Is the Fall changing?

Satellite image of Wisconsin, October 2012

As we enjoyed some of the last really nice autumn weather here in Madison last week, the question arose as to whether the fall has changed in any perceptible way over the past several decades.

We have reported before in this column on the observed trends in the areal extent of lower tropospheric cold air over the Northern Hemisphere since 1948. During the winter months, that areal extent has systematically shrunk over the last 70 years, consistent with a modest but detectable warming of the planet.

A more recent analysis has considered the onset of the winter from the same perspective. We have measured the areal extent of minus 5 degrees Celsius air at about 1 mile above the surface for every September and October since 1948. Adding up each day’s areal extent, we have determined on what day the sum of these values first reaches 1 billion square kilometers.

It turns out that it used to be around Oct. 16 that we first met that criteria in 1970 and that now it is more like Oct. 23 — meaning the fall advances about a week slower today than it did 50 years ago. This year, we will accrue our first billion square kilometers of cold air on Oct. 21 — slightly early compared to the current trend.

Locally, the warm autumn weather is likely to end rather abruptly on Tuesday and we will remain cold for much of next week. The transition to winter is about to begin.

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: Uncategorized

Comments Off on Is the Fall changing?

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

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?

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?

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?

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?