Road Weather Glossary

Plain-English definitions for 26 key weather terms, how they affect driving conditions on your route, and exact locations where each metric appears in Wayther.

Wind

More trips are changed by wind than by rain

Wind speed: The sustained or average movement of air, distinct from short-lived gusts. Wind forecasts and observations are usually referenced to a standard height above the ground, commonly 10 metres, but the wind you experience at road level can differ. Buildings, trees and terrain slow or redirect the flow, while bridges, ridges and open plains can leave a vehicle more exposed.; In Wayther. Wind speed is shown at forecast points along your route, helping you identify where conditions may be more challenging than at the start or destination.
Gust: A brief increase in wind speed above the prevailing wind. Gusts can create sudden changes in the force acting on a vehicle, making them particularly important for motorcycles, vans, motorhomes and vehicles towing trailers. The effect depends on the vehicle, wind direction, road exposure and how abruptly the gust arrives.; In Wayther. Wind gusts are shown alongside wind speed at forecast points, so you can compare sustained conditions with the stronger peaks expected along the route.
Crosswind, headwind and tailwind: Wind direction describes where the wind comes from, and its effect depends on how it meets your direction of travel. A headwind blows towards the front of the vehicle and can increase aerodynamic resistance. A tailwind blows from behind. A crosswind acts from the side and can push a vehicle laterally, particularly when it has a large side area or is travelling across an exposed stretch. Real conditions can change around bends, buildings, bridges and gaps in terrain.; In Wayther. Wind direction is shown at forecast points. Compare it with your direction of travel to understand where crosswinds or headwinds may matter most.
Why tall vehicles are affected more by wind: Wind exerts pressure on a vehicle’s exposed surfaces. The force generally increases with the square of the relative wind speed, although the actual effect depends on the vehicle’s shape, orientation and surroundings. A tall vehicle with a large side area, such as a motorhome, van or trailer, can experience substantial lateral force. Vehicle weight, load distribution, suspension and the height at which the force acts also influence stability. An empty trailer can behave differently from a loaded one, and there is no single wind speed that is safe or unsafe for every vehicle.; In Wayther. Wind speed and gusts are shown at forecast points along the route, helping you identify exposed stretches rather than relying on one wind figure for the whole journey.
Wind direction: The direction from which the wind is blowing, usually expressed as a compass direction or an angle. A north wind comes from the north and blows south. Direction matters because the same wind speed can affect a vehicle differently depending on whether it meets the front, side or rear. Local terrain can also change the direction experienced at road level.; In Wayther. Wind direction is available at forecast points, allowing you to assess it in the context of the road and your direction of travel.
Wind exposure: How directly a stretch of road is exposed to the wind. Open plains, bridges, coastal roads, mountain passes and gaps between hills may offer less shelter than roads lined with trees or buildings. A weather forecast describes atmospheric conditions at forecast locations; it does not necessarily capture every local gust or sudden change caused by the landscape.; In Wayther. Use the route map and point-by-point wind forecast to identify where stronger winds may coincide with exposed terrain. The forecast cannot confirm the exact wind conditions at every bridge or road cutting.

Temperature

Why the top of the hill is not the same as the bottom

Lapse rate: The rate at which air temperature changes with altitude. In the standard atmosphere, temperature decreases by about 6.5 °C per 1,000 metres through the lower atmosphere. This is a reference value, not a rule for every day: actual temperature profiles depend on weather systems, humidity, time of day and inversions. Even so, a mountain pass can be substantially colder than a nearby valley.; In Wayther. Temperature is shown at forecast points along the route, helping you compare conditions in valleys, at higher elevations and near mountain passes.
Freezing level: The altitude at which the air temperature reaches 0 °C in a particular atmospheric profile. It helps indicate where snow or other frozen precipitation may occur, but it does not by itself determine the precipitation type or whether a road surface is icy. Warm layers, cold air near the ground, precipitation intensity and road-surface temperature can all change the outcome.; In Wayther. Compare temperature and precipitation forecasts at points along the route to spot changes that may matter on high roads. These values do not directly measure road-surface temperature or confirm ice.
Wind chill: An estimate of how cold the air feels on exposed skin when wind increases heat loss from the body. It is a human comfort and cold-exposure index, not a measurement of air temperature or the temperature of a vehicle. The standard calculation is designed for cold conditions and has defined limits; it should not be extrapolated to warm weather or used to infer whether a road is frozen.; In Wayther. Wayther shows forecast air temperature rather than a calculated wind-chill value. For motorcyclists, combine the temperature with wind speed and expected exposure when planning clothing and breaks.
Road-surface temperature: The temperature of the road itself, which can differ from the air temperature. A road can cool below freezing on a clear night even when the air is slightly above 0 °C. Bridges and elevated sections can cool faster because they lose heat from more than one side. Forecast air temperature alone cannot confirm whether a surface is icy.; In Wayther. Wayther displays forecast weather conditions, not a direct measurement of road-surface temperature. Treat temperatures near freezing as a reason to check local road and winter-weather information, especially on bridges and mountain roads.
Temperature inversion: A layer of the atmosphere in which temperature increases with altitude instead of decreasing. Inversions can form on calm, clear nights as the ground cools, leaving colder air trapped in valleys beneath warmer air above. They can contribute to valley fog and large temperature differences over relatively short climbs.; In Wayther. Compare temperature and visibility forecasts along your route. Conditions can differ between a valley and higher ground, although a route forecast does not guarantee that every local inversion or fog bank will be captured.

Rain and visibility

What you can be inside, and what you cannot see

Rain and showers: Rain often describes relatively steady precipitation over an area, while showers tend to be intermittent and more localised. This distinction matters on a journey: steady rain may affect a long section of the route, while showers can vary considerably over short distances. Forecast timing and location are estimates, so the precise start and end of a shower may differ from what is predicted.; In Wayther. Precipitation is forecast along the route and associated with your expected arrival times, helping you see where and when rain may affect the journey.
Probability of precipitation: The likelihood that measurable precipitation will occur at a given location during a specified period. It is not the percentage of the route that will be wet, the percentage of time it will rain, or a measure of how heavy the rain will be. Probability, expected amount, duration and timing describe different aspects of a forecast.; In Wayther. Use the precipitation forecast together with its timing and location along the route. A probability alone cannot tell you exactly when a shower will begin or how much rain will fall on a particular road.
Freezing rain: Rain that freezes when it touches a surface at or below freezing, forming a layer of ice. It commonly develops when snow melts while falling through a warmer layer of air and then passes through a shallow layer of subfreezing air near the ground. The road can become icy even when the air feels relatively mild. Other types of freezing precipitation can also create dangerous ice.; In Wayther. Check temperatures and precipitation along the route to identify conditions that could be consistent with icing. Wayther does not directly detect freezing rain or confirm that a road is icy, so consult official winter-weather warnings and road reports.
Visibility: The distance at which objects can be seen and recognised under prevailing conditions. Fog, heavy rain, snow, spray, smoke and dust can all reduce visibility. The change can matter as much as the absolute value: a rapid drop from clear conditions to a short viewing distance can leave less time to react. Fog can form overnight when the air near the ground cools, but its timing and duration depend on local conditions and are not always predictable.; In Wayther. Visibility is charted along the route and listed at forecast points, helping you identify where reduced visibility is expected during your journey.
Fog: A cloud of tiny water droplets at ground level that reduces horizontal visibility. Radiation fog can form overnight as the ground cools, often in calm conditions, and may clear after sunrise. Advection fog forms when moist air moves over a cooler surface. Fog can also develop or persist in valleys and near bodies of water, and it may vary sharply over short distances.; In Wayther. Check the visibility forecast at points along the route and at the times you expect to reach them. Local fog can form or clear faster than a forecast can resolve, so remain alert to actual conditions.
Heavy rain: Rain falling at a high rate over a period of time. It can reduce visibility, increase windscreen-wiper demand and contribute to water accumulating on the road. The risk of surface water depends not only on rainfall intensity but also on drainage, road condition, slope and how much rain has already fallen.; In Wayther. The precipitation forecast helps you spot periods and route sections where rain is expected. It cannot confirm surface water, flooding or the condition of the road itself.
Snow and sleet: Snow consists of ice crystals that fall as flakes. Sleet generally refers to small ice pellets formed when falling precipitation freezes before reaching the ground, although terminology varies by region. Snow can reduce visibility and accumulate on roads; sleet can also make surfaces slippery. Air temperature alone does not determine whether snow or ice will remain on the road.; In Wayther. Compare forecast temperatures and precipitation along the route, especially when travelling through higher elevations. Use official road and winter-weather reports to check for closures, snow accumulation and icy surfaces.

Your route

The numbers on the screen, and what they attach to

Forecast point: A location along your route where forecast conditions are evaluated. It does not have to be a town or a named place. This distinction matters because the weather at your departure point or destination may not represent the conditions you encounter in between.; In Wayther. The map, charts and timeline provide different views of forecast conditions at points along the route.
Departure and arrival time: Departure time is when you begin the journey; arrival time is when you are expected to reach a particular point or your destination. Arrival estimates are based on the planned route and travel-time assumptions, so traffic, stops and changes in driving speed can affect the actual timing. Changing the departure time shifts the estimated arrival times and can reveal whether another travel window has a more favourable forecast.; In Wayther. Plan departure times up to 10 days ahead and review the forecast at expected arrival times along the route. Actual arrival times may differ from the estimates.
Day and night driving: The portion of a journey that takes place in daylight or darkness. Sunrise and sunset vary with date and location, and a long route can cross different sunrise and sunset times. Darkness can reduce visibility and make unfamiliar roads, changing weather and fatigue more difficult to manage.; In Wayther. The trip summary shows the day-and-night split, and sunrise and sunset are listed at forecast points along the route.
Alternative route: A different path between the same departure and destination, potentially with a different distance, travel time, elevation profile and weather forecast. The shortest route is not necessarily the most suitable for every journey. A longer route may encounter different weather, but a forecast alone cannot establish that it is safer or that road conditions will be better.; In Wayther. Compare available route alternatives side by side, including their forecast conditions, distance and travel time, to understand the trade-offs before choosing a route.
Severe weather warning: An official warning issued by a recognised weather authority when specified hazardous conditions are expected or occurring. Warning systems, severity levels and geographic coverage vary by country. A general weather forecast or an app’s summary of conditions is not necessarily an official warning, and the absence of an in-app warning does not mean that no warning has been issued.; In Wayther. Wayther highlights forecast conditions such as strong winds, extreme temperatures, heavy precipitation and poor visibility. Check the relevant official weather service for formal warnings and their latest updates.
Forecast uncertainty: The unavoidable uncertainty in predicting future weather. Forecasts are generally more reliable for broad patterns than for the precise timing and location of local showers, fog patches or gusts. Uncertainty usually increases further into the future, and complex terrain can make local conditions harder to predict.; In Wayther. Wayther helps compare forecast conditions along a route and across departure times, but these are predictions rather than guarantees. Recheck the forecast before leaving, particularly when conditions are changing.
Estimated travel time: The predicted duration of a journey along a selected route under the assumptions used by the routing service. Actual travel time can change because of traffic, stops, roadworks, weather, speed limits and driving conditions. A travel-time estimate should not be treated as a guarantee of when you will arrive.; In Wayther. Wayther uses the planned route and selected travel settings to associate weather forecasts with estimated arrival times. Allow for stops and unexpected delays when planning your trip.
Weather along a route: A view of forecast conditions at different locations and times along a planned journey, rather than a single forecast for the departure city or destination. It combines location and timing: the weather expected halfway through the drive may be different from the weather at either end.; In Wayther. This is Wayther’s core purpose: view weather on the map, explore forecast points in charts and a timeline, and compare route alternatives or departure times.

Methodology & Data Standards

Meteorological Sources & Excluded Thresholds

Definitions across this page follow standard meteorological principles. Specific parameters are grounded in international scientific standards:

Lapse Rate (6.5 °C per 1,000 metres):Reflects the International Standard Atmosphere (ISA) temperature gradient defined by ICAO, WMO, and ISO.
Wind Chill Index:Calculated for temperatures below 10 °C and wind speeds above 3 mph, using the model established by the US National Weather Service and Environment and Climate Change Canada.

Arbitrary driving thresholds—such as unsafe wind speeds, critical visibility distances, heavy rainfall definitions, or bridge closure wind limits—are intentionally excluded. These variables depend on vehicle type, road exposure, forecast provider criteria, and local transportation authority policies.

Notice: This page serves solely as an educational glossary. It does not generate live forecasts or replace official emergency weather alerts and road closures.

Before your next journey

Check the forecast for the road ahead.

Wayther is a road trip weather app for drivers, available on iPhone and iPad. It provides hour-by-hour road trip weather at every point on your route, timed to when you'll be there. Compare routes, find the best time to leave, plan 10 days ahead. Try it free.