Temperature shifts - how do they happen?

This is a good question.  Without knowing where exactly you observed this shift in the weather...I will answer it in a general way.  There are two common ways that the weather can change rapidly (usually for the worse)...with temperatures dropping...clouds arriving...and precipitation falling.  The first explanation is the arrival of a cold front.  Fronts signify a change in air mass...say for example a change from a warm humid tropical air mass to a cold Canadian air mass.  Since the United States is located in the "middle latitudes" on the Earth...wedged between the tropics and the polar regions...cold fronts are common in the continental United States...even as far south as Miami and the Keys.  Temperatures can fall from the 80s to the 30s in only a few hours when a strong cold front passes.  Thunderstorms are common along cold fronts...as the lighter warmer air mass is lifted over the denser colder air mass.  When warm humid air is lifted vigorously...you can get severe weather.  There have been a couple recent dangerous severe weather episodes this February in the Deep South.

 The second way weather can change so rapidly...especially if you live along the coast...is a shift in the winds from a land breeze to a sea breeze.  In early summer...when the land is warming up but the waters are still icy from the winter...the coastal temperatures all depend on whether the wind is blowing onshore or offshore.  San Diego is a perfect example of this...where temperatures along the immediate coast stay in the 60s and 70s in summer...yet father inland it can get into the 100s. 

 

 

Let's talk pressure...something we feel everyday at our jobs...ok...a different kind of pressure.  First the basics...there are two important measurements when it comes to pressure--the pressure itself and the pressure tendency (or how much the pressure has changed over the past 3 hours).  Typically...pressure is measured in inches of mercury or measured in millibars.  Meteorologists almost always work with pressure in millibars.  A high pressure center can typically have pressure ranging from 1010millibars (mb) to 1040mb (often found in Canada during the winter).  Low pressure centers usually get below 1000mb and strong No'easters can get down around 970mb.  Hurricane Wilma in 2005 broke the lowest recorded pressure in the Atlantic with a pressure of 882mb.

Now the pressure reading on a barometer is much less important than the pressure tendency...or how the pressure has changed over the past few hours.  Pressure tendency can help you determine what kind of weather you are in store for...up to about 12 hours in advance.  In fact...sailors before the age of satellites and computers...relied heavily on the barometer to make their own short range forecasts and attempt to avoid dangerous storms.

So...how does this all work??  First...on a barometer you should have two arrows...one measures the pressure...while the other is just a dial that you can turn (doesn't change when the pressure changes).  When you turn the dial so that the arrow lays ontop of the pressure arrow...you will be able to see if the pressure changes.  For example...after 3 hours...if the pressure drops significantly (say 3-6 millibars)...then there is the possibility of bad weather....if the pressure rises...chances are that conditions will improve.  The rule of thumb in meteorology is that high pressure clears skies out and there is little chance of precipitation...and low pressure brings clouds and precipitation.

A bit long winded...the take home message is that it is much more important to determine the pressure tendency when taking pressure measurements...if the barometer is dropping...a low pressure storm might be on the horizon...if the pressure is rising...expect smooth sailing!