A weather map summarizes the distribution of atmospheric pressure, fronts, and observation data on a map. By learning how to read it, we can predict the weather.
Basic Knowledge
The basic elements of a weather map are isobars (usually drawn every 4hPa, with bold lines every 20hPa), high-pressure systems (H), low-pressure systems (L), front symbols, and weather symbols.
Weather at each location is represented by symbols: ○ clear, ◯ one line = fair, ◎ mostly cloudy, ● rain, ※ snow, and so on.
Wind is shown with wind barbs. The direction of the arrow indicates the direction from which the wind is blowing, and the number of feathers indicates wind force (0–12).
Weather is analyzed by combining satellite visible images (cloud thickness can be inferred from brightness), infrared images (cloud-top temperature and altitude), and water vapor images (the amount of water vapor in the middle atmosphere). AMeDAS (the automated meteorological data acquisition system) observes rainfall, wind direction and speed, temperature, and other data in real time at about 1300 locations.
Weather map（A map showing pressure, fronts, observation values, and other data）
Isobar（A line connecting places with the same pressure. Usually every 4hPa）
Front symbol（▲ warm／▼ cold／▲▼ stationary／overlapping occluded）
Weather symbol（○=clear, ◎=cloudy, ●=rain, ※=snow, etc.）
AMeDAS（Automated meteorological data acquisition system. About 1300 locations）
Himawari weather satellite（Observes the area around Japan from geostationary orbit. Visible, infrared, and water vapor images）
Further Study (Background and Significance)
The history of weather forecasting is long. In Japan, the first weather forecast was issued in 1884. At that time, forecasts were vague, such as “The wind direction will be unsettled throughout the country, and the weather will change easily; however, rainy weather is likely.”
Today, computer simulations called numerical weather prediction are central. They incorporate observation data from around the world and calculate future conditions by solving equations of atmospheric motion. Japan's supercomputer “Fugaku” is also used for weather prediction.
The ability to read weather maps protects lives and property in many situations, including mountaineering, shipping, agriculture, and sports. The ability of a hiker to judge that “tomorrow will be rainy because a low-pressure system is approaching from the west” is an important part of disaster-prevention education.
- Isobars are drawn every 4hPa (bold lines every 20hPa)
- H = high-pressure system (clear) / L = low-pressure system (rain)
- Front symbols show the type of front
- Wind barb direction = direction from which the wind blows
- Wind force has 13 levels, from 0 to 12
- AMeDAS observes at about 1300 locations
- Himawari is a geostationary satellite (altitude about 36000km)
Points to Note (Easily Confused)
① Wind barbs indicate the direction from which the wind is blowing. “A northerly wind” blows from the north. ② Close isobar spacing means strong wind. ③ Distinguish visible images (daytime only・cloud thickness) from infrared images (available 24 hours・cloud-top altitude). ④ AMeDAS does not observe weather conditions (this is difficult to automate); it observes only rainfall, wind direction and speed, temperature, and humidity.
Practice
- At what interval are isobars usually drawn on weather maps?
- In what type of orbit is the weather satellite “Himawari” located?
- From where to where does a “northerly wind” blow?