7.4: Accessible Descriptions

Figure 7.2.4 Surface Weather Conditions on October 23, 2016  

This is a surface weather analysis map of the continental United States, valid at 12 Z (7:00 AM EDT) on October 23, 2016. 

Key Elements:

1. Isobars:

• Shown as thin blue contour lines drawn every 4 millibars (mb).
• Pressure values range from 1000 mb to 1040 mb.
• Isobars form closed loops around high and low-pressure centers.
• Closely spaced isobars indicate stronger pressure gradients, suggesting windy conditions—especially in the northeastern U.S.

2. Pressure Centers:

• High-pressure systems (H) are centered in Southern California (1020 mb), Texas (1028 mb), and Florida and nearby Atlantic waters (1024–1028 mb).
• Low-pressure systems (L) are located in the Pacific Northwest and California coast (1008–1012 mb), the Great Plains (1004–1016 mb), and the Upper Midwest and Northeast (1004–1008 mb).

3. Frontal Boundaries:

• Cold fronts are shown as blue lines with triangles pointing in the direction of motion.
• Warm fronts are red lines with semicircles.
• Occluded fronts are purple, combining triangles and semicircles.
• Stationary fronts are alternating red/blue.

Notable features:

• A cold front extends from Nebraska to Texas.
• A stationary front lies across the central Appalachians.
• An occluded front spirals around a deep low in the far northeast U.S. near Maine and southeastern Canada.

4. Radar Reflectivity:

• Shaded regions with color scales show precipitation intensity, primarily in:

• Northeast (New York, Vermont, Maine)
• Great Lakes
• Central Plains and Rockies
• Pacific Northwest

Intensity is color-coded on the left, ranging from light (10 dBZ) to very heavy (over 70 dBZ).

5. Station Plots:

• Each station model displays:

• Temperature (°F), Dew point (°F), Wind speed and direction, Sea-level pressure, and Cloud cover.
• For example, New York reports 44°F with overcast skies and northeasterly winds.

Regional Summary:

• West Coast: Mixed precipitation in Oregon and Washington. High pressure dominates California.
• Rockies and Central Plains: Multiple low-pressure areas with frontal boundaries, widespread cloud cover, and precipitation.
• Northeast U.S.: A strong low-pressure system brings wind and rain, with closely packed isobars suggesting high wind speeds.
• Southeast and Gulf Coast: Generally clear, under high pressure with light winds.

Figure 7.2.5 A zoomed-in version of Figure 7.2.4

This is a zoomed-in portion of Figure 7.2.3, focusing on the northeastern United States and southeastern Canada at 12 Z (7:00 AM local time). The map shows isobars, surface fronts, precipitation, and weather station observations, indicating a strong low-pressure system.

Key Features:

• Low-Pressure System (L):
  • A large “L” symbol is centered over northern New England or southern Quebec.
  • Surrounding the low are tightly packed isobars (lines of constant pressure), labeled 980 mb, 984 mb, 988 mb, and so on.
  • The tight spacing indicates a strong pressure gradient, suggesting high wind speeds in the region.
• Fronts :
  • An occluded front (line with alternating semicircles and triangles) extends from the low eastward.
  • A cold front (triangles) and a warm front (semicircles) are attached and wrap around the low-pressure center.
  • A stationary front (alternating symbols) appears to the west.
• Precipitation :
  • Precipitation is mostly located in the New England states and southern Canada, extending southward through New York and into Pennsylvania.
• Weather Stations (station models):
  • Several circular plots show wind barbs, cloud cover, temperature, and pressure data.
  • Most stations report overcast conditions, temperatures in the 40s °F, and wind from the northwest or west.
  • Wind barbs indicate strong winds, consistent with the pressure gradient.

Figure 7.3.3 The Coriolis Force turns the parcel to the right

This labeled diagram illustrates how an air parcel responds to the pressure-gradient force and the Coriolis force in the Northern Hemisphere.

Diagram Structure:

• The image consists of three horizontal lines, each representing a different isobar (a line of equal atmospheric pressure):
  • Bottom Line (Point B): 1016 millibars (mb)
  • Middle Line (Point A): 1012 mb
  • Top Line (Point C): 1008 mb
• The vertical distance between each isobar is labeled as 50 kilometers.
• A northward-pointing arrow on the far left indicates directional orientation (with north at the top).

Air Parcel Motion:  

• A red circle represents an air parcel starting at Point A (1012 mb).
• The parcel initially moves northward toward lower pressure (Point C) due to the pressure gradient force, which is shown as a thick red upward arrow labeled PGF.
• As the parcel accelerates northward, the Coriolis force (CF) begins to deflect its path to the right, as shown by a blue arrow pointing eastward.
• Three red dots trace the curving path of the air parcel:
  • First directly northward.
  • Then deflected slightly northeast.
  • Finally curving more to the right as the Coriolis force increases.
• The rightward deflection continues until the Coriolis force balances the pressure-gradient force, at which point the parcel moves in a straight line parallel to the isobars, achieving geostrophic balance (not yet fully reached in this diagram).
• A text label on the right side explains: "As the particle accelerates, the Coriolis Effect amplifies, and the particle turns further right."

Figure 7.3.4 Geostrophic Flow is achieved

This is a labeled atmospheric diagram showing the path of an air parcel under the influence of the pressure gradient force and the Coriolis effect, culminating in geostrophic balance.

Diagram Structure:

• The diagram contains three horizontal lines representing atmospheric isobars, or lines of equal pressure:
  • Bottom Line (Point B): 1016 millibars (mb)
  • Middle Line (Point A): 1012 mb
  • Top Line (Point C): 1008 mb
• Vertical spacing between isobars is labeled as 50 kilometers, showing equal pressure intervals from the surface upward.
•  On the left side, an upward-pointing arrow labeled "N" denotes North, establishing that the top of the diagram is the direction toward lower pressure.

Air Parcel Motion and Forces:

• A red dot at Point A shows the initial position of the air parcel.
• The parcel moves northward from 1012 mb to 1008 mb due to the pressure gradient force (PGF), represented by a large red arrow pointing straight up.
• As the parcel moves, it is deflected to the right (eastward) by the Coriolis force (CF), shown as a blue arrow pointing rightward.
• The parcel's motion path is illustrated by several sequential red dots that curve from vertical to horizontal.
• Eventually, the parcel reaches a point where the Coriolis force exactly balances the pressure-gradient force, and it begins to move in a straight path parallel to the isobars, indicated by a horizontal black arrow.
• This final condition is labeled with the explanation: "Eventually, PGF = CF, and the particle travels in a straight path, parallel to isobars."