When you look at a weather map, or listen to the weather report on the news, you probably wonder: What are the forces that control our daily, weekly, or monthly weather patterns? Weather forecasters use their understanding of these forces to predict the upcoming weather events, and with a little practice, so can you.
Over time these weather patterns become the averages that we know as climate. On the map to the right you can see the average annual precipitation patterns for the past 30 years in the United States. Areas colored blue receive more rainfall on average than areas colored orange or reddish brown. Can you predict why some areas might receive more rainfall than others?
In addition to the global circulation patterns that you reviewed in the first part of this lab, six continental- and regional-scale forces work together to control weather and climate in your part of the country. These forces, called "Synoptic-scale drivers," each contribute to the weather that we experience on any given day, week, or month. The region of their influence can be several hundreds to thousands of kilometers wide covering large sections of the continents. They occur on a timescale of days to weeks. Think of these climate drivers as the ones that you see on your local weather forecast maps. The six major regional weather drivers are listed below.
What are the predominant forces that are controlling your regional weather today, and ultimately your long-term climate?
As you work through the weather drivers described below, refer to your local region for each. Some questions to ask yourself: Do you live near a coast, or are you inland? Are you upwind or downwind of a mountainous region? And which type of air mass is over your region most often? Use the map (linked below) to help you record information as you work through these six drivers. Build a list of the drivers influencing your regional weather this week; number the list in order of size of influence (with 1 being the most influential). You will use this list, your maps, and a chart in a discussion of regional weather climate at the end of the activity.
Begin your study of the climate in the United States by downloading and analyzing maps of United States: Climate from Education Place.
Click the link to access the United States: Climate map. (Note: your teacher may have already provided you with a copy of this map, in which case you do not need a second one.)
Locate your state or region on the map.
Record the average temperature and precipitation of your home state, notice how it compares to other states.
Use the map of the USA: Average Precipitation and Temperature to record your notes and observations while completing this lab. Continue to use the organizing chart and World Climate map from Lab 3A as well.
Air masses
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Source: NWS JetStream
In the graphic to the right, you can see the major air masses that affect North America. Air masses form over large geographic regions, such as the ocean, continents, and polar regions. They are characterized by their temperature and humidity, which are a result of their place of origin or source region. Air masses are one of the primary forces that control a region's weather. For example, an Arctic air mass is very cold and typically dry. In contrast, an air mass whose source region is more tropical, is warm. An air mass would be moist if it originated over the ocean and dry if its source region was over a continent.
Air masses are slow moving and relatively stable; they influence weather on a time scale of several days to a few weeks. As they move to a new region, they gradually are transformed by the characteristics of the new region. For example, an air mass that moves from the polar regions southward will gradually warm and lose its "punch."
First, watch this short video to get a better understanding of air masses.
Air Masses by Dan Guthrie
Learn about the air mass classification scheme. The classification system is relatively simple: each air mass is represented by a two-letter classification scheme based on its moisture and temperature characteristics. This code tells its place of origin and type. The first letter in lowercase — m (maritime) or c (continental) — describes its moisture content. The second letter in uppercase — E (Equatorial), T (Tropical), P (Polar), or A (Arctic) — provides information about its temperature.
Next, check your understanding of these symbols, and learn more about the characteristic of air masses with the following air mass characteristics table.
Air Masses of North America. Click to view larger.
Then, draw the air mass most commonly found over your state on your U.S. climate map.
Finally, watch this satellite visualizations to see how warm and cold air masses move across the United States and bring related weather patterns with them.
from Boise NWS
This video shows infrared satellite images combined with modeled air masses overlaid with colors indicating temperature. Cooler temperatures are in blues, warmer temperatures in reds. A warm air mass is building along the West Coast and a cold air mass is moving south from Canada to impact the Great Plains.
Explore more
Try this Air Masses(Acrobat (PDF) 1.2MB Feb2 22) on a globe demonstration.
Fronts are boundaries between air masses. A front is described by the temperature of the incoming air mass; a cold front is the boundary before an advancing cold air mass, while a warm front is the boundary before an advancing warm air mass. Fronts separate air masses with differences in temperature and humidity. Warm fronts are shown on maps with red, semi-circular shapes; cold fronts are marked by blue triangles. Cold fronts tend to move at faster rates than warm fronts; cold fronts move in a scale best measured by hours, while warm fronts are slower and are better measured by days. Often you will notice that the wind speed and direction change on either side of a front. Visit the National Weather Service's Current Surface Map to see what frontal boundaries there are today in the contiguous United States.
Watch the two animations, below, for a view of how fronts move. The regular passage of warm and cold fronts dominate the weather and climate in most of the contiguous United States (the lower 48 states). The equatorial / polar regions to our South and North do not commonly experience fronts.
Cold Fronts from Animating Geography
Warm Fronts from Animating Geography
Watch the animations several times. While watching, note the the cloud patterns associated with each type of front. Become a cloud watcher; the changing cloud types over your head can help you to predict the weather for the next few hours (and days).
Cold fronts are characterized by cumulonimbus clouds. Warm fronts, which move in move slowly, have a series of clouds that form; the leading edge is usually signaled by cirrus-type clouds, which change to stratus-type clouds as the front advances.
Look outside your window; what types of clouds do you see today, and what do they tell you about the weather? Refer to the cloud chart in Lab 1A for more information about cloud types. If you have access to a weather map, view the location of today's fronts NWS forecast map. Note their location on your U.S. Climate map.
Semi-permanent pressure systems are a result of global circulation patterns (especially Hadley Cells). They contribute to the dominating air mass in the region in which they reside. Areas where air is rising, such as around the Equatorial Intertropical Convergence Zone (ITCZ), are low-pressure regions, characterized by clouds and stormy, wet weather. On the other hand, regions that are dominated by descending air, resulting in high pressure, are typically dry. In the case of Arizona and New Mexico, the descending air is relatively warm and dry, creating hot desert-type climate conditions. However, there are cold deserts as well. Use the diagram, pictured right, to think a location of a cold desert. Record your notes on your world climate map.
The size and shape of semi-permanent air pressure systems can change slightly from season to season, and year to year. They are influenced by oceanic conditions, such as El Niño and La Niña, as well as land surface types.
Return to the Moving Heat Interactive in Lab 3A to review the relationship between Hadley cells, wind patterns, and areas of semi-permanent high and low pressure.
Air circulation patterns around high (H) and low (L) pressure centers. Source: NWS JetStream
The atmosphere, like any fluid, is in constant motion. We sense this atmospheric motion as wind. Wind moves from areas of high pressure to areas of low pressure. At any given time, the centers of high and low pressure initiate the local and regional wind patterns. Except for the semi-permanent pressure systems, these centers of high and low pressure systems are constantly migrating around the Earth, giving us a never-ending variety of wind and weather patterns.
In the Northern hemisphere, wind rotates counter-clockwise and into low pressure centers, and clockwise and out of high pressure areas. These patterns of rotation are called cyclones (into low pressure) and anti-cyclones (away from high pressure). First, view this video showing NASA Earth Observatory's animated series of Geostationary Operational Environmental Satellites images. The images show a cyclonic (low pressure) storm rotation near the Great Lakes from September 25-27, 2011.
Note the counter-clockwise direction of the rotation of the clouds in the storm over the Midwest.
Next, follow this link to view an amazing wind map for the entire conterminous (lower 48) United States. While watching the animated map, compare it with the High and Low pressure locations on the mixed surface analysis map. Adjust the windows so you can see both maps side-by-side, and then investigate how winds move from areas of high pressure to areas of low pressure, along the isobars isobars: a line connecting points of equal atmospheric pressure. of pressure (white lines on the fronts map). Also take note of the clockwise and counter-clockwise patterns around the highs and lows. Note: Be sure to compare maps from the same day and time.
Explore more
Try this hand-twist model(Acrobat (PDF) 842kB Feb2 22) demonstration. You will also need to print this base map(Acrobat (PDF) 194kB May10 12) to accompany the demonstration.
Wind-driven surface ocean currents close to coasts play a strong role in the weather of coastal regions. Areas on western coasts of continents, where the prevailing westerly winds bring cool ocean air on shore, are generally cooler and wetter than the interior regions of continents or areas on eastern coasts. On the western side of continents, the surface ocean currents are carrying cold polar water toward the equator; therefore the air above these currents tends to be cool, moist, stable, and fog-prone. Examples of U.S. cities influenced by cold surface ocean currents include Seattle and San Francisco, which is famous for its fog!
Where warm surface ocean currents are moving warm, equatorial water toward the poles, the air above tends to be warm, moist, and unstable. This creates more turbulent and changeable weather because the warm ocean water is evaporating and adding water vapor to the atmosphere. Generally, the eastern sides of continents are warmer than their western counterparts. The eastern seaboard of the United States is an example of a region with highly turbulent and changeable weather due to the influence of the warm Gulf Stream current.
View the ESRI story map What Causes Ocean Currents? to learn more. Read through and play the videos in the first four sections:
What Causes Ocean Currents?
Why are Ocean Currents in Motion?
Coriolis Effect
Geographic Patterns of Ocean Currents Systems
Note the warm and cold currents that run along the coasts of continents. These currents influence weather and climate patterns.
Ocean currents do not directly influence the weather areas in the middle of the continent. The weather in these interior regions is controlled by the prevailing wind patterns. For example, use your U.S. climate map to compare the weather of three cities on the same latitude: one in California, a second in the Midwest or Plains States, and one on the East Coast.
Using the diagram, right, as your guide, draw the warm and cold surface currents on your world climate map.
Mountain ranges also influence regional weather and climate. As air flows upward over a mountain range, it expands, cools and condenses. The windward (upwind) sides of mountain ranges are moist, rainy and cool. As the air descends on the other side of the mountain range, it compresses and warms. These warm—and dry—downslope winds can be extremely strong and so are often are given their own unique names. As a result of these winds, areas on the leeward (downwind) side of mountains are generally arid. For example, compare the climate of Southern California to that of Nevada.
Using the diagram, right, as well as your knowledge of U.S. geography, as your guide, sketch the approximate location of the large western mountain ranges on your U.S. climate map.
There are different names for downslope winds around the world. The U.S. uses two names for well-known downslope winds in the western states. Watch the video below to learn more about these winds and the process that creates them. Then try the experiment below.
How wind warms up weather in a hurry from 9News Denver, CO
Now that you have worked through these regional weather drivers, try to answer the question: What are the predominate forces that are controlling your regional weather today? Make a list of the drivers influencing your regional weather. Number the list in order of size of influence.
After you have generated your list, visit the NOAA/NCDC State Climate Summaries site to download a PDF file of the descriptive climatology for your state.
On the site, scroll down to the map of the U.S. and click on your state to be taken to your state's information. At the top of the climate summary page, below your state's name, you will see three buttons. Click the one labeled Downloads. From the popup screen, select the Summary (pdf) option to download the pdf or view in your browser. Work with your lab team to identify the major climate drivers for your region.
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