Appendix A: Glossary of Selected Meteorological Terms -Cont.

APPENDIX A

AVIATION WEATHER

OBSCURED SKY The condition when the entire sky is hidden by a surface-based obscuration.

OBSCURATION Any aggregate of particles in contact with the Earth's surface that is dense

enough to be detected from the surface of the Earth. Also, any phenomenon in the atmosphere,

other than precipitation reducing the horizontal visibility.

OVERCAST A layer of clouds whose summation amount of sky cover is 8/8.

PARTIAL A descriptor, PR, used only to report fog that covers part of the airport.

PARTIAL FOG Fog covering part of the station and which extends to at least six feet above

the ground and apparent visibility in the fog is less than 5/8 SM. Visibility over parts of the

station is less than or equal to 5/8 SM.

PARTIAL OBSCURATION The portion of the sky cover (including higher clouds, the moon,

or stars) hidden by weather phenomena in contact with the surface.

PATCHES A descriptor, BC, used only to report fog that occurs in patches at the airport.

PATCHES (OF) FOG Fog covering part of the station which extends to at least six feet above

the ground and the apparent visibility in the fog patch or bank is less than 5/8 SM. Visibility in

parts of the observing area is greater than or equal to 5/8 SM, when the fog is close to the point

of observation, the minimum visibility reported will be less than 5/8 SM.

PEAK WIND SPEED The maximum instantaneous wind speed since the last METAR that

exceeded 25 knots.

PRECIPITATION DISCRIMINATOR A sensor, or array of sensors, that differentiates

between different types of precipitation (liquid, freezing, frozen).

PRESSURE FALLING RAPIDLY A decrease in station pressure at a rate of 0.06 inch of

mercury or more per hour which totals 0.02 inch or more.

PRESSURE RISING RAPIDLY An increase in station pressure at a rate of 0.06 inch of

mercury or more per hour which totals 0.02 inch or more.

RADIOSONDE A balloon-borne instrument used to measure the temperature, pressure and

humidity aloft.

RAIN Precipitation of liquid water particles, either in the form of drops larger than .02 inch

(0.5 mm) or smaller drops which, in contrast to drizzle, are widely separated.

PREVAILING VISIBILITY The visibility considered representative of conditions at the

station; the greatest distance seen throughout at least half the horizon circle, not necessarily

continuous.

A-6

Glossary of Selected Meteorological Terms