Why Weather Forecasts Are Probabilistic
Before learning to read a forecast, it helps to understand what forecasts are actually saying. Weather prediction is probabilistic, not deterministic — forecasters cannot specify exactly what will happen but can quantify how likely different outcomes are. A "70% chance of rain" does not mean it will rain 70% of the day. It means that, given current atmospheric conditions, rain is expected in 70% of similar situations. On a given day it may rain all day, for five minutes, or not at all — all consistent with a 70% probability forecast.
This probabilistic nature becomes more pronounced at longer time ranges. A 24-hour forecast can be highly accurate; a 7-day forecast is a useful guide to broad trends rather than precise conditions. Understanding this prevents the frustration of "the forecast was wrong" when conditions fall within the expected probability range.
Temperature: High, Low and Feels Like
Most forecasts show a daily high and low temperature. The high is typically reached in the early-to-mid afternoon; the low occurs just before sunrise. These are measured in the shade at standard height — direct sun exposure can feel significantly warmer.
The "feels like" (apparent) temperature accounts for two modifying factors: wind chill and heat index. Wind chill describes how much colder wind makes exposed skin feel by accelerating heat loss — a 5°C day with 50km/h winds can feel like -5°C. The heat index describes how humidity makes hot temperatures feel hotter by impeding the body's evaporative cooling — 32°C at 90% humidity can feel like 41°C. Always check the feels-like temperature for outdoor planning.
Precipitation: Type, Amount and Probability
Precipitation forecasts typically show three elements: type (rain, drizzle, snow, sleet, hail), amount (millimetres or inches), and probability (percentage). Probability of Precipitation (PoP) indicates the likelihood that measurable precipitation will occur at a given location during the forecast period.
Rain amounts matter for planning: 1–5mm is light, 5–20mm is moderate, 20–50mm is heavy, and 50mm+ in 24 hours indicates severe conditions with potential for flooding. Snowfall is measured differently — a common rule of thumb is that 10cm of snow equals approximately 10mm of liquid water, though this varies greatly with snow density.
Wind: Speed, Direction and Gusts
Wind forecasts typically show average wind speed and direction, plus gusts. Wind direction is reported as where the wind is coming FROM — a northerly wind blows from north to south. Speed may be expressed in km/h, mph, m/s, or knots depending on the platform.
Wind gust values are important for outdoor planning — they represent short bursts significantly stronger than the average. A forecast of "20km/h with gusts to 45km/h" means sustained wind at 20km/h with intermittent gusts at more than double that speed. For sailing, outdoor events or structural concerns, gust values are often more relevant than average speed.
Humidity and Dew Point
Relative humidity expresses the amount of water vapour in the air as a percentage of the maximum it could hold at that temperature. 100% humidity means the air is fully saturated — at which point dew forms or fog develops. 30% humidity feels dry and comfortable; above 70% starts to feel muggy.
Dew point is a more absolute measure of atmospheric moisture — it is the temperature at which air would become fully saturated. A dew point above 20°C feels oppressively humid; below 10°C is comfortable; below 0°C is very dry. Professional meteorologists often consider dew point more useful than relative humidity because it doesn't change with temperature the way relative humidity does.
UV Index: Understanding Sun Exposure Risk
The UV index quantifies the intensity of ultraviolet radiation from the sun on a scale from 0 to 11+. UV levels of 0–2 are low (minimal protection needed); 3–5 are moderate (wear sunscreen); 6–7 are high (sunscreen, hat, and shade recommended); 8–10 are very high (extra protection essential); and 11+ are extreme (avoid outdoor exposure at peak hours).
UV index varies with time of day (peaks around solar noon), season, latitude, altitude and cloud cover. Thin cloud cover can reduce UV by 50%; thick cloud cover can still allow 80% of UV through. SunorSnow's weather dashboard displays the current UV index for all tracked cities, helping you plan sun exposure appropriately.
Reading Hourly vs Daily Forecasts
Daily forecasts summarise the full day with a high, low and general description. Hourly forecasts break the day into individual hours, showing how temperature, precipitation chance and wind will evolve throughout the day — essential for planning time-sensitive outdoor activities. If you're planning an outdoor event, always check the hourly forecast and look for the timing of any precipitation windows, not just the daily probability.
SunorSnow's dashboard shows current conditions for up to 6 cities simultaneously, making it easy to compare conditions across locations in real time and decide which city offers the best window for your plans.
Frequently Asked Questions
What does 40% chance of rain mean?
A 40% probability of precipitation (PoP) means that, given current atmospheric conditions, there is a 40% chance that measurable rainfall will occur at your specific location during the forecast period. It is not a statement about how long it will rain or how much — just whether any rain is likely.
What is the difference between weather and a forecast?
Current weather is what the atmosphere is doing right now, measured by instruments. A forecast is a scientific prediction of future atmospheric conditions based on numerical weather models, satellite data and historical patterns.
How accurate are 7-day weather forecasts?
Modern 7-day forecasts are generally reliable for broad temperature trends and major weather patterns but have lower accuracy for specific precipitation timing and amounts. Accuracy drops significantly beyond 5 days. The first 24–48 hours of a forecast are usually highly accurate.
What does 'dew point' mean in a forecast?
Dew point is the temperature at which air becomes fully saturated with moisture, causing condensation to form. A high dew point (above 18°C) indicates humid, muggy air; a low dew point (below 10°C) feels dry and comfortable. Dew point is a more stable humidity measure than relative humidity.