|BACKYARD METEOROLOGY: FROPA
METEOROLOGIST JEFF HABY
FROPA stands for FROntal PAssage. Being in the backyard at the moment a frontal passage occurs is an interesting experience. Some fronts
are most shallow at their edge thus at frontal passage ground level will experience the frontal passage first. The two distinct changes
that occur at the moment of frontal passage are a shift in the wind and a change in temperature. The smell of the air will often change
also since the air behind the frontal passage comes from a different region.
A cold front separates a cold air mass from a warm air mass. Cold fronts can bring a dramatic change in temperature during the first few
minutes to hours of frontal passage. The edge of the cold front can also bring precipitation due to the lifting that takes place at and
near the cold front boundary. This precipitation can be heavy rain when the front is lifting warm-humid air.
A dryline passage can also be a remarkable experience. The dryline will separate warm and moist air from warm and dry air. During
dryline passage, the dewpoint drops dramatically. The relative humidity will drop also. For example, the dewpoint could drop from
the 60s to the teens during passage and the relative humidity could drop from 75% to 10%. The dryline passage is sometimes accompanied
by gusty winds and lowering visibility due to blowing dust. Typically skies clear and precipitation chances decrease after dryline
passage, although strong storms can from near the dryline boundary due to lifting and access to warm-moist air.
A warm front passage can also result in a dramatic change in weather. The temperature can quickly rise after frontal passage and skies
will often be sunnier. The dewpoint will often increase and winds will blow from warmer latitudes.
Below are a variety of backyard experiences associated with FROPA:
1) Note how the temperature changes with time on a backyard thermometer especially during the first few minutes and first hour
2) Note convection and precipitation intensity at the time of frontal passage
3) Note the change in the smell of the air during frontal passage
4) Note how dramatic the wind direction change is during frontal passage
5) Note changes in visibility
6) Note changes in how the air feels (i.e. wind chill, humidity)
7) Note wind gusts behind the front