Xweather Horizon Glossary

Pressure within the atmosphere, also known as barometric pressure.

Temperature of ambient air.

Absolute value of the amount of chemical present. The liquidus and solidus temperatures depend both from the amount of salt, and of water and ice. The water content may change very fast, and usually will do so when an event, such as the start of snowfall, rain, or accumulating of frost takes place, so the amount of chemical salt is a more reliable safety indicator.

Sub-surface temperature at a defined depth.

The temperature to which a given parcel of air must be cooled for water vapor to condense into water. As the air temperature is cooled towards the dew point, the air becomes more saturated and this is represented by an increase in relative humidity. Once the air and dew temperature are the same, 100 % relative humidity is reached and condensation occurs. The greater the difference between the air and dew temperature the drier the air mass is and the lower the relative humidity. Dew forms on roads when the road surface temperature is below the dew point of the air directly above the road. Frost forms on roads when the road surface temperature is below freezing and is also below the dew point of the air directly above the road.

The grip readings display the level of grip between the vehicle tire and road surface. They can be used for

• Safety assessments: They provide an objective measure of the likelihood that vehicles are able to start, stop, or maneuver without losing control. Road maintenance crews can use this data to make informed decisions about where and when to apply treatments like grit or salt.
• Evaluating treatment effectiveness: Post-treatment grip levels can be monitored to assess how effective a particular maintenance strategy was. This information is then a key factor in the decision to treat again if needed.
• Proactive measures: Regular monitoring of grip levels can provide an early warning of deteriorating conditions. It allows maintenance operations to react before roads become hazardous, rather than responding after problems are reported.
• Legal and insurance implications: Documented grip levels can be important during legal disputes or insurance claims related to road accidents. They can serve as evidence as to whether appropriate road maintenance was carried out.
• Public information and communication: This objective measure can be communicated to the public for real-time road condition updates. It is valuable information for drivers planning their routes and can increase overall trust in winter road maintenance operations.

Average depth of water on road surface if all the ice were melted.

The liquidus temperature is the temperature at which ice crystals first start forming on the road surface. This is generally 0 °C (32 °F) at sea level under standard atmospheric pressure. When adding chemical, if it goes into solution, the liquidus temperature decreases. The stronger the concentration, the lower the temperature goes.

The solidus temperature is the temperature at which the salt solution on your pavement surface becomes completely frozen, and no liquid solution remains.

In between liquidus and solidus temperatures, the mixture of chemical and water begins to freeze but has not yet become completely solid. It is a slushy mixture of ice crystals and a concentrated salt solution, meaning it is more manageable for removal. Once the road surface temperature goes below the solidus temperature, it is more challenging to remove the ice.

These readings can be used to show evidence that a treatment was made, and that there is some residual chemical on the sensors. However, reference should also be made to the amount of chemical reading.

Rate at which precipitation occurs, expressed in units of depth per hour. It includes liquid precipitation and the liquid water equivalent of any frozen precipitation.

Rate at which rain occurs, expressed in millimeters or inches per hour.

The amount of water vapor within a parcel of air, expressed as a percentage, relative to the maximum amount it could hold at a given temperature. When the air becomes saturated with water vapor and the relative humidity reaches 100%, fog appears.

A network, region, or route is divided into smaller segments. For each segment, Xweather Horizon models what is happening now (assessment) and what will happen in the future (forecast). If observation data is available, it is used that to initialize and verify the assessment and forecast data.

A road segment is a stretch of road that exhibits similar road surface conditions. Therefore, Xweather Horizon creates one assessment or forecast for each road segment. Each segment has unique contextual road data such as longitude, latitude, shading, height above sea level, constructions, traffic flow, and bridge locations.

A road segment length is dynamic and can vary from 30 m to up to 2 km, depending on predicted weather variability. The more variability over an area, the shorter the segments will be. For example, a bridge deck will be a separate segment because the surface conditions on the bridge deck may well be significantly different from the non-bridge segments around it.

Depth of snow as measured with a ruler, from the ground to the top of the layer of snow. Forecast snow depth takes into account melting and traffic.

A user-defined data point on a road network that generates site-specific forecasts . There is no hardware/sensors at the location.

Rate at which snow occurs, expressed in centimeters or inches per hour.

Average depth of water on road surface if all the snow were melted. Typically, 10 cm of snow equals about 1 cm of water.

Amount of snow on the road or runway affected by other factors, such as melting, removal by traffic, and evaporation. For now, treatments are only taken into account at weather stations.

Condition of a road or runway surface (dry, moist, wet, ice, and similar).

Temperature of a road or runway surface.

Greatest distance at which a black object of suitable dimensions, situated near the ground, can be seen and recognized when observed against a bright background.

Average depth of water on road or runway surface.

Ongoing or upcoming time window during which specified weather conditions are present. These conditions are defined in the weather event configuration.

Lowest temperature to which air can be cooled by the evaporation of water into the air in current conditions.

Direction from which the wind blows.

Direction from which wind gust blows.

In forecast, highest 3‑second average wind speed within 10 minutes at a height of 10 meters above the surface. In observations, the height depends on where sensors are installed.

Rate at which air is moving. In forecast, the rate is a 10‑minute average. In observations, it is typically a 10‑minute average, but can vary depending on the sensor.