One of the promises that we repeat daily in our recitation of the Keriyat Shema is that the reward for appropriate behavior is rain in its proper time – yoreh u’malkosh. Our Gemara discusses these terms and their meaning. The yoreh, according to the baraita, is the first rain of the year, which occurs in the month of Marheshvan, and the malkosh is the rain that ends the season in Nisan. In truth, establishing the time that rain normally falls based on the Jewish calendar is inaccurate, at best, given the fluctuation that exists between these months and the solar-based, Gregorian calendar. Historically, rain has fallen in Israel as early as October (which sometime coincides with the end of Tishrei) and as late as the end of April (which sometimes falls out in the middle of Iyyar).
With regard to the meaning of each of these words, several suggestions are made by the Gemara.
Yoreh can be understood to be made up of any one of a number of different root words.
The Sages taught in a baraita: The first rain [yoreh] is called by this name due to the factpeople that it instructs [moreh] to plaster their roofs and to bring in their produce from the fields to their houses and to attend to all their needs in the field before more rain falls. Alternatively, yoreh is referring to the fact that it moistens [marve] the earth and waters it to the depths, as it is stated: “Watering [ravvei] its ridges abundantly, settling down its furrows, You make it soft with showers, You bless its growth” (Tehillim65:11). Alternatively, yoreh means that it falls gently and it does not fall vehemently.
Malkosh appears to come from the root lekesh, an unusual root that means “late.” Rashi says that it is the name of a type of locust – the Schistocerca gregaria – whose appearance coincides with the end-of-season rains. These locusts ordinarily live in small groups and pose no danger whatsoever. Occasionally, when the late rains coincide with warm weather, development of eggs is hastened, large numbers of locusts are hatched at the same time, and changes take place in the creatures’ behavior. They now change from solitary creatures to swarming communities that attack vegetation, causing widespread damage. Under certain circumstances they can reach Israel on South-easterly winds.