. Wind blows in the direction of the arrow with the length of the arrow showing the strength of the wind.
- Temperature - The temperature chart shows areas of low and high temperatures. Isothrems shows lines of constant temperature labeled in degrees Celsius. Warmer lines are shown in red/pink while cooler lines are shown in blue.
- Humidity - This chart shows lines of constant relative humidity in percent. Areas of high humidity can expect poor visibility and low clouds.
- Radar - This chart shows precipitation. Green area depict light rain, while yellow, orange and red indicate heavier precipitation. Snow is shown with progressing toward blue colors.
- Satellite - Shows overall cloud cover and cloud thickness.
- Cloud Bases - Shows the cloud bases with lines marked with cloud bases in tens of feet. Areas marked CLR are clear of clouds.
These weather products are available at two scales "mainland" showing all of SL mainland, and "Blake Sea" showing a closeup of the Blake Sea, Nautilus, most of Corsica and the Northern portion of Satori.
ATIS (Automatic Terminal Information Service) is a pre-recoded message containing current weather information about an airport. It is usually broadcast over a specific frequency for each airport. The frequency can be found in the information box of an airport by clicking that airport's symbol on the Shergood Map. See the documentation for your aircraft on how to select that frequency with your radios.
The information found in an ATIS message is similar to the information found in a METAR. For example, an ATIS message for White Star airport might be:
White Star Airfield information Oscar, time 0151 Zulu observation. Wind 010 at 02. Visibility 10. Light Rain. Overcast at 035. Temperature 25, dew point 10. Altimeter 30.21. Advise on initial contact you have information Oscar.
The message includes the following pieces of information:
- Name of station
- Phonetic Letter Code to identify the broadcast, used to indicate to ATC that you have the correct version of the broadcast.
- Time at which the message was recorded in "Zulu" time (i.e., UTC time)
- Current wind direction and speed
- Visibility at the station
- Any precipitation (rain or snow)
- Cloud conditions
- Temperature and dew point
- The current altimeter settings
- Repeat of the Phonetic letter code