Where does all the moisture come from?
’Tis the season for mixed precipitation, but before we make the move to winter I’d like to answer a question I received from Bob, earlier this month.
“Does a rain storm contain all the rain it drops (from the start of the storm to the end) at the beginning or does it replenish itself as it moves along?”
That is a great question. Let’s start with the cloud… without it, there is no rain.
A cloud is a visible accumulation of minute droplets of water, ice crystals, or both, suspended in the air. When these droplets become large enough or heavy enough, they fall to the ground.
Now to rainfall. Moderate rain is considered to be about five mm/hour. Assuming “moderate” rainfall can be considered “average,” we would assume the amount of time for a cumulonimbus cloud to dispense itself would be the cloud height in kilometres divided by 2.5 km per hour. The cloud height is the distance from the bottom of the cloud to the very top of the cloud, not its altitude. Therefore, a 10-km tall cloud would take four hours to rain itself out completely.
That’s one cloud. Bob was asking about a storm. Storms or large systems that dump huge amounts of precipitation over very large areas must pull in a consistent and strong inflow of warm, moist air from a greater distance. Examples of this happening include the Pineapple Express for rainfall in California, onshore winds during the Indian monsoon and, closer to home, air from off the Gulf Stream with our Nor’easters.
In all these regional large precipitation events, moist air gathered from a great surface area flows into a smaller region. As the air approaches, it is lifted by the low pressure, condenses, and finally falls as rain or snow. This process is often termed “moisture convergence.”
Another example of moisture convergence is the Fujiwhara effect. That’s when two low pressure systems orbit each other and close the distance between the circulations of their corresponding low-pressure areas. The effect was named after Sakuhei Fujiwhara, the Japanese meteorologist who initially described the effect. We saw that happen late last week off the east coast of Labrador. The result was the merging of two low-pressure systems into a single extratropical cyclone – with more rain than the initial storm started out with!
So Bob, large storms can be refueled in a variety of ways and that can change the moisture profile of the original system. Cindy Day is Saltwire Network’s Chief Meteorologist.