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Exploring why two cities at the same latitude can have completely different climates.
Why is it that Seattle is famous for its rain, while just a few hours east, the land is a dry, dusty desert? The answer isn't in the clouds—it's in the ground beneath your feet.
Have you ever noticed that high mountain peaks are covered in snow even in the middle of summer? This happens because of air density and pressure. As you move higher into the atmosphere, there are fewer air molecules pushing down on you. When air rises, it expands because there is less pressure. This expansion causes the air to lose energy and cool down. On average, for every meters you climb, the temperature drops by about . This is known as the environmental lapse rate. This is why a city at the base of a mountain might be wearing t-shirts while the hikers at the summit are wearing parkas.
Let's calculate the temperature at a mountain peak. 1. Base Elevation: meters (Sea Level) 2. Base Temperature: 3. Peak Elevation: meters 4. Calculation: Since it drops per m, the total drop is . 5. Final Temperature: .
Quick Check
If you climb a mountain, does the air pressure increase or decrease, and how does that affect the temperature?
Answer
Air pressure decreases as you climb, causing the air to expand and the temperature to drop.
Mountains act like giant barriers for wind. When moist air from the ocean hits a mountain, it is forced upward. This side is called the windward side. As the air rises, it cools, water vapor condenses into clouds, and it rains. By the time the air crosses the peak, it has lost most of its moisture. As it slides down the other side—the leeward side—the air compresses and warms up. This warm, dry air soaks up moisture from the ground like a sponge, creating a dry area called a rain shadow. This is why one side of a mountain can be a lush forest while the other is a desert.
Consider Washington State: 1. Moist air blows in from the Pacific Ocean. 2. It hits the Cascade Mountains (Windward side). 3. Cities like Seattle receive heavy rainfall ( cm/year). 4. The air crosses the peaks and sinks. 5. Cities like Yakima on the leeward side receive very little rain ( cm/year), creating a semi-arid climate.
Quick Check
Which side of a mountain range would be the best place to start a farm that relies on natural rainfall?
Answer
The windward side, because that is where the rising air cools and releases precipitation.
Water is much slower to change temperature than land. This is due to specific heat capacity. During the summer, the ocean stays cool, acting like a giant air conditioner for coastal cities. In the winter, the ocean holds onto the summer's heat, acting like a heater. This is called maritime moderation. Inland cities, far from the water, have a continental climate. Without the ocean to balance them out, they experience 'extreme' seasons: scorching hot summers and freezing winters. The closer you are to a large body of water, the smaller your annual temperature range will be.
Both cities are at similar latitudes (): 1. San Francisco (Coastal): Summer high , Winter low . Range = . 2. Kansas City (Inland): Summer high , Winter low . Range = . 3. Result: The ocean reduces the temperature swing in San Francisco by compared to the heart of the continent.
What happens to a parcel of air as it is forced up the windward side of a mountain?
If a coastal city has a summer temperature of , an inland city at the same latitude is most likely to have a summer temperature of:
The 'Rain Shadow' is located on the windward side of a mountain range.
Review Tomorrow
Tomorrow morning, try to explain to a friend why mountain climbers need oxygen and heavy coats, using the concepts of air density and the lapse rate.
Practice Activity
Open a map and find a mountain range near an ocean (like the Andes or the Himalayas). Look at satellite images to see if you can spot the green windward side and the brown leeward side!