Is there a net loss of water from the upper atmosphere?

Posted on October 16, 2017 by WeatherGuys Editor

Yes, but very little loss occurs.

GOES-16 satellite image of water vapor in Earth’s Atmosphere.

Our planet, along with all planets that have an atmosphere, lose gases to outer space.

The escape velocity is the minimum speed needed for an object to escape from the gravitational influence of Earth. The escape velocity is a function of how close the object is to Earth’s surface and the molecule’s mass.

Different processes drive this escape, and they operate at different time scales. One loss process is through molecular kinetic energy.

Temperature is a measure of the average kinetic energy of a gas. The collisions between molecules in that gas cause the velocities of individual molecules to gain and lose kinetic energy.

The kinetic energy and mass of a molecule determine its velocity. The more massive the molecule of a gas is, the lower the average velocity of molecules of that gas at a given temperature.

Therefore at the same temperature, it is less likely that heavier gases will reach escape velocity than lighter gases. Hydrogen will escape from an atmosphere more easily than carbon dioxide, which has more mass.

If the planet has a high mass, like Jupiter, the escape velocity is greater, and fewer particles will escape. Given Earth’s temperature and mass, our atmosphere does not lose a significant proportion of its atmosphere through molecules reaching escape velocities.

Stripping of the atmosphere by a solar wind is a process that can strip an atmosphere of its gases. Earth’s magnetic field helps to protect us from large losses by this process.

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: Meteorology

Comments Off on Is there a net loss of water from the upper atmosphere?

How does frost form?

Posted on October 9, 2017 by WeatherGuys Editor

Frost on Berries, Frautschi Point, Madison Wisconsin. Credit: Fred Best

Frost on objects is just water vapor in the air that has deposited itself as ice onto a surface. Frost forms on objects close to the ground, such as blades of grass.

At night, a blade of grass loses energy by emitting radiation (a non-lethal kind) while it gains energy by absorbing the energy emitted from surrounding objects. Under clear nighttime skies, objects near the ground emit more radiation than they receive from the sky, and so a blade of grass cools as its energy losses are greater than its energy gains. If the temperature of a grass blade gets cold enough and there is sufficient water vapor in the environment, frost will form on the grass.

Overnight cooling of the air near the ground causes morning frost on grass and car windshields. Frost will form on a surface only where the temperature is at or below freezing. The observed air temperature may be higher than 32 degrees, since those air temperature observations are taken at about 4 feet above the ground, where it can be warmer than the ground.

You may notice that frost forms in an open field but not under a tree. Trees emit more radiation toward the ground than does the clear sky. Energy losses at the ground under the tree are therefore less than those of the grass in the open field. The grass in the open field cools faster and reaches the frost point before the grass blades under the tree.

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: Meteorology, Seasons

Comments Off on How does frost form?

Do trees need cold weather?

Posted on October 2, 2017 by WeatherGuys Editor

Extreme cold weather can kill trees, and cold weather at the wrong time can damage trees. For example, a warm February and March in Michigan in 2012 brought early blooms to apple trees that then were killed by an April frost.

Some trees require cool temperatures, such as some fruit trees (peaches, cherries and blueberries) and nuts (almonds). Cold air along with less sunlight that comes with winter halts tree growth, preparing the tree to withstand freezing temperatures and then resume their growing the following spring.

The amount of time the temperature is between 32 and 45 degrees F is called “chill hours.” If these fruit and nut trees do not get their required number of chill hours, buds are delayed and the fruit can be small and underdeveloped.

You may have noticed the small crop of Georgia peaches this year. That is because the 2016 and 2017 Georgia winters were warm. As a result, the region lost as much as 85 percent of its peach crop. If winters continue to warm, these fruit trees will be less productive.

Georgia was not the only state with a chill hour deficit last year. Most of the U.S. had fewer chill hours than average.

Farmers have always been dependent on good weather and have learned to adapt to bad crop weather. Warming winters as a result of climate change bring new challenges.

Category: Meteorology, Seasons

Comments Off on Do trees need cold weather?

What’s up with all the wild weather?

Posted on September 27, 2017 by WeatherGuys Editor

Another week of destructive weather around our country culminated in the passage of Hurricane Maria over the island of Puerto Rico on Wednesday of last week. Some of the rainfall totals from automated stations on the island were of truly biblical proportions.

Though still subject to verification and a check on accuracy, 12.20 inches of rain fell in one hour at Gurabo Abajo, one of several stations on the southwest side of El Yunque, the high mountainous area in the northeast.

Such unimaginable rainfall is consistent with the swelling of the Rio Grande de Manati and Rio de la Plata (at least 10 to 16 feet above flood stage) on the island in the immediate wake of Maria’s passage. It is likely that the full extent of the flooding will not be known for many days, perhaps even weeks.

Meanwhile, here at home we have experienced a sudden resurgence of summer with last Wednesday’s high of 91, only the third high at or above 90 all summer, as both the daily record and the latest day in Madison history to be at 91. The old record for September 20 was 89 set last in 1908 after occurring both in 1891 and 1895.

Record highs on both Friday (91) and Saturday (92) gave us three records in four days — the first record highs registered in September in Madison since 1978! With our 92 on Saturday, the warmest day of this year ironically fell on the first full day of autumn! What a month!

Category: Meteorology, Seasons, Tropical

Comments Off on What’s up with all the wild weather?

When does fall begin?

Posted on September 18, 2017 by WeatherGuys Editor

GOES-16 True Color image from September 21, 2017. Credit: Kaba Bah, CIMSS/SSEC
(click to enlarge)

Even the least observant person has probably recognized by now that the days are growing shorter.

In fact, on Friday at 3:02 p.m., we will reach the autumnal equinox and the night will be as long as the day for the first time since late March.

At the North Pole, the situation is more dire, as Saturday morning will be the first of 182 straight days during which the Sun will not rise there! As each day goes by, the portion of the polar regions that gets no sunlight continues to expand until late December. Thus, larger and larger areas of the high latitudes have continual night over the next 3 months.

This circumstance drives the production of increasingly extensive cold air masses near the surface which gradually begin to spread southward as the season progresses.

In North America, the cold air production during this period can be enhanced if early season snow falls over wide areas of northwestern Canada because air over a snow-covered surface can get colder overnight than air over a bare surface.

So, how cold the fall and early winter will be in Madison can be strongly connected not only to when the snow begins to fall to our far northwest but also how much actually falls.

Keep an eye on northwest Canada!

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: Meteorology, Seasons

Comments Off on When does fall begin?