Your street decides when you get your first freeze — if you’re in a city, you can expect it as much as a month later than in the suburbs.

The first freeze of fall isn't only a matter of how far north you live. It's also a matter of how much concrete is next to you.

Here’s how it works.

Examples of urban vs. suburban first frost dates

For some real life examples, we can take a look at NOAA's 1991–2020 data for urban and suburban points near two locations: Washington, D.C., and New York City.

Typical first frost dates:

• Washington urban: Washington Reagan Airport Nov. 18
• Washington suburban: Washington DC Dulles Airport Oct. 24
• New York urban: Central Park Tower Nov. 20
• New York suburban: Essex Fells Service Building Oct. 24

And this doesn’t just apply to the first frost in fall, we see the effect mirrored in spring with the last frost, too.

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Typical last frost dates:

• Washington urban: Washington Reagan Airport March 24
• Washington suburban: Washington D.C. Dulles Airport April 15
• New York urban: Central Park Tower March 30
• New York suburban: Essex Fells Service Building April 18

(MORE: Frost versus freeze: What the difference means for you)

In Washington, D.C., the western suburb of Dulles sees a gap of almost a month between the first frost there and downtown — neighborhoods barely 25 miles apart.

New York shows the same pattern. Central Park’s average first frost lands about four weeks later than Essex Fells, a suburb in nearby Essex County, New Jersey.

How does this happen?

That's the city's urban heat island at work.

Concrete, steel and water absorb heat all day, then release it slowly overnight, so built-up areas stay a few degrees warmer after dark than the open land around them. Tall buildings and narrow streets add to the effect by blocking heat from escaping upward.

(MORE: Fall's first freeze: Here's when it arrives in your area)

There’s also typically less plant life in urban areas too, so they benefit less from “nature’s air conditioner,” transpiration. That's the process of plants taking in water through their roots before it eventually is released into the air.

Urban building materials, like asphalt and concrete, are often darker in color, absorbing heat, rather than reflecting it. Many of these materials are also impervious surfaces, meaning water can’t flow through them. Without a cycle of flowing and evaporating water, these surfaces have nothing to cool them down.

What does this mean for you?

That means "first freeze" is a lot more localized than you might have initially thought since it’s not a blanket date for an entire metro area. It depends on how much pavement and steel surround a specific spot. 

(MORE: Should you protect tomato plants from frost? Here's what gardening experts say)

That's worth knowing because the first freeze is the marker a lot of fall planning hangs on — when to bring plants inside, when a frost advisory becomes a freeze warning, when to get the furnace checked.

A homeowner in the suburbs may need to act weeks before a neighbor near the city center does.

(MORE: Should you water your garden before a freeze? Here's what the pros say)

And yes, this effect is applicable year round — meaning urban areas will be a bit hotter than suburban ones in summer, too.

Content writer Ada Wood enjoys exploring the stories that science and climate teach us about our natural world and how it influences the way we live in it.