SME
Grade 12 Science · Problem type
Highs, lows and the pressure gradient
Use this for “A 1,024 hPa high and a 996 hPa low are 700 km apart. What is the pressure gradient, and which way does the wind blow round the low?”
Diagram
Winds blow from high to low pressure, turned right by the Coriolis effect: clockwise out of the high, counterclockwise into the low.
6 isobars lie between them.
This is an example. Type your own values to start a new problem.
G (hPa per 100 km)
=
(
H (hPa)
−
L (hPa)
)
÷
D (km)
×
100
ΔP
Pressure difference
worked out (not typed) · hPa
Formulas
ΔP
=
H
−
L
28 = 1,024 − 996
G
=
ΔP
D
×
100
4
=
28
700
×
100
Assumptions
•
Wind blows from high to low, faster where isobars are closer.
•
The Coriolis effect turns it right in the Northern Hemisphere, so it circles a low counterclockwise.
•
Friction near the ground turns it partway back toward the low.
Step-by-step
WE KNOW
H = 1,024 hPa, L = 996 hPa, D = 700 km
FIND
pressure difference (ΔP), pressure gradient (G)
Step 1 · Find pressure difference (ΔP)
Use ΔP = H − L
How much higher the pressure is at the high.
ΔP = 1,024 − 996
ΔP = 28 hPa
Step 2 · Find pressure gradient (G)
Use G = ΔP ÷ D × 100
The pressure change per km, times 100 km.
G
=
28
700
×
100
G
=
0.04
×
100
G
=
4
hPa
per
100
km
Check
Put the numbers back in. Both sides should match.
28 = 1,024 − 996 ✓
4
=
28
700
×
100
✓
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