By Jan Null
Special to the Mercury News

The winds are shifting in Iraq -- and with them, as U.S. and British troops have already learned, comes trouble.

The sandstorms in Iraq during the past few weeks have been the result of a prevailing desert wind called a Sharqi. However, in the next two months, the wind pattern will shift to a wind called the Shamal. Though different in character, both of these winds lift the fine particles of dust and sand into dangerous clouds that restrict visibility and make breathing and other activity hazardous.

The Sharqi wind pattern, which usually persists through the month of May, is characterized by occasionally strong, gusty, warm southerly winds. These winds become especially strong as weather systems move eastward out of the Mediterranean and across Turkey along the northern border of Iraq.

Winds as strong as 50 mph are possible during April and May, and to a lesser extent in the September-through-November time frame.

During the months of June, July and August, the prevailing Shamal wind shifts to northerly. These winds originate in the mountains of Turkey and Kurdistan, and they sweep down the valleys of the Tigris and Euphrates. They are not as strong as the Sharqi but are more persistent, which results in more hours of restricted visibility and other problems due to dust and sand in the air.

Despite the fact that these winds are from a cooler northerly direction, the maximum temperatures average more than 105 degrees for the entire period.

The dust and sandstorms that can accompany these winds can rise to heights of several thousand feet, causing hazardous conditions that effectively shut down travel on both the air and ground.

Additional information about the climate and weather in Iraq and the rest of southwest Asia can be found at http://ggweather.com/sw_asia.htm.

Q I have an e-mail friend in Australia, and I know their seasons are the opposite of ours, but I would have expected their equinox to occur on the same day of the year as ours, even if theirs is autumn and ours is spring. What am I missing? Lorna Norbeck - San Jose

A The equinoxes and solstices are astronomical events not related to local times but to our position relative to the sun. Consequently, the equinox will occur at the same moment at all places on the earth. But in Australia, the beginning of their meteorological seasons is designated as the first of the month. Consequently, fall begins on March 1, winter on June 1, spring on Sept. 1 and summer on the 1st of December.

Q As we all know, pure water is odorless and tasteless. Why, then, is there an unmistakable smell to the air right after a rain shower has started? I find this particularly noticeable if the shower occurs after a period of dry weather. Frederick Clegg - Cupertino

A The aromatic ``smell of rain'' can be caused by a number of factors. One of the most common and pleasant smells is actually caused by bacteria. The culprit is a filamentous bacterium called Actinomycetes that grows in soil that's damp and warm. As the soil dries, the bacteria produce tiny spores that can be disturbed by the force and wetness of rain. These spores, which are common worldwide, can be easily carried on the moist air so that we breathe them in as well as smell them as the distinctive, earthy smell we often associate with rainfall. The smell is indeed typically most acute after a rain that follows a dry spell.

There are other less prominent smells associated with rain interacting with other organic materials, sometimes made stronger by rain that is slightly acidic.

Q Could you please explain or define the terms HDD and CDD? Rolf Druskat - Marion, Mass.

A These acronyms refer to Heating Degree Days and Cooling Degree Days, respectively, and are ways of quantifying temperatures in regards to heating and cooling demands. HDDs are the cumulative number of degrees in a month or year by which the mean temperature falls below 65 degrees and when there might be the need for heating buildings. Conversely, CDDs are the cumulative number of degrees in a month or year when the mean temperature exceeds 65 degrees. For example, consider a day when the high temperature was 77 degrees and the low was 43 degrees. The average for the day would be 60 degrees. Because this average is 5 degrees below 65 degrees, the day would accumulate 5 heating degrees. These are then summed up for the month or year to estimate energy usage.

Q In New England, where I grew up, passage of a cold front is almost always followed by rapid clearing and fine, dry weather, sometimes lasting for several days. But here in the Bay Area, a cold front is usually followed by a long period of unstable, showery weather. What is the reason for this difference? Ed Taft - Mountain View

A The main reason is the character of the cold air masses that affect these regions. In the Northeast, the cold fronts are a result of cold and relatively dry continental air from Canada. The relatively dry air behind these storms leads to mostly clear skies. The cold fronts we see in Northern California are usually the result of cool, moist maritime air masses from the north Pacific. The moist air behind these fronts often results in the clouds you note, and sometimes additional precipitation.