How fast do raindrops fall?

The typical speed of a falling raindrop depends on the size of the drop. A large raindrop, about the size of a house fly, has terminal fall speeds of about 20 mph. (Photo Credit: John Hart, State Journal Archives)

Gravity pulls everything downward. As an object falls, it experiences a frictional drag that counters the downward force of gravity.

When the gravity and frictional drag are balanced, we have an equilibrium fall speed that is known as the terminal velocity of the object. The terminal velocity depends on the size, shape and mass of the raindrop and the density of the air. Thus, it is worth talking a bit about the shape and size of raindrops.

While cartoonists typically draw raindrops in a teardrop or pear shape, raindrops are not shaped in those forms. They are drawn as teardrops to give the image of falling through the atmosphere, which they do.

As the raindrops fall they are flattened and shaped like a hamburger bun by the drag forces of the air they are falling through. Raindrops are at least 0.5 millimeters (or 0.02 inches) in diameter.

You will not find a raindrop any bigger than about one-quarter of an inch in diameter; larger than that, the drop will break apart into smaller drops because of air resistance. Precipitation drops smaller than 0.02 inches in diameter are collectively called drizzle, which is often associated with stratus clouds.

The terminal velocity of cloud droplets, which are typically about 10 microns in radius or 0.0004 inches, is about 1 centimeter per second, or about 0.02 miles per hour. Tiny cloud droplets can stay in the atmosphere because there is upward moving air that overcomes the force of gravity and keeps them suspended in the cloud. Only a very gentle upward movement of air is required to keep them afloat.

Raindrops are larger. A large raindrop, about one-quarter of an inch across or about the size of a house fly, has terminal fall speeds of about 10 meters per second or about 20 mph. That kind of speed can cause compaction and erosion of the soil by the force of impact.

Raindrops are of different sizes, and the smaller raindrops are traveling about 2 mph.

Category: Meteorology

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Does lightning add nitrogen to the soil?

Lightning mapped by the GOES-16 satellite in April 2017. Each lightning bolt carries electrical energy powerful enough to break atmospheric nitrogen bonds.

Yes, lightning adds nitrogen to soil, but not directly.

The atmosphere’s composition is 78 percent nitrogen, but the nitrogen in the air is not available to our bodies. The two atoms in the airborne nitrogen molecule are held together very tightly. For our bodies to process that nitrogen, the two atoms must separated.

Our bodies need proteins that contain nitrogen. The way our bodies normally get nitrogen is by eating plants or animals that eat plants.

Plants absorb nitrates in the soil and when we eat plants, we get the nitrogen in a form that our bodies can use. Plants also cannot make use of the nitrogen in the atmosphere so fertilizer is one way to add nitrogen to the soil.

Lightning is another natural way. Nitrogen in the atmosphere can be transformed into a plant-usable form, a process called nitrogen fixation, by lightning.

Each bolt of lightning carries electrical energy that is powerful enough to break the strong bonds of the nitrogen molecule in the atmosphere. Once split, the nitrogen atoms quickly bond to oxygen in the atmosphere, forming nitrogen dioxide.

Along with the lightning in the cloud are cloud droplets and raindrops. Nitrogen dioxide dissolves in water, creating nitric acid, which forms nitrates. The nitrates fall to the ground in raindrops and seep into the soil in a form that can be absorbed by plants.

Lightning does add nitrogen to the soil, as nitrates dissolve in precipitation. This helps plants, but microorganisms in the soil do the vast majority of nitrogen fixation.

Steve Ackerman and Jonathan Martin, the ‘Weather Guys’, are professors in the UW-Madison department of Atmospheric and Oceanic Sciences, and guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.
Category: Meteorology

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Is warmer air ‘heavier’?

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We are now in the heart of the baseball season and even the casual fans begin to tune in a bit more regularly to the summer game. One of the long-standing pieces of baseball wisdom suggests that the heat and humidity of oppressive summer heat waves render the air “heavy” and lead to a decrease in offensive power, particularly in home runs.

The veracity of this “wisdom” is testable.

The air we breathe is a mixture of many gases (but mostly molecular nitrogen and oxygen). Since we know the constituents and their fractional amounts, it is fairly easy to determine the molecular weight of the mixture.

The most highly variable constituent of the mixture is water vapor, so it is useful to calculate the molecular weight of so-called “dry air” — air without any water vapor in it. That weight is 29.87 grams/mole. The molecular weight of water vapor (water as a gas) is only 18.02 grams/mole.

Consequently, when any amount of water vapor is mixed into dry air, the resulting mixture is less massive than the purely dry air. On the very humid days that attract our attention in the summer, this effect is at its height and the weight of the humid mixture is often much less than that of dry air. Sir Isaac Newton was the first to demonstrate this truth.

Thus, this piece of baseball “wisdom” is utterly false — it is much more likely that any decrease in power production observed during a mid-summer heat wave is a function of the human body’s inability to perform at its peak efficiency in the heat and humidity. It is definitely not because the air is “heavier.”

Category: Meteorology, Phenomena

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Is climate science becoming a political issue?

Last week a group of four Republican U.S. senators — Ted Cruz, Rand Paul, James Lankford and Jim Inhofe — wrote to the inspector general of the National Science Foundation attacking its support of the Climate Matters program.

The program offers short courses, webinars and graphical data for broadcast meteorologists to incorporate discussion of climate change when appropriate in their broadcasts.

By doing so, the program seeks to leverage the general public’s frequent and familiar contact with atmospheric science, through television weather broadcasts, as a means to contributing to public education on this important topic.

The senators argued that the foundation’s support of the Climate Matters program goes beyond its mandate to fund basic research and represents a violation of the 1939 Hatch Act, which prohibits government agencies from engaging in partisan activities.

These senators, and large numbers of their colleagues, are now many years into a relentless effort aimed at convincing the country that the carefully argued, peer reviewed scientific evidence and conclusions regarding the reality and accelerating threat of global warming is a hoax.

They have essentially encouraged the mistaken belief that the science is politically motivated and now argue that the foundation is in the business of funding “partisan” efforts in science education.

This is rather like asserting that 2+2=5, counter to the analytical conclusions reached by nearly all mathematicians, and then demanding that public funding of mathematics education and outreach be stopped on the basis that the 2+2=4 crowd is “partisan.”

The scientific consensus on climate change is not a political viewpoint — a reality perhaps best illustrated by the recent statements of the new NASA Administrator James Bridenstine, formerly a congressman from Oklahoma and climate change skeptic.

Only a month into his new job, when asked about his stance on the issue, he said, “I … know that the climate is changing. I also know that we human beings are contributing to it in a major way. … We’re putting (CO2) into the atmosphere in volumes that we haven’t seen, and that greenhouse gas is warming the planet.”

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.
Category: Climate

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Did weather forecasting play a role in D-Day?

Had the Allies delayed the D-Day invasion of Europe that began June 6, 1944 with the landings on the beaches at Normandy, the combination of lunar cycle, tides and weather almost certainly would have postponed the invasion for more than a month likely costing the effort the tremendous advantage of secrecy. (Photo Credit: Associated Press Archives)

Wednesday was the 74th anniversary of the Allied invasion of Europe that began with the landings on the beaches at Normandy.

The combined land, air and sea assault of June 6, 1944, remains the largest such event in history. The success of the invasion was extraordinarily dependent on weather conditions.

More than three months before the invasion, a combined British and American forecasting team began rigorous forecast exercises designed to iron out the physical and logistical kinks of such a coordinated effort.

As June drew near, the nature of this collaboration was still problematic as the two groups employed vastly different methods in fashioning the requisite three- to five-day forecasts. The British were attempting to make such forecasts based upon the understanding of atmospheric dynamics that had grown substantially during the war. The Americans were employing a method based on a statistically based search through old weather data for historical analogues that could be used to guide the forecast.

To maintain secrecy, a large portion of the Allied fleet was squirreled far away in northern Scotland. Consequently, five days of lead time was required to mobilize these forces. Thus, Gen, Dwight D. Eisenhower needed to know by May 31 whether the first week of June, the prospective target for the invasion, would provide favorable weather.

The forecasters foresaw a break in that year’s unusually stormy late spring and suggested June 5 would work. As the day approached, the team realized that a one-day postponement would offer better conditions, prompting Eisenhower to make the fateful decision to invade on June 6, under barely acceptable conditions.

Had the Allies delayed, the combination of lunar cycle, tides and weather almost certainly would have postponed the invasion for more than a month — likely costing the effort the tremendous advantage of secrecy.

Category: Meteorology

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