Two fundamental assertions about weather and why there is weather:
Air Mass is an extremely large body of air whose properties of temperature and moisture content (humidity), at any given altitude, are fairly similar in any horizontal direction.
Source Regions are simply geographic areas where an air mass originates. Should be:
The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below.
Classification: 4 general air mass classifications categorized according to the source region.
We can then make combinations of the above to describe various types of air masses.
cP continental polar cold, dry, stable
cT continental tropical hot, dry, stable air aloft--unstable surface air
mP maritime polar cool, moist, and unstable
mT maritime tropical warm, moist, usually unstable
Air masses in the U.S. include
cP -- wintertime bitter cold can extent to Southern US and even Florida causing crop damage. Require long, clear nights, which means strong radiational cooling of air near the surface. A stable air mass. Little moisture added so air is dry
mP -- Winter cP air moves over a region such as the NE Pacific, picking up some warmth and moisture from the warmer ocean. In the case of the Pacific NW mountains force the air to rise (orographic lifting) causing rain.
mT -- wintertime source for the SW US is the subtropical East Pacific Ocean. mT air that influences weather east of the Rocky Mountains comes from the Gulf of Mexico, but only influences winter weather in the SE states. Occasionally, slow moving weather systems in SW flow aloft can draw up moisture at mid and low levels producing precipitation.
cT -- Continental tropical air usually only influences the US in summertime as warm, dry air is pumped up off of the Mexican Plateau. It is usually fairly stable and dry, and if it becomes stagnant over the midwest, results in a drought. Deaths associated with the 1995 heat wave in the midwest were the result of cT and mT air which stagnated over the central and eastern part of the US this last summer.
Air masses can control the weather for a relatively long time period: from a period of days, to months. Most weather occurs along the periphery of these air masses at boundaries called fronts.
Front is the transition zone between air masses with distinctly different properties.
The differences in density are most often caused by temperature differences. Separate air masses with different humidities as well. We identify fronts by the movement of this transition zone and the properties that move over a geographical location. What weather changes do you expect when TV weather person says a cold front is moving through the area?
How do you identify a front on a surface weather map or by your own weather observations? Look for:
A stationary front is characterized by no movement of the transition zone between two air masses.
A cold front is cold air displacing warm air.
Steep leading edge -- friction slows surface advance, moves quickly--25 knts up to 40 knts faster=steeper
General weather characteristics of a Cold Front Weather Feature Before Frontal Passage Region of Front After Frontal Passage Winds S-SW gusty W-NW Temperature Warm Sudden decrease Steady Cooling Dew Point high; remains steady Steady Decreases Pressure Falling steadily Minimum; rapid rise steady rise Visibility Fair to Poor Poor then improving good Clouds Ci, Cs Cb Cb Cu Precipitation showers heavy precip. clearing
A warm front is warm air displacing cool air diagram. Shallow leading edge warm air must "overrun" cold air--cold air recedes moves slow 10-15 knts
General weather characteristics of a Warm FrontWeather Feature Before Frontal Passage Region of Front After Frontal PassageWinds S-SE Variable S-SW Temperature Cool, slowly warming steady rise warmer Dew Point Steady Rise Steady Increases than steady Pressure Usually Falling Levels off slight rise, followed by fall Visibility Poor Improving fair Clouds Ci, Cs As Ns St fog Stratus clearing with scattered Sc Precipitation light to moderate drizzle or nothing usually none
There are two kinds of occluded fronts
Cold occlusion-cold front catches up with warm front. Ns, Tcu,Cb warm very cold
Warm occlusion mostly in NW. Warm cool mP off ocean cold cP warm occlusion. Precipitation is similar to the cold occlusion