Earth as a System: Energy, Matter, and Life · Lesson 6 of 13
Uneven Heating Causes Wind and Ocean Currents
“Warm air rises, cool air slides in, and suddenly the valley has a schedule.”
• Explain how uneven heating creates pressure differences. • Relate air temperature to density and vertical movement. • Describe valley breeze during the day. • Describe mountain breeze during the night. • Compare the direction and cause of the two local winds. • Connect local winds with daily life in mountain regions.
Wind
Before sunrise, the ground and mountain slopes are usually quite cold because they have been losing heat throughout the night. The air touching these cold slopes also becomes cooler. Cool air is denser and heavier than warm air, so it tends to flow downhill and collect in the valleys below. This movement of cold air can create gentle winds blowing from the mountain slopes toward the valley.
After sunrise, the situation gradually changes. Sunlight heats the mountain slopes, and the air in contact with them becomes warmer. Warm air expands, becomes less dense, and begins to rise. This creates differences in air pressure between the slopes and the surrounding areas. As a result, air begins to move toward the warmer slopes, and the direction of the wind may reverse.
The mountains and valleys have not changed their position. What has changed is the way different parts of the landscape are being heated. These differences in temperature change the density and pressure of the air, causing it to move from one place to another. This simple example shows how uneven heating of Earth’s surface can produce local winds and changing air circulation patterns.
Wind is the horizontal movement of air from a region of higher pressure toward a region of lower pressure.
When air is heated, its particles spread farther apart and the air becomes less dense. It tends to rise. Cooler, denser air sinks and moves into the space left behind. This produces a circulation in which vertical motion and horizontal pressure differences work together.
Local Winds
A local wind is an air movement produced by temperature and pressure differences over a relatively small region.
Problem
Trace the daytime circulation on a sunny mountain slope.
- 1.The mountain slope receives sunlight and warms.
- 2.Air touching the slope warms, expands and becomes less dense.
- 3.The warm air rises along the slope.
- 4.Relatively cooler valley air moves upslope to replace it.
- 5.This upslope movement is called a valley breeze.
Problem
Trace the circulation after a clear sunset.
- 1.The exposed slope loses heat rapidly.
- 2.Air touching the slope cools and becomes denser.
- 3.Gravity helps the dense air flow down the slope.
- 4.The valley receives this cooler descending air.
- 5.This downslope movement is called a mountain breeze.
| Feature | Valley breeze | Mountain breeze |
|---|---|---|
| Typical time | Day | Night |
| Slope condition | Warmer than nearby air | Cooler than nearby air |
| Air movement | Upslope | Downslope |
| Main density change | Heated air becomes less dense | Cooled air becomes more dense |
These winds can influence frost, cloud formation, smoke movement and farming. Cold night air collecting in a low valley can increase frost risk, while daytime upslope air can carry moisture and help clouds form near mountain ridges. Local observations therefore matter when choosing crop sites or interpreting weather.
Rising and sinking are vertical movements. Wind is usually described as horizontal movement caused by pressure differences. A complete circulation contains both.
Quiz
Which description best matches Wind?
Which description best matches Local Wind?
Which term matches this description: Wind is the horizontal movement of air from a region of higher pressure toward a region of lower pressure.
Which term matches this description: A local wind is an air movement produced by temperature and pressure differences over a relatively small region.
Which statement is a key takeaway from this lesson?
Practice Problems
- Explain how unequal temperature can produce wind.
- Draw and label a valley breeze without using section numbers.
- Compare the density of rising daytime air and descending night air.
- Predict where smoke from a small valley fire may move during a sunny afternoon.
Key Takeaways
• Uneven heating creates density and pressure differences. • Air moves horizontally from higher to lower pressure. • Valley breezes move upslope during the day. • Mountain breezes move downslope at night.