SL Helicopter Flying Handbook/Weather

SECTION 12. Weather

1 Introduction

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, 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.

2.1 Pressure

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.

2.1.2 Pressure Variation from Weather

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.

3 Weather Services

Preflight preparations should always include a review of current and expected weather conditions along the route of flight.

3.2 METARS

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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.
9. 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.
10. 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 mont, 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.

3.2.1 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.

3.3 ATIS

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:

1. Name of station
2. Time at which the message was recorded in "Zulu" time (i.e., UTC time)
3. Current wind direction and speed
4. Visibility at the station
5. Any precipitation (rain or snow)
6. Cloud conditions
7. Temperature and dew point
8. The current altimeter settings
9. A code name for the message used to indicate to ATC that you have the current weather