Reading Weather Maps

Before reading a wind symbol, find north on the map and read the key. Different symbols can describe direction in different ways.

A standard wind barb starts at a station's position. Its staff extends toward the direction the wind comes from. The small speed marks are near the outer end. If that marked end lies west of the station, the wind comes from the west and moves east.

Speed marks add together. A short mark stands for 5 knots, a full mark for 10 knots, and a filled triangular flag for 50 knots. Knots measure speed. One flag, two full marks, and one short mark give 50 + 10 + 10 + 5 = 75 knots.

An arrowhead may instead show the direction air moves toward. Do not read an ordinary arrow as though it were a wind barb. Follow the symbol rules in that map's key.

A wind rose is a circular chart of wind observations. Its heading should identify the place and the period covered. Some cover one day; others cover weeks or years.

Read the compass directions around the chart. A spoke extending toward east represents observations of wind coming from the east. That air moves west.

Use the chart's scale to compare how often the wind came from each direction. The longest complete spoke marks the most common direction during the recorded period. Check whether the scale uses percentages, hours, or another measure.

Colored sections may divide a spoke into wind-speed ranges. The key explains those colors and gives the speed unit. A color has no fixed speed meaning without its key.

The chart summarizes the stated observations. Even if east was the most common direction, the wind did not have to come from the east every time. The chart alone does not tell you what the wind is doing now.

A rain gauge measures rainfall. A useful rainfall record pairs each date with an amount and a unit.

Use this invented practice record:

Here, mm means millimeters. All four amounts use the same unit, so you can compare them. Tuesday had the most rain: 5 millimeters. Thursday's zero means no rain was recorded that day. Keep that day in the table.

A graph with one bar per day would show four separate daily amounts. It would not show the running total unless it was made for that purpose.

To find the total for these four days, add their amounts: 2 + 5 + 1 + 0 = 8 millimeters. The wettest day's amount is 5 millimeters; the four-day total is 8 millimeters. Those answers describe different things.

These numbers are practice data, not measurements from a historical storm.

Suppose you want to compare a town's usual April and July rainfall. Records from many years give more useful evidence for that question than one wet afternoon.

Imagine those records show larger rainfall totals in April than July in most years. You could use that pattern to expect a wetter April than July in that town. It would not tell you whether rain will fall during a particular picnic.

Check what each record measures. Total rainfall tells you how much rain fell. A rainy-day count tells you how many days had rain. Many days of light rain and a few days of heavy rain can give different patterns in these two measurements.

Also check the place and dates. A record for another town or another season may answer a different question.

Climate describes patterns over many years. A few recent days describe a short spell of weather. For tomorrow's plans, check the latest forecast as well as any pattern you notice.

A weather map can show observations or a forecast. Observations describe recorded conditions. A forecast describes conditions expected when the forecast was made. Look for an observation or forecast label in the title, key, or notes. Use the date and time to find when the conditions apply. A past date alone cannot identify the map type: an old forecast can refer to a time that has already passed.

Find the location you need. Then check its symbols against the map's key. Read nearby numbers with their labels and units. Temperature, rainfall, and wind speed are different measurements, even when their numbers happen to match.

To look for change across several maps, compare the same place and measurement in time order. A temperature falling across several dates is a downward trend. One symbol on one map cannot show how conditions changed over several days.

A trend gives a clue about what may happen next. Meteorologists also combine measurements with radar, satellite images, weather balloons, and computer models. New information may point to a change in the pattern.

For outdoor plans and safety, use the latest official forecast and weather warnings. A practice reading of old maps does not replace that current information.