Review of weather charts Flashcards
Isobars are
the solid lines (they are NOT height contours)
Unlike the upper air charts, this chart is
not at a constant pressure level
Frictional force is significant on this chart. Turns wind about
30 degrees toward low pressure. This causes convergence into low pressure regions.
Strong fronts will cause
“kinking” of isobars
WHAT TO LOOK FOR:
- Fronts: cold fronts, warm fronts, troughs, outflow boundaries, occluded fronts, stationary fronts, drylines
- Pressure: High pressure regions, low pressure regions
- Convergence, divergence
- Temperature and moisture gradients
- Influence of topography upon the weather conditions
……………………….. will cause the low levels of the atmosphere to become increasingly unstable
- Warm air advection and moisture advection near the surface
- The temperature must be increasing rapidly and the dewpoint increasing rapidly
. If the morning temperature and dewpoint are 60/47 and the afternoon temperature and dewpoint are 87/70,
a large amount of warm air advection (WAA) and moisture advection have taken place throughout the day
The temperature must be increasing rapidly and the dewpoint increasing rapidly for the atmosphere to rapidly become unstable. If the morning temperature and dewpoint are 60/47 and the afternoon temperature and dewpoint are 87/70, a large amount of warm air advection (WAA) and moisture advection have taken place throughout the day. If a trigger mechanism is nearby
strong storms are very likely
The temperature must be increasing rapidly and the dewpoint increasing rapidly for the atmosphere to rapidly become unstable. If the morning temperature and dewpoint are 60/47 and the afternoon temperature and dewpoint are 87/70, a large amount of warm air advection (WAA) and moisture advection have taken place throughout the day. If a trigger mechanism is nearby, strong storms are very likely. If a mT airmass extends from
the surface to the upper levels of the atmosphere, severe weather is not as likely
The temperature must be increasing rapidly and the dewpoint increasing rapidly for the atmosphere to rapidly become unstable. If the morning temperature and dewpoint are 60/47 and the afternoon temperature and dewpoint are 87/70, a large amount of warm air advection (WAA) and moisture advection have taken place throughout the day. If a trigger mechanism is nearby, strong storms are very likely. If a mT airmass extends from the surface to the upper levels of the atmosphere, severe weather is not as likely. Look for
differential advection for severe weather (mT air in low levels with cT or cP air in midlevels of atmosphere.)
- The 850 mb vhart
What do I need to look for on this chart?
- Chart is good for assessing low level warm air and cold air advection
- Region of strong thermal gradient gives indication of 850 millibar front and regions of convergence
- Use dew-point depression to determine if atmosphere is near saturation or dry at this level
- Determine intensity of highs and lows
Advection is a function of
height contour spacing, the temperature gradient, and the angle isotherms cross height contours
Low level warm air advection contributes to
to synoptic scale rising air
; Low level cold air advection contributes to
synoptic scale sinking air
Air rises due to
low level convergence and confluence
Deep high (surrounded by several height contours) covering a
large spatial area
Determine intensity of highs and lows
- Deep low (surrounded by several height contours)
- Deep high (surrounded by several height contours) covering a large spatial area
- Disregard highs and lows not surrounded by at least one isohypse
- Several highs located near each other indicates one broad area of high pressure and not a scattering of individual highs
Thermal advection is a function of three factors:
(1) the temperature gradient,
(2) the height contour spacing and
(3) the angle the isotherms make with the height contours.
Thermal advection is maximized by the combination of:
- Closely spaced isotherms
- Closely spaced height contours
- Isotherms perpendicular to height contours
Thermal advection is minimized by the combination of:
- Widely spaced isotherms
- Widely spaced height contours
- Isotherms parallel to height contours
- The 700 mb Chart
look for
The trough/ridge pattern becomes more defined at the 700 mb levels as compared to lower levels
if trough and ridge appear in 700 mb it will be
deep on 500 mb
- The 500 mb chart
warm advection in 500 mb means that there is
upward motion
areas bellow the trough are
cooled
A
CAA
B
WAA
Therefore, in general warmer than average temperatures can be expected
underneath ridges
colder than average temperatures can be expected
underneath troughs
The more pronounced the ridge (or trough), the
more above (or below) average the temperatures will be.
The terminology “trough” and “ridge” is related to the fact that
the contour lines often look like waves
One other feature in the 500 mb pattern worth pointing out are
closed lows and closed highs
A closed low indicates
a pool of colder air surrounded by warmer air
Closed highs generally indicate
warm and fair conditions.
Having said all that, in the summer months, there is generally …………….. in the 500 mb height pattern as compared with the colder months
much less structure
Having said all that, in the summer months, there is generally much less structure in the 500 mb height pattern as compared with the colder months, thus
features like troughs, rigdes, and closed highs and lows are often harder to pick out
Having said all that, in the summer months, there is generally much less structure in the 500 mb height pattern as compared with the colder months, thus features like troughs, rigdes, and closed highs and lows are often harder to pick out
Therefore, in winter there is
a very large difference in 500 mb heights between the warm tropics and very cold Arctic
Therefore, in winter there is a very large difference in 500 mb heights between the warm tropics and very cold Arctic, while in summer,
there is a much smaller difference in temperature and hence 500 mb heights between the warm tropics and the now relatively warm Arctic.
To be a little more precise in estimating expected temperature compared to average on any given day, we should
compare the actual 500 mb heights from a daily map to the long-term average or “climatological” 500 mb heights
For a given location, if the 500 mb height on the map is close to average, then
the temperature is expected to be about average
. If the 500 mb height is lower than the average height, then
lower than average temperatures are expected.
If the 500 mb height is higher than the average height, then
higher than average temperatures are expected
Thus, using the 500 mb heights to estimate surface temperatures is
not exact.
Thus, using the 500 mb heights to estimate surface temperatures is not exact. However, as you will see, the 500 mb maps often provide
a very good overview of the pattern of warm and cold conditions near the ground surface
Vorticity
The three elements that produce vorticity are
SHEAR, CURVATURE, and CORIOLIS.
SHEAR-
A change in wind speed over some horizontal distance. Determined at 500 millibars by examining the spacing (and rate of spacing change) of height contours.
CURVATURE
A change in wind direction over some horizontal distance. This change will result in either a counter-clockwise or clockwise curvature.
POSITIVE / INCREASING VORTICITY
*Wind speed increasing when moving away from center point of trough. (positive shear vorticity)
*A counterclockwise curvature in the wind flow. This occurs in troughs and shortwaves. (positive curvature vorticity)
*A south to north movement of air. Coriolis increases (becomes more positive) when moving from the equator toward the poles. (increasingly positive earth vorticity)
NEGATIVE / DECREASING VORTICITY
*Wind speed decreasing when moving away from center point of trough. (negative shear vorticity)
*A clockwise curvature in the wind flow. This occurs in ridges. (negative curvature vorticity)
*A north to south movement of air. Coriolis decreases (becomes less positive) when moving from the pole to the equator. (decreasingly positive earth vorticity)
The highest values of vorticity are found often just to the
south or east of a highly amplified trough.
The highest values of vorticity are found often just to the south or east of a highly amplified trough. To the right of the trough, winds will be
from a southerly direction.
The highest values of vorticity are found often just to the south or east of a highly amplified trough. To the right of the trough, winds will be from a southerly direction. This makes the coriolis term
increasingly positive
The highest values of vorticity are found often just to the south or east of a highly amplified trough. To the right of the trough, winds will be from a southerly direction. This makes the coriolis term increasingly positive. Winds are generally
light near the center of a trough with increasing winds away from the base of the trough.
Winds are generally light near the center of a trough with increasing winds away from the base of the trough. This makes the shear term
positive
Winds are generally light near the center of a trough with increasing winds away from the base of the trough. This makes the shear term positive. If the trough is highly amplified,
this will give a positive curvature vorticity term
JET STREAM
what to look for
- This is the best chart to assess the magnitude of vorticity. Vorticity can be generated in three different ways. They are:
- Curvature vorticity
- Shear vorticity
- Earth vorticity (Coriolis)
High vorticity is an indication of ageostrophic flow and upper level divergence.
(2) This is the best chart in assessing the trough/ ridge pattern . A trough is an indication of cooler weather and possible precipitation while a ridge is an indication of warmer weather and fair conditions. Greatest storminess is found to right of 500 mb trough axis.
- Zonal flow - air flow is generally west to east
- Meridional flow - highly amplified troughs and ridges
- Use height falls and height rises to predict movement of troughs and ridges. Lows tend to develop toward regions with the greatest height falls while large height rises indicates a ridge is building into the area.
- Temperatures at 500 mb are rarely above 0° Celsius. Temperatures can be above 0 ° Celsius at 500 mb in a hurricane due to the warm core nature of the storm.
- Look for shortwaves within the longwave flow. The atmosphere will be unstable in association with shortwaves (baroclinic instability, ageostrophic flow). Precipitation is most likely to right of shortwave axis. The 500 and 700 mb charts are the best to use when locating shortwaves.
RULE OF THUMB
If a jet streak exists on the left side of a trough and winds are stronger to the left of the trough (as it is in our example in “Time 1”), the trough will become more amplified with time and will dig in a southerly direction
Certain regions of jet streaks are more favorable for
rising or sinking air.
The jet stream is a powerful forecasting tool.
- Not that it can give exact highs/ lows/ and precipitation chances, but because it gives information such as when to expect the next storm system and whether temperatures will be above or below normal.
- It gives clues to how the upper levels will promote rising air or sinking air. It gives clues to the character of the next storm system.
- Jet streaks alone provide much information of how a trough or ridge will develop over the next couple of days.
WHAT TO LOOK FOR ON 300/250/200 chart:
jet stream
jet streaks
general trough ridge pattern
