Passage of a cold front: Meteogram

The following shows a meteogram for Barre/Montpelier from 2 UTC 11 March and 2 UTC 12 March, 2009 (10 PM Tuesday to 10 PM Wednesday). Wind barb and cloud cover symbols show southerly winds and overcast conditions early in the period. The temperature (top line in top panel) stays relatively constant at 35 F for most of the evening. With sunrise at 11Z the temperature gradually climbs to a little over 40 degrees at 19Z.

At 20Z (4 PM EDT) the clouds begin to dissipate and the winds begin to shift towards the west as the occluded/cold front moves in. The dewpoint (lower line on upper panel) plummets as drier air moves in, although the temperature actually rises to a sharp peak of 49 F at 22 Z. After this, cold advection takes over and the temperature begins to fall rapidly.

The pressure trace is also consistent with the passage of a low pressure trough. Precipitation occurs between 5Z and 13 Z early in the morning as pressures drop. Pressure bottoms out at 20Z with the arrival of the front.

Finally, the weathercam image for Wednesday shows how dramatically the frontal passage changes sky conditions. For most of the day, a low deck of clouds moves rapidly from right to left (south to north) across the screen. At around 21Z, the front arrives and the clouds rapidly dissipate. They also begin to move into the page, indicating that the wind has shifted to the west. The setting sun can be seen briefly lighting Burke montain and the underside of the dissipating cloud. Nice sunset.