Fair Weather for the Holiday


A relatively dry airmass over the island for Thursday and Friday is expected to slowly erode over the weekend and be replaced by a very humid airmass from the west and southwest. An approaching cold front from the northwest on Sunday loses its frontal characteristics and continues its approach as a trough. The trough then taps into some of that very humid air and convergence along and out ahead of that trough is expected to bring showers Sunday night into Monday. Isolated chances for thunder are also possible in the area.

20150612 Ascent
The Temperature and Dew Point (both in degrees C) profile from the morning of June 12th. Where the Temperature (red) and Dew Point (green) are further apart, the humidity is low. Aside from near the surface/bottom of this plot, the temperature and dew point are far enough apart for only a few clouds to form. The vertical axis is height above the surface in meters. The black line represents the temperature profile a parcel of air would have if it were lifted from the surface to 250mb or ~18,000 m.

However, this dry airmass led to mostly clear skies overnight Thursday into Friday. That, plus light winds courtesy of a ridge of high pressure just about overhead, allowed a decent radiative cooling event. During the day, solar radiation comes in (warming the surface) and terrestrial radiation goes out (cooling the surface); where the solar radiation exceeds the terrestrial radiation, there is net warming. At night, however, there is no solar radiation and so there is often net cooling. The calm winds keep the air near the surface from mixing with the warmer air above – enhancing this cooling process. Clouds act to both scatter terrestrial radiation back to the surface, and emit their own radiation down to the surface – a lack of clouds removes this extra process.

Land cools much faster than the ocean, Bermuda is surrounded by the ocean. So here, coastal areas have the added influence of the often warmer waters to moderate the nighttime cooling. Further, the island’s hills can shelter small areas from this effect and allow the cooling process to go unmoderated. This can lead to temperatures near the coast being over 5 F higher than temperatures less than 1/8mile away in a valley. This is what happened last night:

Temperature trace with time from the airport (exposed and coastal - black) and my PWS (sheltered and inland - orange). Temperatures were near 5F different overnight, yet similar the next day because of the clear night with light/calm winds.
Temperature trace with time from the airport (exposed and coastal – black) and my PWS (sheltered and inland – orange). Temperatures were near 5F different overnight, yet similar the next day because of the clear night with light/calm winds.

For the latest official forecast for Bermuda, see the Bermuda Weather Service.

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