|TILTING OF TROUGHS ON A
METEOROLOGIST JEFF HABY
The best chart to use when examining the trough / ridge pattern is the
500-millibar chart. As you know,
are synonymous with cooler than normal temperatures while
ridges are synonymous with warmer than normal temperatures.
Troughs can be tilted in one of three ways which include:
positive tilt, neutral tilt, and negative tilt. In the
Northern Hemisphere, a positively tilted trough tilts from the northeast toward the southwest, a neutrally tilted
trough tilts from north to south, and a negatively tilted trough tilts from northwest toward southeast.
As a mid-latitude cyclone develops, it tends to begin as a positive tilted trough and end as a neutrally or
negatively tilted trough.
The atmosphere is most
unstable when a large trough in association with a strong
mid-latitude cyclone becomes NEGATIVELY TILTED. Why? Because on the right side of the trough, the negative
tilt causes cold air advection in the upper levels of the atmosphere while the
PBL is warm and humid (especially
if this situation occurs east of the Rocky Mountains in the fall or spring). Cold air above warm air creates
thermodynamic instability and
convective instability. A strong
jet streak can cause a trough to become negatively
tilted and contributes to
dynamic lifting. It is the
jet stream and jet streaks that are responsible for causing
troughs to become more amplified or less amplified. The jet streaks also contribute to the tilt of a trough. Look
at the 500-mb chart each day and see if the troughs over the U.S. are highly or weakly amplified and positive, neutral,
or negatively tilted.