Chapter 9 -- Weather Patterns & Fronts
The primary weather producer in the middle latitudes (for our purposes, the
region between southern Florida and Alaska, essentially the area of the westerlies)
is the middle-latitude or midlatitude cyclone. Middle-latitude cyclones are
large low pressure systems with diameters often exceeding 1000 kilometers (600
miles) that generally travel from west to east. They last a few days to more
than a week, have a counterclockwise circulation pattern with a flow inward
toward their centers, and have a cold front and frequently a warm front extending
from the central area of low pressure. In the polar front theory, (also called
the Norwegian cyclone model), wave cyclones develop in conjunction with the
Power Point Presentations
|Fronts||Clouds and Precipitation||4 Ways Air Raises||Chapter 9 Fronts|
Fronts are boundary surfaces that separate air masses of different densities,
one usually warmer and more moist than the other. As one air mass moves into
another, the warmer, less dense air mass is forced aloft in a process referred
to as overrunning. The
five types of fronts are
(1) warm front, which occurs when the surface (ground) position of a front moves so that warm air occupies territory formerly covered by cooler air,
(2) cold front, where cold continental polar air actively advances into a region occupied by warm air,
(3) stationary front, which occurs when the air flow on both sides of a front is neither toward the cold air mass nor toward the warm air mass,
(4) occluded front, which develops when an active cold front overtakes a warm front and wedges the warm front upward,
(5) a dryline, a boundary between dry, dense air and less dense humid air often associated with severe thunderstorms during the spring and summer. The two types of occluded fronts are the cold-type occluded front, where the air behind the cold front is colder than the cool air it is overtaking, and the warm-type occluded front, where the air behind the advancing cold front is warmer than the cold air it overtakes.
WEATHER MAPS THRU NEXT 48 HRS
0-6 hr Forecast WX Map
06-12 hr Forecast WX Map
12-18 hr Forecast WX Map
18-24 hr Forecast WX Map
30-36 hr Forecast WX Map
42-48 hr Forecast WX Map
LOCAL TEMPERATURE FORECASTS FOR NYC & LONG ISLAND THRU 48HR
Today’s High Temperatures
24hr High Temperatures
48hr High Temperatures
24hr Low Temperatures
48hr Low Temperatures
|Data at Surface Station
Temp 45 °F, dewpoint 29 °F,
overcast, wind from SE at 15 knots,
weather light rain, pressure 1004.5 mb
|Station Plots - NE Surface - USA Surface Plot Map||
How to read 'Surface' weather maps
How to Read Station Plots
Shaft is direction wind is coming from
|Fronts and Radar||Selected
|1/8||1-2 knots (1-2 mph)||warm front||
|scattered||3-7 knots (3-8 mph)||stationary front||
|3/8||8-12 knots (9-14 mph)||occluded front||
|4/8||13-17 knots (15-20 mph)||trough||
|5/8||18-22 knots (21-25 mph)||squall line||
|broken||23-27 knots (26-31 mph)||dryline||Freezing Rain|
|7/8||48-52 knots (55-60 mph)||Radar Intensities
|overcast||73-77 knots (84-89 mph)||Fog|
|obscured||103-107 knots (119-123 mph)||Haze|
|Dust or Sand|
Middle Latitude Cyclone
primary weather producer
large Low pressure system, travels west to east
lasts a few days to more than a week
counter-clockwise rotation, net flow inward
cold front and sometimes warm front extend from center
cloud development, precipitation
Middle Latitude Cyclone - Weather Conditions
CROSS SECTION OF CYCLONE
A Falling pressure, high cirrus clouds. Clouds lower and thicken as warm front approaches.
B As front nears, precipitation increases, temperature rises, and winds change from E to SE.
C S winds, warm temperatures, clearing skies or fair weather cumulus.
D Gusty winds, precipitation along cold front. Possibly severe weather, thunderstorms. Winds change from SW to NW, N.
E Rising pressure, cooler dry air, clearing skies as high pressure builds in.
F-G Temps remain cool, continual drop in pressure, increasingly overcast. This area often generates snow storms in winter.
|Before Passing||While Passing||After Passing|
|Temperature||warm||sudden drop||steadily dropping|
|Pressure||falling steadily||minimum, then sharp rise||rising steadily|
|Clouds||increasing: Ci, Cs and Cb||Cb||Cu|
|Precipitation||short period of showers||heavy rains, sometimes with hail, thunder and lightning||showers then clearing|
|Visibility||fair to poor in haze||poor, followed by improving||good, except in showers|
|Dew Point||high; remains steady||sharp drop||lowering|
|Before Passing||While Passing||After Passing|
|Temperature||cool-cold, slow warming||steady rise||warmer, then steady|
|Pressure||usually falling||leveling off||slight rise, followed by fall|
|Clouds||in this order: Ci, Cs, As, Ns, St, and fog; occasionally Cb in summer||stratus-type||clearing with scattered Sc; occasionally Cb in summer|
|Precipitation||light-to-moderate rain, snow, sleet, or drizzle||drizzle or none||usually none, sometimes light rain or showers|
|Visibility||poor||poor, but improving||fair in haze|
|Dew Point||steady rise||steady||rise, then steady|
According to the polar front model, cyclones form along fronts and proceed through a generally predictable life cycle. Along the polar front, where two air masses of different densities are moving parallel to the front and in opposite directions, cyclogenesis (cyclone formation) occurs and the frontal surface takes on a wave shape that is usually several hundred kilometers long. Once a wave forms, warm air advances poleward invading the area formerly occupied by colder air. This change in the direction of the surface flow causes a readjustment in the pressure that results in almost circular isobars, with the low pressure centered at the apex of the wave. Usually, the position of the cold front advances faster than the warm front and gradually closes the warm sector and lifts the warm front. This process, known as occlusion, creates an occluded front. Eventually, all the warm sector is forced aloft, and cold air surrounds the cyclone at low levels. At this point, the cyclone has exhausted its source of energy, and the once highly organized counterclockwise flow ceases to exist.
Guided by the westerlies aloft, cyclones generally move eastward across the United States. As an idealized midlatitude cyclone moves over a region, the passage of a warm front places the area under the influence of a maritime tropical air mass and its generally warm temperatures, southerly winds, and clear skies. The passage of a cold front is easily detected by a wind shift, the replacement of a south or southwesterly airflow with winds from the west or northwest. There is also a pronounced drop in temperature. A passing occluded front is often associated with cool, overcast conditions, and snow or glaze during the cool months.
Airflow aloft (divergence and convergence) plays an important role in maintaining cyclonic and anticyclonic circulation. In a cyclone, divergence aloft does not involve the outward flow of air in all directions. Instead, the winds flow generally from west to east, along sweeping curves. Also, at high altitudes, speed variations within the jet stream cause air to converge in areas where the velocity slows, and to diverge where air is accelerating. In addition to speed divergence, directional divergence (the horizontal spreading of an air stream) and vorticity (the amount of rotation exhibited by a mass of moving air) also contribute to divergence (or convergence) aloft.
For example, if a region of diverging winds at upper levels is stronger than the converging winds of a suface low pressure center below it, the low will deepen (intensify). This is because more air is being removed from the vertical column of air above the low than flowing into it, causing the pressure at the surface to decrease. A drop in pressure means an intensification of the low pressure center.
In contrast, if a region of diverging winds at upper levels is weaker than the converging winds of a suface low pressure center below it, the low begins to fill (weaken). This is because more air is flowing into the vertical column of air above the low than flowing out of it, causing the pressure at the surface to increase. An increase in pressure means a weakening of the low pressure center.
During the colder months, when the temperature gradients are steepest, cyclonic storms advance at their greatest rate. Furthermore, the westerly airflow aloft tends to steer these developing pressure systems in a general west-to-east direction. Cyclones that influence western North America originate over the Pacific Ocean. Although most Pacific storms do not cross the Rockies intact, many redevelop on the lee (eastern) side of these mountains. Some cyclones that affect the United States form over the Great Plains and are associated with an influx of maritime tropical air from the Gulf of Mexico. Another area where cyclogenesis occurs is east of the southern Appalachians. These cyclones tend to migrate toward the northeast, impacting the eastern seaboard.
Due to the gradual subsidence within them, anticyclones generally produce clear skies and calm conditions. One to three times each winter, large highs, called blocking highs, persist over the middle latitudes and deflect the nearly zonal west-to-east flow poleward. These stagnant anticyclones block the eastward migration of cyclones, keeping one section of the nation dry for a week or more while another region experiences one cyclonic storm after another. Also due to subsidence, large stagnant anticyclones can produce a temperature inversion that contributes to air pollution episodes.
In the spring, Earth's pronounced north-south temperature gradient can generate intense cyclonic storms. At a midlatitude location, as a spring cyclone with its associated fronts passes, temperatures can change quickly from unseasonably warm to unseasonably cold, and thunderstorms with hail can be followed by snow showers.
Sample Station Plot
For more information about an item marked with a (*), click on the appropriate link:
[Weather] [Wind] [Sea-Level pressure] [Pressure trend] [Sky cover]
Click here if you are interested in sample ship or buoy observations.
A weather symbol is plotted if at the time of observation, there is either precipitation occurring or a condition causing reduced visibility.
Below is a list of the most common weather symbols:
Wind is plotted in increments of 5 knots (kts), with the outer end of the symbol pointing toward the direction from which the wind is blowing. The wind speed is determined by adding up the total of flags, lines, and half-lines, each of which have the following individual values:
Flag: 50 kts
Line: 10 kts
Half-Line: 5 kts
If there is only a circle depicted over the station with no wind symbol present, the wind is calm. Below are some sample wind symbols:
Sea-level pressure is plotted in tenths of millibars (mb), with the leading 10 or 9 omitted. For reference, 1013 mb is equivalent to 29.92 inches of mercury. Below are some sample conversions between plotted and complete sea-level pressure values:
410: 1041.0 mb
103: 1010.3 mb
987: 998.7 mb
872: 987.2 mb
The pressure trend has two components, a number and symbol, to indicate how the sea-level pressure has changed during the past three hours. The number provides the 3-hour change in tenths of millibars, while the symbol provides a graphic illustration of how this change occurred. Below are the meanings of the pressure trend symbols:
The amount that the circle at the center of the station plot is filled in reflects the approximate amount that the sky is covered with clouds. Below are the common cloud cover depictions:
Depiction of frontogenesis and frontolysis
Frontolysis is depicted as a dashed line with the graphical representation of the weakening frontal type drawn on every other segment. Below is an example of a dissipating warm front.
Precipitation Areas and Symbols
24 Hour Precipitation Total - Day 1
6 Hourly Precipitation Amounts - Day 1