Climate

Weather is what the atmosphere is doing right now. Climate is the long-term pattern of weather at a place. That means its average temperature and precipitation, and just as important, how much those swing over a year. A cold snap in July does not change New York's climate, and neither does one warm winter. Climate comes from decades of data.

Two places can share the same average temperature and still have very different climates if one swings wildly between seasons and the other barely changes. This chapter is about the handful of factors that set those patterns, and every one of them comes down to location.

Nothing matters more than latitude. Near the equator the sun climbs high overhead all year and delivers intense, direct energy. That gives hot climates with very little seasonal change. Toward the poles, sunlight arrives at a low angle and spreads thin across the surface, which gives cold climates. The middle latitudes, where New York sits, get strong seasons because the sun's angle changes a lot through the year.

Latitude also places you in the planetary wind and pressure belts charted on your Reference Tables. Moist rising air at the equator makes a rainy belt. Dry sinking air near 30 degrees makes the world's deserts. New York sits in the prevailing southwest wind belt, which steers its weather in from the west and matters for every factor that follows.

Water heats up and cools down far more slowly than land. A large body of water steadies the temperature around it. So a coastal, or marine, climate has cooler summers, milder winters, and a small temperature range across the year. Deep inside a continent, far from that steadying effect, a continental climate swings hard, with hot summers, cold winters, and a big range. Compare mild coastal Seattle to Minneapolis, which sits at nearly the same latitude.

Water also feeds moisture into the air, so coastal and downwind-of-water locations tend to be wetter. New York gets a signature version. In late fall and winter, cold dry air crossing the still-warm Great Lakes loads up with moisture. It then buries the towns just east of the lakes, like Buffalo and Syracuse, in lake-effect snow.

The oceans move heat around in currents, which are mapped on your Reference Tables. A warm current flowing along a coast warms the air above it and adds moisture. A cool current chills the coast and can even dry it out. The Gulf Stream carries warm tropical Atlantic water north past our coast and on toward Europe. That keeps northwestern Europe much milder than you would expect from its latitude. The cold California Current does the opposite and cools the U.S. west coast.

Here is the rule of thumb. Currents flowing away from the equator carry warm water. Currents flowing toward the equator carry cool water.

Climbing higher has the same effect as traveling toward a pole. Air thins and cools with altitude, so higher places are colder. That is why snow-capped mountains stand right on the equator. In New York, the Adirondack high country runs several degrees colder than the lowlands around it, with a shorter growing season and heavier snow.

A mountain range in the path of the prevailing wind splits the climate in two. Air forced up the windward side expands and cools. It reaches its dew point, condenses, and drops its rain or snow on that slope. The air spilling down the other side is dry, and it warms as it sinks. That creates a rain shadow, which is a belt of dry climate behind the range. The deserts east of the Sierra Nevada sit in exactly this kind of rain shadow.

Everything so far in this chapter explains why one place is warmer or wetter than another right now. But climate also changes over time, and the longest, steadiest cycle is the ice ages. Over the last few million years, huge ice sheets have advanced and retreated across the Northern Hemisphere again and again, roughly every hundred thousand years. What triggers them is not on Earth at all. It is Earth's orbit.

Three slow orbital changes, together called the Milankovitch cycles, set the pace. The shape of Earth's orbit stretches from more circular to more elliptical and back over about 100,000 years. The tilt of Earth's axis nods between about 22 and 24.5 degrees over roughly 41,000 years. And the axis itself wobbles like a slowing top, tracing a full circle every 26,000 years. None of these changes the total sunlight Earth receives by much. What they change is how that sunlight is distributed across the seasons and latitudes.

How that sunlight is spread out is what matters for ice. An ice age does not take hold because winters get especially cold. It takes hold when northern summers are cool enough that the previous winter's snow survives instead of melting. Year after year the snow piles up and compresses into ice. Once ice covers the ground, the albedo feedback from the last unit takes over and cools things further. When the orbital cycles swing back toward warmer northern summers, the ice retreats. The orbit sets the timing, and feedbacks make the swings much larger.

Place an imaginary city by choosing its latitude, its distance from the ocean, its elevation, and its side of a mountain range. The lab describes the climate those choices build. Try moving the same city from the coast to the interior and watch the annual range stretch.

Climate is the long-term pattern of average temperature and precipitation, plus how much they range across a year. Latitude sets the baseline through sun angle and the planetary wind belts. Large bodies of water steady the temperature, so marine climates have small ranges and continental interiors have big ones. Lake-effect snow is New York's local version of that effect. Warm currents like the Gulf Stream carry heat toward the poles, and cool currents chill the coasts they run along. Higher elevation means colder air. Mountains split the climate at the crest, with wet conditions on the windward slope and a dry rain shadow on the other side.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.