Local Winds
Learning Goal 10a: Explain how the following local winds work and how they apply to sailing:
sea breezes, land breezes, katabatic winds, and coastal (barrier) jets.
Sea and Land Breezes
Fundamentals
A sea breeze is a shallow cool wind that blows onshore (from sea to land) during daytime (see figure below). It occurs in large-scale high-pressure regions of weak or calm synoptic-scale wind under mostly clear skies.
Similar flows called lake breezes form along lake shorelines, and
inland sea breezes form along boundaries between adjacent land regions
with different land-use characteristics (e.g., moist irrigated fields
of crops adjacent to drier land with less vegetation).
The sea breeze is caused by a 5 °C or greater temperature difference
between the sun-heated warm land and the cooler water. There is also a weak return flow aloft
from land to sea (see sketch below). The reason that the ocean
warms more slowly than the land during a sunny day is that sunlight is
absorbed and spread (diluted with cooler deeper water) by ocean
turbulence over several meters or more of ocean depth, but on land all
the solar heat is concentrated in the top few centimeters.
Source: R. Stull, 2017: Practical Meteorology. Used with permission.
A sea-breeze front
marks the leading edge of the advancing cool
marine air over land and behaves similarly to a weak advancing cold
front or a thunderstorm gust front. If the updraft ahead of the front
is humid enough, a line of cumulus clouds can form along the front,
which can grow into a line of thunderstorms if the atmosphere is
statically unstable. Kelvin-Helmholtz waves (KHW) can form at the
interface between the cool onshore sea breeze and the warm return flow
aloft. The sea-breeze front can advance 10 to 200 km inland by
the end of
the day, although typical advances are 20 to 60 km unless inhibited by
mountains or by opposing synoptic-scale winds. Even without mountain
barriers, the sea breeze will eventually turn away from its advance due
to Coriolis force.
As the cool marine air flows over the land, a thermal internal
boundary layer (TIBL) forms just above the ground (see figure above).
The TIBL is a region of warmed air that grows in depth with
increasing distance from the shore as the marine air is heated by the
underlying warm ground
In early morning, the sea-breeze circulation does not extend very
far from the coast, but spreads out further over land and over water as the
day progresses. When fully developed, near-surface wind speeds in the marine, inflow portion of the sea breeze at
the coast are 4 to 40 km/h (=1 to 10 m/s) with typical values of about
22 km/h (=6 m/s).
At the end of the day, the sea-breeze circulation dissipates and a weaker, reverse circulation called the land-breeze
forms in response to the nighttime infrared cooling of the land surface
relative to the sea (assuming clear skies). The reason is that
all of the heat loss at night is concentrated in the top few cm of soil
over land causing the land surface to get relatively cold, but in the ocean the cooling is spread over tens of meters at night.
In the land breeze, low altitude winds flow from land toward ocean, and
the return flow aloft is from the ocean toward land.
In the vertical cross section normal to the coastline (as in the
figure above), the surface wind oscillates back and forth between
onshore (coming from the sea during the sea breeze) and offshore (coming from the land as a land breeze),
reversing directions during the morning and evening hours. The Coriolis
effect causes the horizontal wind direction to rotate throughout the
course of the day. Rotation is clockwise in the northern hemisphere and
counterclockwise in the southern hemisphere. The idealized sea-breeze
hodograph has an elliptical shape (see Figure below). For example,
along a meridional coastline with the ocean to the west in the Northern
Hemisphere, the diurnal component of the surface wind tends to be
westerly (onshore) during the mid-day, northerly (alongshore) during
the evening, easterly near midnight, and southerly (alongshore) near
sunrise.
Source: R. Stull, 2017: Practical Meteorology. Used with permission.
The sea-breeze wind and the mean (24 h average) synoptic-scale
surface wind are additive. If the synoptic-scale wind (i.e., wind
driven by larger-scale pressure patterns like Lows and Highs) in the
above example is blowing from the north, the surface wind speed will
tend to be higher around sunset when the mean wind and the diurnal
component are in the same direction, than around sunrise when they
oppose each other.
Sailing Tips
- Sea breeze is strongest during clear sunny daytimes under High
pressure. Under these conditions, the sea breeze is strongest
mid-day through mid-afternoon local time. Caution, the sea-breeze can die in the late evening BEFORE the sun sets, so plan ahead to return to port before the winds die.
- In synoptic high-pressure regions where the synoptic-scale winds are
light to calm (bad sailing), you can still get enough sea-breeze near
the coastline to have winds for good sailing.
- The surface winds (white arrows in figure below) caused by the
sea breeze (cyan color) are in a band (outlined with dashed purple
line) parallel to the coastline. This band of good sailing is
roughly 10 to 50 km wide, depending on local conditions.
- The sea-breeze is strongest at the coastline and is weaker the further away from shore. Caution, beware
of shallow waters and rocks too close to shore. But don't venture
too
far from shore where there is no sea breeze. So you need to find
a
compromise sailing distance from shore, roughly within the purple
region outlined in the figure below. (You can anticipate similar
good sea-breeze sailing regions parallel to other coastlines.)
Note for non-sailors. Modern sailboats can travel in almost
any direction relative to the wind, except they cannot head directly
into the wind (see
Sailing Basics).
Thus, for the image below with sea-breeze winds (white arrows)
perpendicular to the coastline, sailboats can easily sail parallel to
the coastline if they wish. Thus, the long region of sea-breeze
winds outlined in the purple dashed line below means that sailboats
within that region can travel long distances parallel to the coastline
when a sea-breeze exists.
Source: Google maps, annotated by R. Stull.
Dashed purple area outlines the region of good near-surface sea-breeze
winds (white arrows) for sailing along the west coast of Vancouver
Island in summer. The cyan rectangle shows the vertical
circulation of the sea-breeze, including the return flow aloft.
But the actual wind directions of the white arrows will change during
the day, as suggested in fig 17.14.
Many coasts have complex shaped coastlines with bays or mountains,
resulting in a myriad of interactions between local flows that distort
the sea breeze and create regions of enhanced convergence and
divergence. The sea breeze can also interact with boundary-layer
thermals, and urban circulations, causing complex air flows near the
shore. If the onshore synoptic-scale (large-scale) wind is too strong,
only a TIBL develops of air warming with increasing distance from the
coastline (see first figure above) with no sea-breeze circulation.
In regions such as the west coast of the Americas, where major mountain
ranges lie within a few hundred kilometers of the coast, sea breezes
and terrain-induced winds (such as katabatic winds, described next)
appear in combination.
Remember, many winds are named by the direction FROM which they blow,
e.g. easterly winds blow from the east. So, sea breezes blow from sea
to land, while land breezes blow from land to sea.
Katabatic Winds
When mountain slopes
experience surface cooling during cloud-free nights (due to infrared
radiation to space), the surface air touching these cold mountain
slopes becomes cooler
than the air at the same elevation away from the mountain. As the
air cools, it becomes denser and sinks due to gravity (buoyancy),
creating katabatic
winds (recall Learning
Goals 6a and 6b).
The speed at which these cold winds fall downhill can vary from
hurricane force (near the long steep slopes at Greenland and Antarctic
coastlines) to a light breeze. Katabatic winds are typical of
night-time surface cooling mountainous regions, or can develop at any
time of day over bodies of ice or snow on mountain slopes.
Source: R. Stull, 2017: Practical Meteorology. Used with permission.
If mountain slopes are adjacent to coast lines, then the cold
downslope winds from the mountain will descend to the sea surface,
where the cold air will spread out and cause a cold wind blowing
offshore (away from the mountains).
If the cold downslope winds flow into a fjord, the cold air can pool
and become stagnant, causing bad air pollution for towns in that fjord,
or can cause ice fog, depending on conditions.
If the valley floor has some slope to it, then the cold air can flow as a mountain wind
down the valley floor in the same direction that streams of water would
flow. If these valleys open into coastlines, then the cold
mountain winds can "gush" out over the coastal waters in narrow regions
from these valley mouths. Such an outflow wind can affect shipping along the coast - - outflow winds are discussed as Learning Goal 10b.
Synoptic winds (meaning the larger-scale winds in the region) and pressure gradients can also influence
katabatic winds to determine the speed and direction in which these
winds will travel (see Katabatic Winds of Greenland video below).
In the image below, you can clearly see the cold air falling off the
ice shelf and creating a local wind in the Bellingshausen Sea,
Antarctica.
Source: fruchtzwerg's world - Flickr, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=3735232
Sailing Tips
- As katabatic winds are mostly a nighttime phenomenon that occurs
very close to the coastline, hopefully few sailors are out at night
dangerously close to shore. But if you are in a larger ship with
radar and detailed charts, just be aware that you could experience
these cold outflow winds every time you pass in front of a valley mouth
or associated inlet.
Coastally-trapped low-level (barrier) jet
Sometimes, a Pacific Low with a cold front hits the Coast Mountains of British Columbia
from the west. This creates a shallow layer of cold air trapped
against the west side of Coast Mountains. It also leaves lower
pressure along the north coast, and higher pressure along the south
coast.
Source: R. Stull, 2017: Practical Meteorology. Used with permission.
The resulting pressure gradient (change of pressure with distance)
along the coast creates a low-altitude jet of fast-moving cold air
blowing from south to north over the coast (just west of the
mountains). Similar low-altitude jets of air can occur near other
mountainous coastlines, such as portions of the west coast of the USA.
Source: R. Stull, 2017: Practical Meteorology. Used with permission.
Sailing Tips
- If you are lucky enough to be sailing northbound when such a
low-altitude wind sets up, then you can have a nice brisk
tailwind. Caution,
some of these coastally trapped low-level jets can be associated with
clouds and rain, while others are associated with fog and low stratus
clouds.
Additional Resources: (non-required material)
UIUC ww2010 guides: http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/sea/htg.rxml
MBNMS Climate and Meteorology: http://montereybay.noaa.gov/sitechar/clim2.html
Videos: (non-required material)
Animation of sea breeze: http://www.classzone.com/books/earth_science/terc/content/visualizations/es1903/es1903page01.cfm
Katabatic Winds of Greenland: https://www.youtube.com/watch?v=pHYb36LzxnI