Recording Data

An accurate observation is essential in noting trends in weather patterns. Ideally, under changing conditions, trends will be noted in some weather parameters. However, this may not always be the case. A minor shift in the winds may signal an approaching storm.

a. Wind Direction. Assess wind direction as a magnetic direction from which the wind is blowing.

b. Wind Speed. Assess wind speed in knots.

(1) If an anemometer is available, assess speed to the nearest knot.

(2) If no anemometer is available, estimate the speed in knots. Judge the wind speed by the way objects, such as trees, bushes, tents, and so forth, are blowing.

c. Visibility in Meters. Observe the farthest visible major terrain or man-made feature and determine the distance using any available map.

d. Present Weather. Include any precipitation or obscuring weather. The following are examples of present weather:

• Rain—continuous and steady liquid precipitation that will last at least one hour.

• Rain showers—short-term and potentially heavy downpours that rarely last more than one hour.

• Snow—continuous and steady frozen precipitation that will last at least one hour.

• Snow showers—short-term and potentially heavy frozen downpours that rarely last more than one hour.

• Fog, haze—obstructs visibility of ground objects.

• Thunderstorms—a potentially dangerous storm. Thunderstorms will produce lightning, heavy downpours, colder temperatures, tornadoes (not too frequently), hail, and strong gusty winds at the surface and aloft. Winds commonly exceed 35 knots.

e. Total Cloud Cover. Assess total cloud cover in eighths. Divide the sky into eight different sections measuring from horizon to horizon. Count the sections with cloud cover, which gives the total cloud cover in eighths. (For example, if half of the sections are covered with clouds, total cloud cover is 4/8.)

f. Ceiling Height. Estimate where the cloud base intersects elevated terrain. Note if bases are above all terrain. If clouds are not touching terrain, then estimate to the best of your ability.

g. Temperature. Assess temperature with or without a thermometer.

(1) With a thermometer, assess temperature in degrees Celsius (use Fahrenheit only if Celsius conversion is not available). To convert Fahrenheit to Celsius: C = F minus 32 times .55. To convert Celsius to Fahrenheit: F = 1.8 times C plus 32.

(2) Without a thermometer, estimate temperature as above or below freezing (0oC), as well as an estimated temperature.

h. Pressure Trend. With a barometer or altimeter, assess the pressure trend.

(1) A high pressure moving in will cause altimeters to indicate lower elevation.

(2) A low pressure moving in will cause altimeters to indicate higher elevation.

i. Observed Weather. Note changes or trends in observed weather conditions.

(1) Deteriorating trends include:

• Marked wind direction shifts. A high pressure system wind flows clockwise. A low pressure system wind flows counterclockwise. The closer the isometric lines are, the greater the differential of pressure (greater wind speeds).

• Marked wind speed increases.

• Changes in obstructions to visibility.

• Increasing cloud coverage.

• Increase in precipitation. A steady drizzle is usually a long-lasting rain.

• Lowering cloud ceilings.

• Marked cooler temperature changes, which could indicate that a cold front is passing through.

• Marked increase in humidity.

• Decreasing barometric pressure, which indicates a lower pressure system is moving through the area.

(2) Improving trends include:

• Steady wind direction, which indicates no change in weather systems in the area.

• Decreasing wind speeds.

• Clearing of obstructions to visibility.

• Decreasing or ending precipitation.

• Decreasing cloud coverage.

• Increasing height of cloud ceilings.

• Temperature changes slowly warmer.

• Humidity decreases.

• Increasing barometric pressure, which indicates that a higher pressure system is moving through the area.

j. Update. Continue to evaluate observed conditions and update the forecast.

Continue reading here: Subjective Hazards

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