Understanding Weather and Climate The Atmosphere (Cont.)

The Atmosphere Atmospheric pressure is produced by molecules hitting a surface. Since more molecules are found near the surface of the Earth, the pressure is highest there. Higher up, there are fewer and fewer molecules present, and so pressure is less and less. As we change our altitude, or height, we respond to the change in pressure. Our ears seem to “pop” because our inner ear is adjusting to the change in atmospheric pressure.

Atmospheric pressure can be measured by using a barometer. The weight of the mercury in the tube is balanced by the force of the air on the mercury in the dish. The mercury will rise or fall depending on the pressure. When a big storm is approaching, the mercury will drop significantly. After the storm has passed, the mercury will rise again.

The Greenhouse Effect Our atmosphere is warmed in a way that it is similar to the inside of a greenhouse. Sunlight that reaches the surface of the Earth is absorbed. The energy that is released back into the atmosphere is then absorbed by water and carbon dioxide in the atmosphere. The warmth is “trapped” in the lower atmosphere like the warmth “trapped” inside a greenhouse. This is why our atmosphere is said to have the greenhouse effect.

The Greenhouse Effect (Cont.)

Air that moves horizontally is called wind. Air that rises is an updraft. Air that sinks is a downdraft. Unequal heating and cooling of air often makes a pattern of rising air, sinking air, and winds called a convection cell. A convection cell is a part of the atmosphere where air moves in a circular pattern because of unequal heating and cooling.

Convection Systems The air around you is constantly moving. A strong storm can make trees seem to dance in the air. This movement is called wind. Have you ever noticed that basements seem colder than the rest of the house? This is because cold air moves downward. Warm air rises, so the highest part of your house will always be the warmest.

Convection Systems (Cont.) Air expands as it is warmed. In other words, the molecules scatter and become active. Cooler air does the opposite. It becomes denser and less active, therefore losing energy. When cool air meets warm air, the denser, cool air will flow beneath the warm air, forcing the warm air upward.

When the Earth absorbs the sun’s radiation, some parts of the Earth get hotter than others. Some places, like city streets, absorb a lot of energy. You can see the radiation coming up from the street on a hot day. On the other hand, other places do not warm up nearly as much. Forests remain relatively cool when compared to the city streets.

As the cooler, denser air moves from an area of high pressure to low pressure (the forests to the streets), it pushes the warm, less dense air upward. This process of warmer air being pushed upward is called convection.

Convection Cell When the warmer air rises and circles back down on the cooler surface, a convection cell has formed. This movement of air in a convection cell creates wind.

Daytime sea-breeze conditions One side of the room has a heater; on the opposite wall is a window. On a cold winter day, when the heat is on, air near the heater will warm up. What happens to hot air? It expands, becomes less dense, and rises. On reaching the ceiling, it is pushed along by more hot air rising behind it. The heated air starts to cool down the farther it drifts from the heater, and this process is speeded up when it meets the cold window. As the air cools, it becomes more dense, sinks to the floor, and eventually completes a circuit of the room. A circular convection current is set up. Circular currents like this are called convection cells. Daytime sea-breeze conditions During the day, land gains heat energy faster and becomes warmer than the water offshore. Because warm air is less dense, it rises and triggers an onshore flow of cooler marine air to replace the rising warm air—the flow is usually strongest in the afternoon.

At night, inland areas cool (radiate heat energy) faster than offshore waters. As a result, the cooler air over the land subsides and flows offshore over the warmer water, where the air is lifted. This night pattern reverses the process that develop during the day.

Sea and Land Breezes A wind that blows from the sea toward the land is called a sea breeze. A land breeze blows from land toward the water.

Convection Cells Also Occur Along Mountains As the sun shines on a mountain during the day, the slope heats up faster than the valley below. Air over the slope warms and rises.

Daytime Valley Breeze Cooler air over the valley replaces the rising warm air, creating a valley breeze that blows up the slope.

Nighttime Mountain Breeze At night the mountain slope cools rapidly. This causes a mountain breeze to blow down the slope.

Developing Vocabulary Sea breeze – The name of the breeze indicates the direction the wind is coming from: the wind blows from the sea. Land breeze – The wind blows from the land. Can you describe a situation when you have experienced breezes from the land or sea?

Remaining Classtime: Read pages D54 through D63. THE END!!!