|
E. Linacre |
11/'97 |
The invisible appearance of dew at night, from nowhere, without rainfall, has led to many folklore sayings, right and wrong. For instance -
When dew is on the grass,
rain will never some to pass.
When grass is dry at morning light,
look for rain before the night.
Dew occurs when the daytime dewpoint depression (Tmax -Td) is small, and/or considerable ground cooling occurs at night. The latter is possible only with still air and a cloudless sky which allows upwards longwave radiation to escape to space. Cloudless (and rain-free) conditions typically arise in the vicinity of a surface high pressure or mid-tropospheric subsidence. Therefore dewfall may be an indication that rainfall is unlikely. On the other hand, dry lawns in the morning suggest wind, cloud and/or warmth, which tend to presage a cold front and therefore the possibility of frontal rain.
The above maxim applies best to mid-latitude coastal regions. It does not apply to desert climates, where the (rare) occurrence of dew indicates an above-normal dewpoint temperature, and an increased probability of rainfall, especially in the form of thunderstorms. Nor does it apply to tropical moist climates, where the (rare) absence of dew indicates a below-normal dewpoint temperature, and a decreased rainfall probability.
Readers are invited to collect, interpret and assess the merit of other dew sayings.