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# Fronts: What are Fronts?

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Author: Janice VanCleave

What are Fronts?

### Materials

• 9-inch (22.5-cm) glass pie plate
• Modeling clay
• 1 cup (250 ml) tap water
• Blue or any dark food coloring
• Spoon
• 1 cup (250 ml) liquid cooking oil
• Timer
• Helper

### Procedure

1. Set the pie plate next to the edge of a table.
2. Use modeling clay to form a barrier across the center of the pie plate, dividing it into two equal parts.
3. Pour the water into the left side of the pie plate.
4. Add three drops of food coloring to the water and stir.
5. Pour the oil into the right side of the pie plate.
6. Kneel on the floor in front of the plate so that you are eye level with the plate.
7. Ask a helper to raise and remove the clay barrier from the plate.
8. Observe the movement of the two liquids for one to two minutes.
9. Look at the liquids as often as possible for 5 minutes or until no changes are seen.

### Results

The colored water slowly moves under the layer of oil.

### Why?

The density ("heaviness" of materials; mass compared to volume) of air masses (large bodies of air, each with about the same temperature and humidity throughout) varies in that different air masses have different temperatures and humidities. Cold air masses are more dense than warm air masses. Since water is more dense than oil, the colored water in the experiment represents a cold air mass and the oil represents a warm air mass. Just like the oil and water, cold and warm air masses do not mix with each other. The colder, more dense air mass generally moves under and lifts the warmer, less dense air mass.

The boundary between two air masses is called a front. The leading edge of a cold air mass advancing into an area occupied by warmer air is called a cold front. A warm front is the leading edge of a warm air mass moving into an area occupied by colder air. The weather is usually unsettled and stormy at a front. Precipitation is common, especially on the cold air side.

### Let's Explore

Does the amount of each liquid affect the results? Repeat the experiment twice, first using one-half the amount of water, then using one-half the amount of oil. The results indicate whether the density of an air mass depends on its size.

### Show Time!

1. Generally a cold front wedges under and pushes up a warm air mass, while a warm front rises up and over a cold air mass. When neither air mass has enough power to move into the area occupied by the other, a stationary front occurs. An occluded front is one in which a cold front overtakes a warm front Prepare drawings similar to the diagram shown here to compare the movement of air masses in the four types of fronts.
2.
1. Collect newspaper weather maps and watch a local media weather broadcast for one week to determine how fronts affect the weather from day to day. Make an enlarged drawing of a map and plot the most westerly cold front. Use pens of various colors to show the location of the front each day as it moves east. Use the maps to record the weather conditions, such as pressure, temperature, winds, and clouds before and after the front passes through an area.
2. Repeat the above procedure, plotting the movement of any of the other three types of fronts that are present during the week.

### Check it Out!

1. The formation of warm and cold air masses occurs when air remains in a region long enough to take on the temperature and humidity of that region. Find out more about air masses. How long does it take an air mass to form? In what general direction do air masses move?
2. The weather before and after a front arrives can change subtly or very dramatically. Fronts are often part of larger weather systems. Find out more about the effect of fronts on the weather. What types of clouds are associated with each type of front? How do storms form?