THE CAP

The infamous cap; where shall I begin. Over the coming months you will most likely see me writing a lot about the cap. The cap is a critical forecasting element for severe weather season here in central Texas which runs from March 1st through mid June. So, I'm sure you're asking yourself, what is the cap? In this blog I hope to help you all better understand the cap and its effects on the weather here in central Texas.

Weather Scenario: 

It is mid May here in Austin and it is HOT and HUMID...the temperature is at 90ºF and the dew point [measure of the amount of moisture in the air] is high at 72ºF. Gusty southeasterly surface winds continue to pump in plentiful amounts of sticky, moist Gulf of Mexico Air feeding into a strong area of surface low pressure over southwestern Oklahoma and northwestern Texas; extending southwestward across west Texas is a strong frontal boundary separating the hot and humid air over central, north and south Texas from the cool, dry air across west Texas. Basically what I am getting at is that it is the perfect set up for severe thunderstorms to form.

There are three key ingredients needed for thunderstorm development. The first ingredient is moisture, the second is instability (warm rapidly rising air into cooler air aloft), and lift (trigger for air at the surface to rise...several factors cause air to rise, the most common of these triggers is convection). Remember, warm air rises because it is less dense than cool air, therefore, hot air at the surface will rise...as that air rises it is forced to expand and cool, when the air reaches the saturation level [level where the air temperature is equal to the dew point temperature] the relative humidity jumps to 100% [meaning the air is saturated] it cannot hold any more moisture. When that rising air passes the saturation level, air is condensed and forms clouds. As that air continues to rise into even colder air aloft, eventually passing the freezing line, the growing cumulus cloud is able to strengthen into a thunderstorm.

So, where does the cap fit into all of this? Remember my description of the perfect severe weather setup in the second paragraph? Watching the latest visible satellite imagery, I see towering cumulus clouds trying to develop across west Texas along the frontal boundary, however, as fast as they rise they drop. Something is inhibiting those towering cumulus clouds from building into thunderstorms. That "something" is the cap [a.k.a. lid on the atmosphere]. 

What is the Cap? Where does it come from?

GEOGRAPHY OF MEXICO (SHADED RELIEF MAP):

 

In order to understand the Cap, we need to understand the geography of our neighbor to the south, Mexico, first. There are two major mountain ranges in Mexico running from northwest to southeast; The Sierra Madre Oriental occupies the eastern side of Mexico and the Sierra Madre Occidental occupies the western side of Mexico...in between these two mountain ranges is a high and dry plateau that consists mainly of deserts. That desert air is generally trapped in between the two mountain ranges for much of the year, however, when strong mid-latitude cyclones (low pressure systems) blow across the United States during the winter and spring air is forced to move north into the low pressure system from areas of higher pressure around it. Remember, high pressure flows into low pressure. Therefore, that desert air is forced to move out of Mexico to the north...keep in mind, the plateau is at least 4,000 feet in elevation while Austin sits at 750 feet above sea level, therefore, that warm and dry desert air moves south to north at or above 5,000 feet aloft...that layer of desert air that is forced northward is known as the cap (or the lid) on our atmosphere.

Here are some graphics to help better explain this weather phenomenon: 

Wind Direction:

CAP SETUP: HOT, DRY AIR ALOFT INHIBITS THUNDERSTORM FORMATION:

DAY WHEN THE CAP IS PRESENT AND STRONG:


  

REAL IMAGERY EXAMPLE:

 DAY WHEN THE CAP IS NOT PRESENT OR VERY WEAK:

 

It may be helpful to think of the cap and its influence on the weather here in central Texas as a boiling pot of water on the stove. If you put a lid on a boiling pot of water, all of the heat energy (lift) being produced from the boiling water will be trapped under the lid and continue to build up; eventually the increasing pressure and built up energy under the lid will eventually cause the lid to blow off of the pot of boiling water in a violent fashion. If warm, rising air is able to break through the cap, very severe storms are likely to develop which would have the potential to produce extremely large hail, destructive winds, and violent tornadoes. If there was no lid on the pot, the energy produced from the boiling water would be able to freely distribute itself and escape. That warm, rising air into cooler air aloft (above) allows for thunderstorms to develop much more sufficiently.

Unfortunately, the cap does block a lot of our rain chances here in central Texas during the spring months, however, it also saves us from seeing a great deal of severe weather during certain instances. Remember, if the rising air is able to break through the cap, watch out, severe weather is likely.

Here's another great graphic explaining the cap from WFAA-TV Meteorologist Steve McCauley in Dallas:

Updrafts and Downdrafts: