Why is Portland Considered a Mediterranean Climate?

There’s a common misconception that Portland is a temperate rainforest, but the reality is that we’re located in a Mediterranean climate! Understanding the difference between these two climates is crucial for understanding the best gardening practices for our region in addition to the environmental threats that we face.

Köppen Climate Classification System

The first thing to know about Earth’s climates is that German-Russian climatologist Wladimir Köppen (1846–1940) developed a system to classify them into categories based on temperature and precipitation patterns. Later, in the mid-20th century, German climatologist Rudolf Geiger (1894–1981) made changes to the system to modernize it a bit. The system is divided into 5 main categories: A (tropical), B (arid), C (temperate), D (continental), and E (polar). Beyond this, there are two more levels of classification that divide precipitation patterns like dry summer climates from wet summer climates, and warm climates from cold ones. Check out this interactive globe map that shows climate types changing over time.

Köppen Climate Classification System table

Source: sos.noaa.gov

What is a Mediterranean Climate?

Mediterranean climates are some of the rarest and most unique climates in the world. What makes them so interesting is that they are classified by having a wet winter with no summer rain–or “summer dry” under the Köppen Classification System. The regions that have very little to no precipitation during their summer months are:

  • West coast of the USA
  • Parts of Chile
  • The Western Cape Province of South Africa
  • The Mediterranean
  • South western Australia
  • The western coast of South Australia
Map of Mediterranean climates around the world

Source: https://www.pacificbulbsociety.org/pbswiki/index.php/DrySummerClimates

How are Mediterranean Climates Formed?

This climatic unicorn is created when subtropical air moves over the oceans and over these coastal regions, which creates drier, hotter, and clearer summers. This air moves toward the equator during colder months and brings winter rain and mild temperatures similar to those you’d see on the coast. Usually these Mediterranean regions are adjacent to large, north-south mountain ranges such as the Andes, the Cascades, and the Outeniqua, which block the winter rain creating a rain shadow.

Dry Summers and Wet Winters

An important note here is that while Sicily and Portland have the same climate, the difference lies in how much precipitation and how cold each location gets. The Pacific Bulb Society created a helpful chart to compare different dry summer climates:

Comparison chart of Mediterranean and dry summer climate cities

Source: https://www.pacificbulbsociety.org/

Please note that since this was published, the 2023 USDA Plant Hardiness Zone Map went live and shows that most parts of the United States have warmed by 5 degrees Fahrenheit.

Pacific Northwest Climate Map

While Portland, Seattle, and various other parts of the United States west coast are considered Mediterranean, other parts do not fall into this category. Some coastal regions, like southern California, have cool summers with fog due to cold ocean air currents moving warm air inland. The western portion of the Olympic Peninsula of Washington is not considered a Mediterranean climate because of its mild, consistent, and oceanic weather, which is why it’s home to true temperate rainforests.

Map of the Pacific Northwest that depicts where Mediterranean climate conditions are present

Source: https://www.pacificbulbsociety.org/pbswiki/index.php/DrySummerClimates

Oregon Climate Changes

As we see Earth’s average temperature slowly rise, we’re seeing dramatically drier and hotter summers in Portland, Oregon. What was once a more mild summer dry Mediterranean climate, is now becoming more intense and more similar to the climate of northern California. But it’s not just happening to our Mediterranean region, west of the Cascade Mountains. Our semi-arid climate neighbors in eastern Oregon and Washington are also experiencing hotter and drier conditions.

Mediterranean Climate-adapted Planting

Like the rest of the west coast, Oregon struggles with major drought. But one of the best ways to combat drought in western states is to minimize water usage in your garden. Shifting parts of your garden—or your entire plant list—to water-wise plantings will not only help you reduce your water bill, but it will also minimize water usage during the driest months of the year.

Accessible Climate Education for Oregon and Beyond

Here at Portland Botanical Gardens, we are passionate about providing accessible education around climate change, conservation, habitat restoration, and of course botany and horticulture. Part of our vision is to create gardens that teach the principles, feasibility, and importance of climate-adapted gardening with water-wise plantings. These will be located throughout the garden as part of our Geographic Gardens and Demonstration Gardens plans. 

Explore the PBG Vision

To learn more about what we hope to achieve from a sustainability perspective, check out our proposed Green Initiatives. As always, these are visionary plans, but as the Pacific Northwest becomes drier and hotter, it’s our goal to help residents and regional organizations convert to more water-wise plant lists! Portland Botanical Gardens is a collaborative and collective effort focused on lifting all boats, so if you’re someone passionate about our vision and what we stand for, give us a follow and spread the word!

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