p., pp.—page, pages
page numbers—Four-digit page numbers have no comma: pg. 5769; five digits or more take a comma for legibility: pg. 11,275. [revised 2/00—CR]
paleoclimate (lower case)
PC—Do not spell out.
PC-McIDAS—PC version of Man-computer Interactive Data Access System.
PDFs—For online material, always make an effort, first, to avoid linking to a PDF. Instead, use advanced search to find a "site:.edu" or "site:.gov" link to a publicly understandable page. If you can't avoid it, then include "PDF" within the text of the link, as in this example: . . . as noted in a 2007 study by Christopher Landsea (NOAA, view a PDF of this paper). You may substitute the PDF icon for the text whenever appropriate or convenient, such as within a list. See, for example: Publications - NCAR & UCAR News Center.
percent—Use % closed up to the number: l8%.
periods—Followed by one space (only) at end of sentence in print and online publications. (Traditionally followed by two spaces at end of sentence for fixed-width typefaces, such as those available on a typewriter. Printers have always used one space (only) for professional printing with proportional-width fonts. Once proportional-width fonts became widely avaialble for electronic publishing, this style was updated to one space.) [02-2012]
Pg—petagram, a common unit for discussing carbon flux. One Pg equals 1015 grams, or 1,000 trillion kilograms, or 1 trillion metric tons. one metric ton is about 2,205 pounds, or the weight of a small car. [4-2000]
Ph.D.—with periods, no space between (Ph.D.-level scientist). Plural: Ph.D.s—no apostrophe (we no longer follow Chicago). [revised 2-2000]
photo-electron—Use a hyphen.
photo-oxidation—Use a hyphen.
PIREPs (pilot reports)
pitot (tube)—do not capitalize.
Plexiglas—initial cap, one s; it's a registered trademark
plurals, generic—With generic plurals, Chicago says to use lowercase. For example: "...the departments of Defense and Homeland Security," or "...the Mississippi and Missouri rivers."
p.m.—lower case. See "time."
p-mode—hyphenate (solar acoustic oscillations, called pressure modes).
pockel cell—a laser word (not pocket cell)
policy maker, policy making—Takes a hyphen when used as an adjective: policy-making bodies. We've abandoned American Heritage Dictionary style in favor of consistency with decision making. [3/13]
preface—by the author. Foreword is by someone else.
prepositions—never capitalized in titles, regardless of length
press releases—See "news releases."
public service announcements (PSAs)—We do not follow AP style for commas in a series. Dates are written month first, with ordinals indicated (April 28th, not 28 April); time is written as it would be spoken (3 p.m., not 3:00 p.m.). Give pronunciation for difficult names in brackets: NCAR [EN-CAR]. Final copy should be printed in all caps (this is standard radio style). [revised 2/00]
pulse length—two words
pulse width—two words