Editing Weather map (section)

=== Constant pressure charts ===
[[file:Jetstreak.png|thumb|left|An upper-level jet streak. DIV areas are regions of divergence aloft, which usually leads to surface convergence and cyclogenesis]]

Constant pressure charts normally contain plotted values of temperature, humidity, wind, and the vertical height above sea level of the pressure surface.<ref>{{cite web|url=http://amsglossary.allenpress.com/glossary/search?id=constant-pressure-chart1|author=Glossary of Meteorology|date=June 2000|title=Constant-pressure chart|publisher=[[American Meteorological Society]]|access-date=2010-02-06|archive-url=https://web.archive.org/web/20110606084509/http://amsglossary.allenpress.com/glossary/search?id=constant-pressure-chart1|archive-date=2011-06-06|url-status=dead}}</ref> They have a variety of uses. In the mountainous terrain of the western United States and Mexican Plateau, the 850 hPa pressure surface can be a more realistic depiction of the weather pattern than a standard surface analysis. Using the 850 and 700 hPa pressure surfaces, one can determine when and where warm advection (coincident with upward vertical motion) and cold advection (coincident with downward vertical motion) is occurring within the lower portions of the [[troposphere]]. Areas with small dewpoint depressions and are below freezing indicate the presence of icing conditions for aircraft.<ref name="upper">{{cite book|url=https://books.google.com/books?id=kGHFWOqyiksC&pg=PA120|pages=129–134|title=Aircraft icing: a pilot's guide|author=Terry T. Lankford|isbn=978-0-07-134139-4|publisher=McGraw-Hill Professional|year=1999}}</ref> The 500 hPa pressure surface can be used as a rough guide for the motion of many [[tropical cyclone]]s. Shallower tropical cyclones, which have experienced vertical [[wind shear]], tend to be steered by winds at the 700 hPa level.<ref>{{cite web|author=[[United States Navy]]|year=2007|url=http://www.nrlmry.navy.mil/~chu/chap4/se100.htm|archive-url=https://archive.today/20120805162928/http://www.nrlmry.navy.mil/~chu/chap4/se100.htm|url-status=dead|archive-date=August 5, 2012|title=Section 1. Influences on Tropical Cyclone Motion|access-date=2010-02-06}}</ref>

Use of the 300 and 200 hPa constant pressure charts can indicate the strength of systems in the lower troposphere, as stronger systems near the Earth's surface are reflected as stronger features at these levels of the atmosphere. Isotachs are drawn at these levels, which a lines of equal wind speed. They are helpful in finding maxima and minima in the wind pattern. Minima in the wind pattern aloft are favorable for [[tropical cyclogenesis]]. Maxima in the wind pattern at various levels of the atmosphere show locations of jet streams. Areas colder than {{convert|-40|C|F}} indicate a lack of significant icing, as long as there is no active [[thunderstorm]] activity.<ref name="upper"/>