Bumps and hollows in the landscape have their own microclimate

As I mentioned above, a group of rocks that provides shelter can allow a pocket of still air to form on an exposed mountain slope. Small bowl-shaped hollows in the landscape, a few meters or even just a few centimeters across, can also act as solar energy collectors (like a parabolic satellite dish which concentrates the signal into the middle), gathering heat into the center to give a warmer microclimate. In tundra—the grassy or shrubby vegetation which exists in very cold Arctic and alpine environments (Chapter 2)—the extra heat concentrated in small hollows in the landscape is crucial to the growth of certain plants, and the survival of certain species of insects. This sort of heat-concentrating effect is also very common among the bowl-shaped hollows in coastal sand dunes at lower latitudes too; temperatures can be many degrees higher on a sunny day in the hollow between several dunes than on the tops of the dunes. In an interesting variant of this heat-gathering effect, the white flowers of the "Arctic rose'' Dryas (a widespread plant of the Arctic) also act as parabolic heat collectors concentrating light into the center of the flower. This warms up the center of the flower increasing the chances that the pollen will grow and fertilize the seeds. It also apparently warms up the bees that visit the flowers, speeding up their activity and helping them to carry pollen between the plants more efficiently.

Where there is a hollow in the landscape (caused by a small valley, or geological features such as a kettle hole or sink hole), it may have warmer sunny days because of the concentration of the sun's heat into the center, sheltered from the breeze. However, it can also have more severe winters. I lived for a while in the Appalachian fold country of east Tennessee, where small, cosy farmed valleys known as "hollers" make up some of the most beautiful countryside I have seen anywhere. My house stood perched on the somewhat cooler sloping side of a holler, and I remember how the short walk down the track to the center of the valley on a summer's day could seem like entering a furnace.

The frosts at the bottom of the holler were also more extreme; the old lady whose house was on the valley floor bemoaned the fact that her tender spring vegetables and flowers would often be hit by late frosts, while those in other people's gardens a few yards higher on the slopes survived intact. In a study of the landscape of part of the Appalachian fold country in Virginia, the date of the last frost in spring was almost a month later in small valleys that formed "frost hollows'', compared with nearby areas up on the ridges. Frost hollows occur because the cold air that forms at the ground surface on the valley slopes and ridge tops during a cold night (as the ground loses infra-red radiation to space) is heavy and drains downslope as a fluid. As it enters a valley bottom, the cold air also tends to pool up to a certain level, producing a sharp transition between frosty air below and the warmer air above (Figure 4.9). One can sometimes see a "burn level'' in small valleys on leafing-out deciduous trees or ferns where frosty air accumulated in a hollow, like water filling a pond; I have seen the

Figure 4.9.

Temperature profile against height on a cold spring morning in a Pennsylvania valley that acts as a frost hollow. Sub-freezing temperatures are only present in the lowermost parts of the valley. Source: Bonan.

i

*

• *

*

4 ♦

T=-2."

'4+0.05

4 E

*

♦ •

♦ ♦

*

*

/

i

>♦♦ *

*

*

20 40 60 80 100 120 140 Relative Elevation (meters)

160 180 200

20 40 60 80 100 120 140 Relative Elevation (meters)

160 180 200

transition from no damage at all to every leaf killed in less than 50 cm vertically, all around the edge of a small valley.

Drainage of cold air also leads to more transient patterns in microclimate. On bare Mediterranean hillsides after a hot sunny day, the air near the top of a hill begins to cool after the sun goes down. Being denser it drains downslope, often forming invisible "rivers" that flow down dry stream valleys and along goat paths. Walking through the garrigue scrub in the evening one often passes through these cool rivers of air and then steps back into warm air within the space of few meters. Local people sometimes explain these patches of air as being the spirits of the dead that wander the hills, chilling or warming the people they encounter during their journeys.

Such microclimate-scale differences brought about by cold air drainage are a miniature version of the same process that can occur in large valleys, producing the mid-elevation warm belt mentioned in Chapter 1. Often, the only distinction between microclimates and mesoclimates is a matter of scale, not the fundamental processes involved.

Was this article helpful?

+1 0
Solar Power Sensation V2

Solar Power Sensation V2

This is a product all about solar power. Within this product you will get 24 videos, 5 guides, reviews and much more. This product is great for affiliate marketers who is trying to market products all about alternative energy.

Get My Free Ebook


Post a comment