How do snowflakes grow?

Posted on December 26, 2011 by Weather Guys Editor

A snowflake can be an individual ice crystal or an aggregate of ice crystals. Ice crystals can grow by water vapor deposition on the crystal or by collisions with other cloud particles.

An ice crystal can grow if the air around it has a relative humidity near 100 percent. The ice particle grows by water vapor deposition. Growth by deposition is generally slow. If you find nicely shaped snowflakes, they likely were produced by vapor deposition.

Collisions can produce large snowflakes. When an ice crystal falls through a cloud it may collide with and collect super-cooled liquid water drops. This process of ice crystal growth by sweeping up water drops is called accretion. When ice crystals collide with super-cooled drops, the drops freeze almost instantly, providing a mechanism for the particle to grow quickly.

An ice particle produced by the accretion process that has a size between 0.04 to 0.2 inches and no discernible crystal shape, or habit, is called graupel. On collision and freezing, the super-cooled water often traps air bubbles. Because of this trapped air, the density of a graupel is low and it can easily be crushed, unlike a hailstone.

Aggregation is the process by which ice crystals collide and form a single larger ice particle. The probability that two crystals will stick together depends on the shape of the crystals. If two crystals with spiky appendices collide, it is likely that their branches will become entangled and the two crystals will stick together. Snowflakes composed of aggregates can sometimes reach 3 or 4 inches in size.

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When is the Winter Solstice?

Posted on December 19, 2011 by Weather Guys Editor

The winter solstice (from the Latin sol, or “sun,” and stice, or “come to a stop”) is the day of the year with the fewest hours of daylight. This year, this occurs for the Northern Hemisphere at 11:30 pm Wednesday.

As Earth orbits the sun, its axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of Earth’s axis to the sun is always changing as it orbits the sun.

As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude. This links the amount of solar energy reaching a particular location to the time of year and causes some months of the year to always be warmer than others — in other words, the seasons.

On the Northern Hemisphere’s winter solstice, the northern spin axis is pointed away from the sun, and latitudes north of the Arctic Circle (66.5 degrees north) have 24 hours of darkness.

The winter solstice is often referred to as the first day of winter. But there are other definitions of winter. For example, the beginning of winter might be defined as the calendar day, on average, when precipitation has an equal chance of falling as rain or snow. For Madison, that calendar day is November 15. Meteorologists often define the three months of winter as December, January and February — the coldest months of the year.

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Why does the moon look red during a lunar eclipse?

Posted on December 12, 2011 by Weather Guys Editor

A total lunar eclipse could be seen in cloud-free regions across most of the United States and Canada on Saturday morning, Dec. 10. In a total lunar eclipse the sun, Earth and moon line up and the Earth casts its shadow on the moon. The moon is always a full moon and it never goes completely dark during a total lunar eclipse. It appears reddish for the same reason that sunsets and sunrises often have a red tint.

The Earth’s shadow has two parts: the umbra and the penumbra. A small amount of the sun’s energy shines directly within the penumbra. Within the umbra, none of the sun’s rays shine directly in this part of the shadow, so it is the darkest part of the shadow. A total lunar eclipse occurs when the moon travels through the Earth’s umbra.

The umbra is not pitch black because of the Earth’s atmosphere. Sunlight is scattered, or redirected, in all directions by the Earth’s atmosphere into the penumbra and umbra. As light passes through the atmosphere, the blue colors are scattered out of the path. Red and orange light pass through the atmosphere and are scattered into the shadow zone. If our planet had no atmosphere, then the moon would be completely dark during a total lunar eclipse.

If you missed seeing this recent total lunar eclipse, the next total lunar eclipse that is viewable from our area occurs in 2014.

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What is the outlook for Wisconsin’s winter?

Posted on December 5, 2011 by Weather Guys Editor

The National Oceanic and Atmospheric Administration’s Climate Prediction Center issues seasonal climate outlook maps for the nation. The organization’s forecast for Wisconsin’s 2011-12 meteorological winter (which started Thursday and runs through Feb. 29) is for below-normal temperatures and above-normal precipitation.

So Madison, for example, will be colder than its average winter temperature of about 20 degrees, with more than the normal 4 inches of water (rain and melted snow) for the three months.

Because they take a longer view, monthly and seasonal climate forecasts look very different from daily or weekly weather forecasts. There are no low- and high-pressure systems, no fronts, and no specific numbers for high and low temperatures. Instead, the maps show chances of above- or below-normal climate and weather conditions.

An important component in determining seasonal forecasts is ocean temperature anomalies associated with El Niño or La Niña. This year, a moderate La Niña in the central and eastern equatorial Pacific Ocean is expected to persist and is the dominant factor that will influence the winter weather in the United States. So, the seasonal forecast reflects typical temperature and precipitation patterns during a La Niña year.

Elsewhere, the forecast for this winter calls for warmer-than-normal conditions from Arizona and southern Utah east into Alabama and the Florida Panhandle. Colder-than-normal temperatures are expected across southern Alaska, along the West Coast, and from northern Idaho east into Michigan.

How accurate are these types of climate forecasts? Overall there is some skill in 90-day forecasts. They are better than a coin flip, particularly in a year with an El Niño or La Niña, but they are not to be taken with the same degree of confidence as a one- or two-day weather forecast.

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When does winter really start in Madison?

Posted on November 28, 2011 by Weather Guys Editor

The most common way to define the start of winter is to appeal to the solstice, the day on which the noontime sun is positioned at its farthest southern point of the year. This is the astronomical start of winter — Dec. 22 this year. On that day, the noontime sun will be directly overhead at 23.5 S latitude.

Of course, by the time Dec. 22 comes around those of us who live in southern Wisconsin have usually already experienced a reasonable dose of winter.

So, is there an alternative way to define the beginning of winter that is more in line with our real experience of the season?

It turns out that there is such an alternative and we can base it on the likelihood of falling snow, perhaps the distinguishing characteristic of winter weather.

Imagine you have 100 years of weather data for Madison. You might reasonably ask what kind of precipitation has historically fallen on Aug. 1 at Madison. In fact, it has never snowed on Aug. 1 in any of those 100 years at Madison. As we move into the fall toward the winter solstice, however, snow begins to show up in the 100 years of precipitation data with greater regularity. By the time we get to Nov. 15, precipitation that falls on that date has been equally split between rain and snow.

This would seem to be a pretty good way to determine the start of meteorological winter, since after this date the most likely form of precipitation is snow. So, though it hasn’t really felt like it yet (but it will by Wednesday), we are already in our meteorological winter season.

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