This map shows change in surface pressure (in whole millibars) during the past three hours at various locations.
- Using colored pencils, you will draw a lines connecting equal values of pressure change for every two millibars. These lines are drawn for the -8, -6, -4, -2, 0, +2, +4, +6, +8, etc. values. Remember, like isobars, these lines (called isallobars) are smooth and do not cross each other.
- Using a blue colored pencil, beginning at any +2 value, lightly draw lines connecting equal values of the +2 millibars pressure change. Remember, you will need to interpolate between values to draw your lines correctly. Your map should look like this.
Draw the remaining "positive" pressure change value(s) at two millibars intervals. Your map should look like this.
Using red colored pencils lightly draw a line connecting equal pressure change values of less than zero (0). Finally, using black, draw a line connecting the zero (0) line. Your map should look like this.
- Cold fronts are often located in areas where the pressure change is the greatest. The front represents the boundary of different air masses. Cold air is more dense than warm air so when a cold front pass your location, the pressure increases. We analyze for pressure change to look for these boundaries. We can also tell where high pressure and low pressure systems are moving by looking where the greatest change is occurring.
- Shade, in red, the region where the surface pressure change is -2 millibars or less.
- Shade, in blue, the region where the surface pressure change is +2 millibars or more. Your map should look like this.