Have you noticed that sometimes the weather will repeat itself several days in a row. For example, the weather could be cold and rainy for several days in a row. Another example is a month of repeating hot and dry weather. Meteorologists refer to repeating weather as a weather pattern. It is common for the weather to become locked in a repeating pattern. The same weather will occur day after day. Then suddenly one day the weather changes and for example dry weather for 2 weeks becomes rainy weather for a week. When this happens it is referred to as a weather pattern change.
The primary source that determines the weather pattern in the middle latitudes is the upper level flow pattern. An examination of the jet stream winds shows regions likely with below normal temperatures and above normal temperatures. If a forecast area repeatedly stays within a trough the weather will tend to be cooler than normal. If a forecast area repeatedly stays within a ridge the weather will tend to be warmer than normal. Troughs and ridges can become locked in position for many days or weeks in a row. When this happens some regions will have droughts while other areas have flooding. Some regions will continually have warm weather while other area much cooler weather.
Regions in which the wind direction continues from a southerly direction will tend to be warmer than normal. Regions in which the wind direction continues from a northerly direction will tend to be cooler than formal. The region in transition from the trough to the ridge with winds from the south tends to have the best rain changes especially if the southerly flow is also bringing up moisture. However, if the southerly flow is from a very dry region such as a desert then the weather will become very dry. The region in the transition from the ridge to the trough with the northerly winds tends to have drier weather since the cooler air does not have as much moisture. Also, if the northerly winds are from the continent they tend to have lower dewpoints.
There are five main components to weather:
- Air pressure
Temperature is how warm or cool the air is in the atmosphere, while humidity measures how much moisture is present in the atmosphere. Humidity also influences the perceived temperature, and anyone who’s been stuck outside on a humid day can attest to how hot it feels compared to a day at the same temperature with low humidity. The high moisture content in the air prevents the sweat from evaporating off of our skin, which would normally keep us cool.
Air pressure is the amount of air molecules packed into an area. Changes in atmospheric pressure create weather patterns, which we’ll discuss later on. High pressure areas bring calm, cool, sunny weather whereas low pressure areas bring warm weather, rain, and clouds.
Wind is the movement of air through the atmosphere, and it helps to move air from high pressure areas to low pressure areas. Changes in air pressure also bring about clouds, collections of dust, and water vapor in the atmosphere. Precipitation is any form of water falling from the clouds; the most common forms that you may be familiar with are snow and rain. These processes bring changes in temperature and greatly influence human activity.
This is the QPF forecast and hazards map for this week – looks to be wet!