Imagine that you’re standing in a field on the plains of Western Russia,
and by some strange quirk of fate, you’ve had to stand there over a whole year. You’ll have
witnessed snowy winters, fresh springs, warm summers and wet autumns. Incredibly
if you had chosen a spot 1,300km west, on the borders of Germany and Poland, or 2,700km east,
deep into the Central Asian Steppe, then your experience wouldn’t have been hugely different.
This is because all these places, spanning 4,000km from east to west
are in the same climate zone – the warm summer continental climate.
If one were to travel from the Atlantic coasts of Mauretania, across the Sahara, the Nile Valley,
Arabia, Southern Iran to arrive at the estuary of the Indus River at Karachi in Pakistan
one would have experienced every day on that 8,000km journey to be more or less the same,
since this whole region is within the Hot Desert climate zone.
The fact that one can experience the same climate across thousands of continuous kilometres
makes their opposites even more incredible. Now imagine that you are on a cool foggy coast
of California, and that you get in your car, and drive for about 20 minutes over a set of coastal
hills, and into bright sun. It’s only when you get out of the car that you are amazed to feel
the hot breeze blowing over your face. In just a few miles the temperature climbed 14 degrees.
How is this sudden change possible? What other areas of earth see dramatic shifts in temperature,
sunshine and rainfall in only a few short miles? And in what other ways? Well, the answers lie
in the subject of Microclimates, the latest case of my Climate Casebook.
A microclimate is defined as a noticeable change in long-term weather patterns over
a short distance, typically within 20-30km.
As climate is principally defined by long term patterns of precipitation
and temperature, we are primarily concerned here with sudden changes in temperature and rainfall,
although, as we shall see, other atmospheric effects, such as fog and cloud enter in.
I have identified five essential ways in which microclimates can come about:
1. Proximity to ocean water 2. Altitude
3. Slope direction 4. Built environment
5. Plant cover Let’s look at each of these in ascending order,
starting with plant cover. Anyone who has walked from a field into in a forest on a
hot day will have felt the sudden cooling of the air, whether it be in the tropics, temperate or
subarctic regions. Forests can drop the air and ground temperatures by a considerable margin
throughout the year in the tropics, and on sunny summer days in temperate and sub-arctic regions.
This cooling is the result of the shade provided by the tree canopy and the cooling effect
of evapotranspiration of water from the trees into the air. In winter, however, the effect is
reversed, with forests being slightly warmer over a 24-hour period than the open lands outside, due
to the dark foliage absorbing more heat from the sun compared to lighter grasslands or indeed snow.
Next there is the built environment, a fancy term to describe buildings within cities.
When large areas of the surface are covered in concrete, asphalt and brick they absorb much
of the heat from the sun during the day, then radiate this out at night as they slowly cool.
This radiative effect also occurs during the day, but is not as strong.
This phenomenon is known as an Urban Heat Island,
and can result in large cities being several degrees warmer than the surrounding countryside.
And then there is slope direction. In valleys that run east-west in the Northern Hemisphere,
the northern slopes that face south receive the sun at a steeper angle
to the ground compared to the opposite slopes, and so will be significantly warmer.
This effect is noticeable in wine-growing regions of Europe, and can affect which grapes would be
grown on which slope, and at what time they would be harvested, since this crop is notoriously
sensitive to temperatures and humidity. Next, we have altitude. As I covered in my
Subtropical Highlands episode of Secrets of World Climate, air cools as altitude rises.
Anyone who has driven up a mountain road will experience this in just a 30-minute drive.
In summer the top is noticeably cooler than the bottom of such a road, while in winter, one can
experience going from a snowless terrain into one in which there are metres-thick snow-drifts
on each side of the road. In some cities built on steep slopes, such as La Paz in Bolivia,
this can result in higher parts of the city being cool enough to be in a different climate zone.
A less obvious effect of altitude occurs in certain low-lying areas surrounded by mountains,
which can produce much colder winter temperatures than is typical. Cold air is more dense than
warmer air and so if left undisturbed in such topographical conditions will sit over an area
in what is known as a Cold Air Pool. In fact the coldest winters outside of Antarctica,
experienced by the lucky town of Verkhoyansk in North-East Siberia, are enhanced by this effect.
Ok, so these effects are all valid as microclimates, but in my view, the most
noticeable and famous microclimates occur in the final category, the proximity of ocean water.
If you’ve been watching my other series on climate, then you’ll know about the moderating
effect that ocean water has on lands nearby, think of the mild climate of the British Isles,
compared the Subarctic climate of Eastern Siberia at the same latitude. But this effect
occurs over thousands of miles, and so is not in microclimate territory. But there are three ocean
effects which can occur over local distances, and we’ll look at each of these in turn.
The first is the ocean providing year-round frost-free conditions in
the Oceanic climate of Britain and other places that otherwise experience regular winter frosts.
Why is this important? Because practically all tropical, and many subtropical species
of plants have evolved in climates that experience no frost, and so if exposed to it, can die-off in
just a couple of nights. This effect occurs in several places in Britain, but only in
sheltered coastal areas, including the southern coasts of the counties of Devon and Cornwall,
and certain spots in North-West Scotland. This has led to the remarkable situation in these places
where palm trees are part of the streetscape, a thousand miles north of the “Palm Line” of
the Northern Mediterranean. The reason for this is that Britain receives the warm Gulf Stream
ocean current which is responsible in general for the mild climate at these high latitudes,
but in particular for local spots right on the ocean that are also sheltered by surrounding hills
that prevent cold winds blowing out from the interior that would otherwise bring frost.
The second type of ocean-generated microclimate occurs on tropical and subtropical islands
subject to trade winds. As mentioned in Episode 2 of my Secrets of World Climate series,
Trade Winds are the consistent winds that blow in tropical and subtropical regions, almost always
coming from one general direction. When such winds hit a mountainous island, this will have
the effect of producing orographic lift of the air on the windward side, resulting in high rainfall,
and leeching the ocean air of its moisture, such that the leeward side of the island is much drier.
If you have ever visited the Hawaiian Islands, the Canary Islands or indeed some Caribbean Islands,
you will see, quite dramatically, the effect of this on the landscape. Windward sides are lush
and forested, while leeward sides are covered in grassland, shrubs or in some cases, even desert.
My own personal experience of this was on visiting Gran Canaria in the Canaries, which
has been dubbed a continent in miniature, owing to its incredibly diverse set of climates and biomes,
despite being only 50km in diameter. On one afternoon I drove from a resort on
the south coast to the capital of Las Palmas in the north. This took only half-an-hour,
but in that time I went from a desert landscape and blistering sun over 30°C to grey skies at 20°C
and lush green landscapes. The final microclimate effect
that I’ll look at in this video is that of a cold ocean meeting hot land. This occurs
exclusively on the eastern margins of the oceans where they meet temperate westerly coastlines.
In these areas we find Mediterranean or Desert climates, both of which experience hot and dry
summers, but the ocean currents passing down these coastlines have come from polar regions and so
carry cold water. Needless to say, putting these two factors together is always going
to create interesting effects. In one of these areas, the coastline of Peru, I discussed this
in detail in the previous Casebook episode, where that city experiences frequent fog and low cloud,
and yet just a few miles inland, the air is clear, and temperatures much higher.
Now the capital of Peru, Lima is no small city, having ten million population, but in the areas of
Greater Los Angeles and the San Francisco Bay Area of California, up to 30 million souls experience
the well-known microclimates of these areas. San Francisco is famous for its rolling fogs,
and anyone who has visited that city will likely have experienced them. They are the result of
a layer of cold moist air rolling in from the Pacific, trapped below warmer drier air above.
This air soon meets the hot dry air of the Central Valley of California and dissipates.
In just a one hour drive from San Francisco to the inland city of Tracy,
the air temperature on any given summer afternoon can rise by as much as 14°C,
with the July average high in Tracy being 33°C while San Francisco is only 19°C.
An even more dramatic effect occurs around the City of Angels, where at the coast in Santa Monica
the August afternoon temperature can be 21°C with low cloud, while Santa Clarita, just a 40 minute
drive north is 14°C warmer at 35°C. The reason for an even more compressed change in temperatures is
that here we have two ridgelines along this journey that separate the cool ocean air,
known locally as the “Marine Layer” from the hot semi-desert interior of Santa Clarita.
I experienced this phenomenon personally on many occasions during my time living in this region.
And no I haven’t forgotten you, San Diego. You experience this effect too, to the degree that
local TV weather spots provide a “Microclimate Forecast” during the hot months of the year.
You Stay Classy, San Diego!
And that's microclimates. I hope you enjoyed this rapid sketch of these peculiar climatic phenomena.
If you did, then please leave a like or a comment, and I'd especially love to hear from you if you
live in an area that experiences a microclimate. Don't forget to subscribe to my channel if you
haven't already done so, and enable notifications, so you don't miss future episodes. If you'd like
to support the development of future videos, please consider becoming a Patreon supporter,
for as little as $2 per month. Thanks again for watching and I'll see you in the next video.