chuồn chuồn bay thấp thì mưa

Explanation:
Dragonflies fly at a low altitude means it's going to tướng rain; if they fly at a high altitude it's going to tướng be sunny; it's going to tướng be shadowy if they fly at a medium altitude.

This idiomatic expression serves as a weather forecast based on people's experience: The altitude at which dragonflies are flying at is an indicator of what the weather would be lượt thích in the very near future. Vietnamese people in the old days were very much reliant on agriculture, and without weather forecasting equipment they came up with excellent natural weather indicators, in this case, the dragonflies.

--------------------------------------------------
Note added at 2 hrs (2009-04-08 10:34:12 GMT) Post-grading
--------------------------------------------------

The levels of moisture in the air affect the dragonflies' ability to tướng fly: When rain is imminent the level of moisture in the air is very high therefore the dragonflies are not able to tướng go to tướng high altitude. On the contrary, when it's going to tướng be sunny the air contains the lowest level of moisture and the dragonflies can fly high thanks to tướng the dry air; etc.

Explanation:
The meaning of the proverb is obvious: it predicts the weather based on what happens in Nature. There is no exact equivalent in English because this kind of traditional weather forecast is quite special of the Vietnamese. However, we can try to tướng create an English version that sounds lượt thích a proverb by making it rythmic in a parallel structure.
"Expect rain as dragonflies fly low, sunshine as they fly high and clouds as they fly neither low nor high."
This translation is in the imperative size. It consists of three equal parts using the same structure; the repetition of "bay" serves as emphasis on the natural phenomena. The verb "expect" indicates that this is weather forecast.