Chapter 5: Weather Observations

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In order to create an accurate weather forecast, you must first start analysing weather observations to create a ‘nowcast’. There are two types of weather observations: manual weather observations, and automated weather observations.

Manual Weather Observations – OGRS Weather Observations

• Standard Observations: Observations taken at regular intervals.
• Interval Observations: Observations taken between the standard times.

Observations Taken at the Study Plot:

• Date
• Time
• Sky conditions
• Precipitation type and intensity
• Air temperature
• Surface snow temperature
• 10cm temperature
• Surface form/size
• Snowpack depth HS
• Snow boards: HN24, H2D, HST
• Snow penetrability: PF, PS, PR
• Snow densities and weights
• Ridge top transport

For more information, please refer to the OGRS Standard Weather Observations page.

Automated Weather Observations

Surface and Upper Air Observations. The surface observation network consists of over 17,000 weather stations on land and sea, reporting weather data typically four times per day (0000, 0600, 1200, and 1800 GMT). Upper air observations are collected by radiosondes attached to balloons, aircraft, satellites, or dropsondes (radiosondes dropped from aircraft). Radiosondes are launched twice per day (0000, and 1200 GMT). They measure temperature, humidity, air pressure, windspeed, and wind direction up to 35km.

Soundings: Atmospheric surrounds are plotted on ‘Skew-T Log-P’ diagrams. They show temperature and dew point through the atmosphere from a certain point above the ground. Isotherms are plotted as straight lines set to 45^o to the right, while isobars are horizontal and slightly curved. Wind speed and direction using a hodograph and/or wind barbs. Hodographs can be used to calculate wind shear, turbulence, and temperature advection.

 

Adiabats: Adiabats are displayed on both diagrams, and represent expected changes in temperature with pressure (elevation) for different starting temperatures. Two types of adiabats are shown: dry adiabats, and moist adiabats.

Dry adiabatic lapse rate: fixed at 9.8^oC per 1000m.

Moist adiabatic lapse rage: approximately 5^oC per 1000m, variable with temperature.

Atmospheric stability: Atmospheric stability can be determined by comparing the lapse rates of the temperature line to the moist and dry adiabats:

• Less than moist adiabatic lapse rate = stable. This condition often occurs below 300m above ground due to surface heating.
• Greater than dry adiabatic lapse rate = unstable.
• Between lapse rates = conditionally stable. Stable if air is unsaturated, unstable if air is saturated.

Isotherms: Isothermal conditions are indicated by the temperature line running parallel to an isotherm. For mountain meteorology, Skew-T Log-P diagrams are the most valuable for assessing the following:

Freezing level: The elevation where the 0^oC isotherm intersects the temperature line.

Degree of saturation: By calculating the dew point depression, which is the difference between the temperature and the dew point. Saturated conditions have approximately zero dew point depression, while dry conditions have a large dew point depression. Precipitation is unlikely if its value is greater than 5 degrees.

Lifted Condensation Level (LCL): The height at which the RH of an air parcel will reach saturation when it is cooled through dry adiabatic lifting. The LCL is a good approximation of the height of the cloud base when air is lifted mechanically. When the dry adiabatic lapse rate line crosses with the dew point temperature clouds will form.

Precipitation type: Look at the temperature curve from the dew point to the ground.

• If the temperature warms above 0^oC, rain will result.
• If the temperature stays below 0^oC, snow will result.
• If the temperature warms above 0^oC, then cools below 0^oC, then freezing rain is likely. This profile is called a ‘warm nose’

Synoptic Weather Maps. Synoptic Weather Maps are a graphical way to show temperature, humidity, air pressure, and air flow for a large area. Data is displayed using isopleths, and weather station plots. Most maps are produced for varying levels of atmosphere, including surface, 850hPa, 700hPa, 500hPa, and 250hPa. When all weather stations are plotted on a map, a ‘picture’ of where the highs, lows, and front are can be obtained. Plots typically include temperature, cloud type, atmospheric pressure, visibility, present weather, total cloud cover, 3-Hhr past pressure trend, wind flow, dew point temperature, and past weather in the previous six hours.

Please refer to How to Read Weather Station Maps.

Weather Satellites. Weather satellites are the best tool for monitoring cloud types and patterns, tracking storms, and gathering atmospheric data. There are two types of satellites:

• Polar-orbiting satellites: Orbits Earth at low-altitude, capturing the entire earth twice each day. Orbit takes 100 minutes, with a 15^oW drift each orbit.
• Geostationary satellites: Orbits Earth over the equator at a high-altitude fixed position, capturing visible, infrared, and water-vapour images for North America every 30 minutes. Views of the polar regions are limited due to the Earth’s curvature.

Visible Spectrum Satellites. Black and white images that show the intensity of light reflected from cloud tops and other surfaces. Clouds, cloud systems, air pollution, smoke, and terrain features are easy to identify. Visible images require daylight, and are of limited use in northern latitudes during winter.

Infrared Spectrum Satellites. Warmer objects appear darker, and cooler objects appear as bright colours. Since temperature decreases with altitude, high clouds are colder than lower clouds and therefore appear brighter. Infrared images are three-dimensional and do not require daylight. These images are valuable for locating towering cumulus or cumulonimbus clouds where heavy precipitation or thunder may occur. However, infrared images do not show opacity, and high clouds can be mistaken for thick clouds.

Water Vapour Satellites. Water vapour images detect water vapour in addition to clouds. These images typically show the upper two-thirds of the atmosphere due to the absorption of heat energy by the atmosphere. It is an excellent tool for showing global scale weather features and flow patterns. Penetration depends on moisture content (cannot see under a high moisture layer).

Radar. Radar is another tool for observing precipitation intensity. Radar systems use a parabolic antenna to 360^o sweep a pulsed radio frequency beam around the radar site, pointing at different elevation angles each time it sweeps around. When the bean strikes a particle of precipitation, a portion of that energy is reflected back. The intensity of this reflection is related to the number, size, and type of the precipitation particles. Sites typically have a range of 250km.

Radar interpretation. A radar image is a picture of precipitation distribution and intensities, called ‘echoes’. Echoes are represented by a series of coloured pixels. The intensity scale, called the ‘reflectivity’ in dBZ’ is on the right, and the rate of fall scale (cm/hr for snowfall in winter, mm/hr for rainfall in summer) is on the right.

Snow reflects less radar energy than rain, therefore moderate to heavy snow may appear light. Also, mountains may block radar beams, leaving gaps in the echo. Other interfering factors may also including large ground objects, virga (precipitation occurring but not reaching the ground), a temperature inversion, and electromagnetic interference.

METAR. A METAR is a format used to report a current surface weather observation within a five-statute mile radius of the reporting station. METARs are issued on the hour, and are submitted in a standard METAR code. The main elements contained are:

• Observation type
• Station identifier
• Date stamp
• Wind information
• Horizontal visibility
• Present weather
• Cloud information
• Temperature / dew point
• Altimeter setting
• Remarks

Decoded in plain language, the above METAR reads as:

TAF. A TAF compliments the METAR. They are presented in similar format, but for a 12-24hr period. TAFs tend to be accurate because they cover a small area, however they are usually a significant distance from the mountains.

Automated Weather Stations. Valley-bottom weather stations can provide valuable data on temperature, humidity, and precipitation. Ridge to stations provide data on wind speed and direction, and temperature. The location of an automated weather station is very important for an accurate weather observation, as a site exposed to strong winds may become rimed during winter (common in maritime climates). Trees or other features may block or alter the wind, affecting the accuracy of data.

Webcams. Webcams can be useful to show traffic conditions, ski area weather, weather approaching your area. They can shoot still or video, be stationary, and have the ability to rotate. Images should be date and time stamped, and always check to make sure the image is current.

Hydrological Data. Hydrological data may be collected to monitor lake levels, stream flow, precipitation, air temperature, and snow-water equivalent. Water temperature, wind speed and direction, solar radiation, and barometric pressure may also be measured. Snowpack data is collected from watersheds for the purpose of water-supply forecasting, but it may also be useful for avalanche forecasting. Snow depth and snow-water equivalents are the primary measurements

Snow Surveys, and Snow Pillow Sites. Snow surveys are manual measurements of snow depth and snow-water equivalents. Measurements are made at the same locations throughout winter. In the case of snow pillow stations, snow accumulates on a large bladder, and the weight of this snow pushes on the anti-freeze solution from the bladder up to a standpipe.