What is the source of energy for hurricanes?

Posted on September 3, 2012 by Weather Guys Editor

Hurricanes are large weather engines, and any engine needs energy to run.

The secret energy source of a hurricane is the large latent heat of water. Air over the tropical oceans is drier than you might think. Although both the air and water may be warm and calm, evaporation can take place because the air is not at 100 percent relative humidity.

Silently and invisibly, water changes from liquid to vapor and enters the atmosphere. The energy required to make this change comes from the sun, and this energy is lying in wait — latent — ready to be released when the vapor is condensed into liquid again. This happens in rising air in a cloud or thunderstorm.

However, this process alone is not enough to power a hurricane. A hurricane adds fuel to its own fire by drawing surface air toward its low-pressure center. The tight pressure gradient nearer the center means that the winds grow stronger as the air approaches the eye. The faster the wind blows, the more evaporation takes place (this is why you blow-dry wet hair or hands instead of merely warming them).

Increased evaporation means more water vapor in the air and more energy ready to be liberated in the hurricane’s thunderstorms as water vapor condenses. In short, evaporation and condensation of water are the keys to understanding the power of tropical cyclones.

How strong is the engine that powers a tropical cyclone? The energy released by condensation in a single day in an average hurricane is at least 200 times the entire world’s electrical energy production capacity. Part of this energy is expended reducing the central pressure of the storm and strengthening the winds.

Category: Tropical
Tags ,
Comments Off on What is the source of energy for hurricanes?

What are sundogs?

Posted on August 27, 2012 by Weather Guys Editor

On a day with high ice clouds, you are likely to see shiny, colored regions at either side of the sun. These are sundogs, an optical effect caused by refraction and dispersion of the Sun’s light through ice crystals. When the light rays strike the boundary between the air and water, like an ice crystal, several things can happen. Some rays are turned back in the direction from which they came, the familiar process of reflection. Other rays are transmitted into the crystal. Some of the transmitted rays change direction, a process known as refraction.

Sundogs appear because ice crystals in the shape of hexagonal dinner plates tend to drift downward with their flat bases parallel to the ground. The sunlight passes through the crystal and refracts sideways. If the Sun is low enough in the sky, you see spots of bright light on one or both sides of the sun, depending on where the clouds are. Refraction causes blue light to be bent more than red light, and so sundogs can show a spectrum of colors with red nearest the Sun.

Sundogs are usually 22 degrees away from the sun, or about a hand width from the center of the Sun when your arm is fully extended. Sundogs are often accompanied by a halo around the sun. A halo is a white ring that encircles but does not touch the sun. It is an optical phenomenon that also owes its existence to refraction of light by ice crystals. Because the light must shine through a fairly uniform layer of ice crystals that are thin enough to let light through, halos are usually associated with high, thin cirrostratus clouds.

Category: Phenomena
Tags
Comments Off on What are sundogs?

Are there fall weather changes beyond turning leaves and falling temperatures?

Posted on August 20, 2012 by Weather Guys Editor

As we head into the second half of August a subtle transition in our weather begins to occur — a transition that is probably hard to detect at first but that eventually becomes very obvious and then lasts for approximately eight months.

We are not talking about the gradual reduction in daytime high temperatures or the increasingly cooler to cold nights, though these are also beginning to invade. Instead, we are talking about the nature of the storms that deliver our precipitation.

Throughout the summer (even in this drought year), most of our precipitation comes in the form of thunderstorms wherein large amounts of precipitation fall in short amounts of time from what we call convective clouds. Most often these storms have lifecycles of only a few hours and drop precipitation over a relatively small area.

As we transition to late summer/early autumn, the thunderstorm frequency abruptly decreases and precipitation tends to occur in persistent, light to moderate rain events that will sometimes last an entire day. This mode of precipitation is associated with the passage of what are known as mid-latitude cyclones — storms that live for over a week during which time they can cover an area the size of 10 states.

As they progress across the country, these mid-latitude cyclones can drop precipitation (rain or snow) over enormous portions of the country. Though not entirely missing from summertime precipitation, such events are definitely the exception rather than the rule in the summer.

Category: Seasons
Tags , ,
Comments Off on Are there fall weather changes beyond turning leaves and falling temperatures?

What is a drought?

Posted on August 13, 2012 by Weather Guys Editor

A drought means different things to different people. Technically, a drought is a period of abnormally dry weather sufficiently long enough in a given area to cause a shortage of water, whether it is for crops, recreation, water supply utilities or other purposes. As you can imagine, a drought for someone who lives in a desert region would be very different than for a person living among Wisconsin’s many lakes.

There are several definitions of drought, but in general there are three types. A meteorological drought is any substantial and prolonged lack of rainfall over a period of time in a given region. A hydrological drought is a deficiency in surface or subsurface water supplies. An agriculture drought exists when there is not enough soil moisture to meet the needs of a crop at a particular time.

Because there are different types of droughts, there are different methods of measuring the severity of a drought. For example, a meteorological drought can be defined in terms of the percent deviation from the normal precipitation in the region, while a hydrological drought is defined in terms of stream flow, lake levels and groundwater levels.

An effective measure of the impact of drought on agriculture is the Palmer Drought Severity Index. This index, developed in the 1960s, is based on the supply and demand of a water balance equation. It takes into account the precipitation deficit of a location, the temperature of the region and the locally available water content of the soil.

Currently, the Palmer Drought Severity Index lists southern Wisconsin as having “severe drought” conditions, though conditions are improving with the recent rains.

Category: Climate
Tags , ,
Comments Off on What is a drought?

Are there different types of lightning?

Posted on August 6, 2012 by Weather Guys Editor

Lightning is a huge electrical discharge that results from the rising and sinking air motions that occur in thunderstorms. Lightning can be either cloud-to-cloud or cloud-to-ground and is accompanied by thunder. Lighting also has different appearances.

Staccato lightning is a cloud-to-ground lightning strike that is a short-duration stroke that often, but not always, appears as a single very bright flash and often has considerable branching. Forked lightning is a name, not in formal usage, for cloud-to-ground lightning that exhibits branching of its path. Ribbon lightning occurs in thunderstorms with high cross winds and multiple strokes. The wind will blow each successive stroke slightly to one side of the previous stroke, causing a ribbon appearance. Bead lightning is a type of cloud-to-ground lightning which appears to break up into a string of short, bright sections. It is relatively rare.

Heat lightning is a common name for a lightning flash that appears to produce no discernible thunder because it occurs too far away for the thunder to be heard. The sound waves dissipate before they reach the observer.

During the past two decades scientists have discovered and confirmed the existence of lightning that shoots upward into the upper atmosphere from thunderstorms. Red sprites and elves occur over cloud-to-ground lightning bolts and can extend to near the top of the atmosphere. They are too quick and weak to be seen by the naked eye. Blue jets, in contrast, are limited to the stratosphere and last long enough to be seen by pilots. Much is still not understood about these electrical phenomena.

Category: Phenomena
Tags , ,
Comments Off on Are there different types of lightning?