Running Weather Terminology Explained: Every Term You Need to Know

Weather forecasts throw around terms that most people vaguely understand but few can precisely define. For runners, this imprecision matters. The difference between relative humidity and dew point affects how you should approach a run. Understanding what "30% chance of rain" actually means changes how you plan. Knowing why "feels like" temperature differs from actual temperature helps you dress appropriately.

This guide defines every weather term that matters for running, explains why each term is relevant, and shows how to use this knowledge for better running decisions.

Temperature Terms

Actual Temperature (Air Temperature)

What it is: The temperature measured by a thermometer in the shade, protected from precipitation and direct sunlight. This is the number you see as the main temperature in forecasts.

How it's measured: Weather stations use thermometers in ventilated enclosures (typically called Stevenson screens) positioned 4-6 feet above the ground.

Running relevance: Actual temperature is your starting point for deciding what to wear and when to run. However, it doesn't tell the whole story—you also need to consider humidity and wind.

Feels Like Temperature

What it is: An adjusted temperature that accounts for how weather conditions actually feel to your body. It combines the effects of temperature, humidity, and wind into a single number.

How it's calculated: In heat, it incorporates humidity (heat index). In cold, it incorporates wind (wind chill). The formula switches based on conditions.

Running relevance: Feels-like temperature is more useful than actual temperature for planning runs. A 75°F day with high humidity might feel like 85°F. A 30°F day with strong wind might feel like 15°F.

Heat Index

What it is: A measure of how hot it actually feels when humidity is factored in. It's the feels-like temperature for warm conditions.

The mechanism: When humidity is high, your sweat can't evaporate efficiently. Evaporating sweat is your body's primary cooling mechanism. When cooling is impaired, the effective temperature your body experiences is higher.

Calculation: Heat index combines air temperature and relative humidity using a complex formula. It's most relevant when temperature exceeds 80°F.

Heat index thresholds for runners:

Below 80°F: Typically labeled as actual temperature
80-90°F heat index: Caution; reduce intensity
90-100°F heat index: Significant heat stress; modify workouts substantially
100-105°F heat index: Dangerous; easy running only if at all
Above 105°F heat index: Extreme danger; avoid outdoor running

Running relevance: Always check heat index, not just temperature, when planning summer runs. The same 85°F day can feel dramatically different depending on humidity.

Wind Chill

What it is: A measure of how cold it actually feels when wind is factored in. It's the feels-like temperature for cold conditions.

The mechanism: Wind strips away the thin layer of warm air your body maintains near your skin. The faster the wind, the faster heat is removed from your body.

Calculation: Wind chill combines air temperature and wind speed. It's most relevant when temperature is below 50°F.

Wind chill thresholds for runners:

Above 25°F wind chill: Generally comfortable with proper layers
10-25°F wind chill: Cold but manageable; protect extremities
0-10°F wind chill: Significant cold stress; limit exposure
-10 to 0°F wind chill: Frostbite risk; cover all exposed skin
Below -10°F wind chill: Dangerous; consider indoor alternatives

Running relevance: Wind chill determines how you should dress and whether outdoor running is wise. A 20°F day with no wind is very different from 20°F with 20 mph wind (wind chill near 0°F).

Dew Point

What it is: The temperature at which air becomes saturated and water vapor condenses into liquid water (dew or fog). It's a measure of the actual moisture content in the air.

Why dew point matters more than relative humidity: Relative humidity changes throughout the day as temperature changes—even with the same amount of moisture in the air. Dew point reflects absolute moisture and remains more stable.

Interpreting dew point:

Below 50°F: Dry and comfortable
50-55°F: Comfortable for most activities
55-60°F: Slightly humid, noticeable for runners
60-65°F: Humid; sweating is less efficient
65-70°F: Oppressive; significant impact on running performance
Above 70°F: Very oppressive; running becomes difficult

Running relevance: Dew point is the best single metric for understanding how humidity will affect your run. A 60°F dew point means reduced sweat evaporation regardless of temperature.

Relative Humidity

What it is: The amount of water vapor in the air expressed as a percentage of the maximum amount the air could hold at that temperature.

Why it's misleading: Relative humidity is relative to temperature. Cold air can hold less moisture, so 70% humidity at 40°F has much less moisture than 70% humidity at 80°F. This makes relative humidity poor for comparing conditions across different temperatures.

The morning problem: Relative humidity is often highest in early morning (when it's coolest) and lowest in afternoon (when it's warmest)—even though the actual moisture content hasn't changed.

Running relevance: Relative humidity is less useful than dew point for running decisions. A high humidity reading early morning might not mean uncomfortable running conditions if the dew point is low.

Wet Bulb Globe Temperature (WBGT)

What it is: A composite measure that accounts for temperature, humidity, wind, sun angle, and cloud cover to assess heat stress. It's the most comprehensive heat metric.

How it's measured: Uses three thermometers: a regular one, one surrounded by a wet cloth (wet bulb), and one inside a black metal globe (to measure radiant heat).

WBGT thresholds:

Below 65°F: Low heat risk
65-73°F: Moderate risk; active individuals should take precautions
73-82°F: High risk; modify intense activity
82-90°F: Very high risk; cancel or postpone intense activity
Above 90°F: Extreme risk; outdoor activity dangerous

Running relevance: WBGT is the gold standard for athletic heat decisions. Many races and sports organizations use WBGT to determine race modifications or cancellations. If you can access WBGT readings, they're more comprehensive than heat index alone.

Wind Terms

Sustained Wind

What it is: The average wind speed over a given period, typically measured over 2 minutes. This is the main wind speed you see in forecasts.

Running relevance: Sustained wind affects your effort consistently. A 15 mph sustained wind means you'll face that wind level throughout your run.

Wind Gusts

What it is: Brief increases in wind speed that exceed the sustained wind speed. Gusts are typically listed as a separate number (e.g., "winds 15 mph with gusts to 25 mph").

Running relevance: Gusts create momentary increases in resistance and can be destabilizing. Strong gusts require more core engagement and can disrupt rhythm.

Wind Direction

What it is: The direction wind is coming FROM, not going toward. A "north wind" or "northerly wind" comes from the north and blows toward the south.

Running relevance: Wind direction determines which parts of your route will have headwind versus tailwind. Plan routes to finish with tailwind when possible.

Calm

What it is: Wind speed below 3 mph. Some systems classify calm as below 1 mph.

Running relevance: Calm conditions are ideal for running. No wind resistance, stable conditions, predictable effort.

Light Wind

What it is: Generally 3-12 mph. You can feel it but it doesn't significantly impede movement.

Running relevance: Light wind has minimal impact on running. You'll notice it but won't need to adjust much.

Moderate Wind

What it is: Generally 13-25 mph. Dust and loose paper are raised. Small branches move.

Running relevance: Moderate wind noticeably affects running. Expect to work harder into headwind and benefit from tailwind.

Strong/High Wind

What it is: Generally above 25 mph. Large branches move. Walking against wind becomes difficult.

Running relevance: Strong wind significantly impacts running. May affect balance, makes hard efforts into wind very challenging.

Precipitation Terms

Probability of Precipitation (PoP)

What it is: The chance that measurable precipitation will occur at your location during the forecast period. "30% chance of rain" means there's a 30% probability of getting at least 0.01 inches of rain.

What it doesn't mean: PoP doesn't indicate how long it will rain, how hard it will rain, or what percentage of the area will see rain. It's purely probability of occurrence.

Running relevance: Understanding PoP helps you decide whether to run with rain in mind. 30% might mean carrying a jacket; 80% means expecting to get wet.

Scattered

What it is: Precipitation affecting 30-50% of a forecast area. If your region has scattered showers, roughly a third to half of the area will see rain.

Running relevance: Scattered precipitation means you might encounter rain or might not. Have a plan for either scenario.

Isolated

What it is: Precipitation affecting less than 30% of a forecast area. Rain will occur somewhere but probably not where you are.

Running relevance: Isolated precipitation is generally ignorable for running planning, though thunderstorms can be dangerous even when isolated.

Intermittent

What it is: Precipitation that stops and starts multiple times.

Running relevance: Intermittent rain means you'll have wet and dry periods during a run. May not need rain gear if willing to get briefly wet.

Steady

What it is: Continuous precipitation without significant breaks.

Running relevance: Steady rain means you will definitely get wet. Plan for rain gear and post-run drying.

Trace

What it is: Precipitation amount so small it can't be measured accurately—less than 0.01 inches.

Running relevance: Trace precipitation often goes unnoticed during running. Barely registers.

Drizzle

What it is: Very light precipitation consisting of small water droplets, typically less than 0.5mm in diameter.

Running relevance: Drizzle is usually comfortable for running—provides slight cooling without soaking you.

Rain

What it is: Liquid precipitation with droplets larger than drizzle.

Running relevance: Rain ranges from manageable to miserable depending on intensity. Light rain is often pleasant; heavy rain creates challenges.

Freezing Rain

What it is: Rain that freezes on contact with cold surfaces, creating ice.

Running relevance: Freezing rain creates extremely dangerous running conditions. Surfaces that look wet are actually icy. Avoid running on paved surfaces during freezing rain events.

Sleet

What it is: Ice pellets that form when rain freezes before reaching the ground.

Running relevance: Sleet stings but usually doesn't create as dangerous surfaces as freezing rain. Can still make footing tricky.

Pressure Terms

Barometric Pressure

What it is: The weight of the atmosphere pressing down at a given location, typically measured in inches of mercury (inHg) or millibars (mb).

Standard pressure: 29.92 inHg or 1013.25 mb at sea level.

Running relevance: Pressure changes slightly with weather patterns but doesn't directly affect running performance significantly at typical elevations.

Rising Pressure

What it is: Barometric pressure increasing over time, typically indicating improving weather—storms moving out, clearing skies.

Running relevance: Rising pressure often means conditions will improve. Good time to plan outdoor runs if current conditions are marginal.

Falling Pressure

What it is: Barometric pressure decreasing over time, typically indicating approaching weather systems—storms, fronts, precipitation.

Running relevance: Falling pressure suggests weather may deteriorate. If planning a longer run, earlier may be better than later.

Cloud and Sky Terms

Clear

What it is: Sky with 0-10% cloud cover.

Running relevance: Clear skies mean maximum sun exposure. Consider sun protection; can be warmer than overcast conditions.

Partly Cloudy

What it is: Sky with 30-50% cloud cover.

Running relevance: Intermittent sun and shade. Generally comfortable conditions.

Mostly Cloudy

What it is: Sky with 70-80% cloud cover.

Running relevance: Limited direct sun. Can make warm temperatures more comfortable.

Overcast

What it is: Complete or nearly complete cloud cover.

Running relevance: No direct sun. Often ideal for running—reduced radiant heat, softer light.

Cloud Ceiling

What it is: The height of the lowest cloud layer covering more than half the sky.

Running relevance: Low ceilings may indicate fog or precipitation. Not directly relevant unless running in mountains where clouds can descend to ground level.

Visibility Terms

Visibility

What it is: The greatest distance at which objects can be identified. Measured in miles or fractions of miles.

What affects visibility: Fog, haze, precipitation, dust, and smoke all reduce visibility.

Running relevance: Low visibility affects safety, particularly when running on roads. Also affects your route choices and navigation.

Fog

What it is: Cloud at ground level that reduces visibility below 5/8 mile. Dense fog reduces visibility below 1/4 mile.

Running relevance: Fog creates visibility concerns for safety and can make conditions feel damper and colder. Can be refreshing for running but requires caution on roads.

Haze

What it is: Suspended dust, smoke, or other particles that reduce visibility and give air a milky appearance.

Running relevance: Haze often accompanies poor air quality. May indicate you should check AQI before running.

Air Quality Terms

AQI (Air Quality Index)

What it is: A standardized scale from 0-500 measuring air pollution levels. Higher numbers mean more pollution.

AQI categories:

0-50: Good
51-100: Moderate
101-150: Unhealthy for sensitive groups
151-200: Unhealthy for everyone
201-300: Very unhealthy
301-500: Hazardous

Running relevance: AQI directly affects whether you should run outdoors and at what intensity. See our Air Quality Basics guide for detailed information.

PM2.5

What it is: Particulate matter 2.5 micrometers or smaller in diameter—small enough to penetrate deep into lungs.

Running relevance: PM2.5 is often the primary pollutant driving AQI. Wildfire smoke is primarily PM2.5. These particles are particularly dangerous for runners who breathe deeply.

Ozone

What it is: Ground-level ozone, formed when sunlight reacts with vehicle emissions and industrial pollutants.

Running relevance: Ozone peaks in afternoon on hot, sunny days. Morning runs avoid ozone peaks.

Putting It All Together

The Essential Numbers for Runners

Before any run, check:

Temperature (starting point)
Feels-like temperature (actual experience)
Dew point (humidity impact)
Wind speed and direction (route planning)
Precipitation probability (gear decisions)
AQI (health consideration)

For summer runs, prioritize:

Dew point (more important than humidity %)
Heat index
AQI (ozone and PM2.5)

For winter runs, prioritize:

Wind chill
Precipitation type
Surface conditions

Converting Knowledge to Decisions

Understanding weather terminology helps you:

Dress appropriately for actual conditions
Choose optimal times to run
Plan routes strategically
Set realistic pace expectations
Stay safe in challenging conditions

Weather forecasts are only useful if you understand what they mean. Run Window translates weather data into running recommendations—but understanding the terminology helps you make smarter decisions even when looking at raw forecasts.