My runs and bike rides this past week have been rather cold and wet. The warm air I had been enjoying was replaced by moister, cooler breezes. This change made me think about the jet streams, or rivers of air in the atmosphere that run between cold and warm air masses, and weather fronts.
Jet Stream Types
There are two primary jet streams in each hemisphere. The polar jet stream type is found in latitudes of 50 to 60 degrees north and south of the equator. Subtropical jet streams are found in latitudes 20 to 30 degrees north and south of the equator. Since I live in Pennsylvania, the polar jet stream affects the weather in my area. The polar jet stream is more forceful and is associated with chillier air swept down from the Arctic region. As the jet stream dips further down into the United States, it pulls more Arctic air with it.
Jet Stream Influences
Jet streams are influenced by the seasons, air temperature, and locations of high and low pressure systems in the atmosphere. In my area, it is spring and the air temperatures had been in the upper 60° F range, but dropped to the low 40° F range for a few days. The change in temperature ushered in moisture in the form of rain that lasted three days. From the weather map in my local paper, I learned a low pressure system had moved into the area.
Source: Pittsburgh Post Gazette, Wednesday, April 22, 2015.
High pressure systems carry with them air that slowly descends. During the descending process, the air becomes warmer, which inhibits cloud formation. This is why most high pressure systems are associated with more pleasant weather conditions. There may be circumstances where high pressure systems do not bring good weather, but these cases are not the norm.
On sunny, clear blue sky days, high pressure dominates. This helps make flowers bloom and plants grow.
In order for the air to rise back up over different altitudes, it must cool. As the air cools, the humidity within it condenses. Depending on the amount of condensing that occurs, rain, snow, or other condensation (also known as precipitation) may fall. This is why low pressure systems are typically associated with cloudy, stormy, or more inclement weather.
Changes Just Like the Weather
Fortunately, jet streams and weather fronts move through areas. Though some fronts may stall, or stay in an area for a few days, they do not stay forever. A quick look at the weather map this morning shows high pressure moving into my area once again. The signal for this to me was the sunrise yesterday and today’s clearer skies. Hopefully, this front will stay a while and allow me to experience more enjoyable runs and bike rides for the next few days.
Resources and Related Links
Cloud formation – http://www.srh.noaa.gov/srh/jetstream/clouds/cloudwise/learn.html
Condensation – https://water.usgs.gov/edu/watercyclecondensation.html
High and low pressure fronts- http://usatoday30.usatoday.com/weather/tg/whighlow/whighlow.htm
Jet stream – http://www.livescience.com/27825-jet-stream.html
Subtropical jet stream – http://www.britannica.com/EBchecked/topic/571160/subtropical-jet-stream