MICROCLIMATE
1. GLOBAL VEGETATION • Global climatic regions relate to: • Latitude • Continental location (E
or W) • Regional climates relate more to: • Altitude • Ocean currents, winds • Distance from
sea MICROCLIMATE DESERT RAINFOREST DESERT Colder - higher altitude, polar, and
more continental. FOR INSTANCE Rainforest is close to the equator Deserts are along the
tropics and in the interior of major continents Ice is at high latitudes NW Europe temperatures
in January Warmer - Gulf Stream takes warm water polewards Warmer - southerly, lower
altitude, oceanic, Warmer - southerly, oceanic
2. On a smaller scale, weather and climate is affected by smaller scale variations in: •
Topography (relief) • Albedo • Aspect • Urban Areas • Vegetation • Moisture and humidity •
Pollution, human activity Farmers alter albedo by plastic sheeting. Bare earth gains more
radiant heat, but loses more heat through evaporation loss and wind at night. Prevention of
frost at night can be crucial to early growth. MICROCLIMATE (2) Low lying valleys and
hollows collect cold and humid air (Frost Hollow); hilltops are exposed to wind; south-facing
slopes (in Europe) are warmer, with longer days, effectively, than north-facing slopes
(Aspect). East or west aspect may affect rainfall or snow coverThis in turn may affect
vegetation, humidity, evaporation rates Built-up areas are 2/3°C warmer than rural areas,
especially at night. This is an Urban Heat Island. The Greenhouse Effect due to human
pollution is not intentional...
3. FROST HOLLOW 1 In hollows, humidity is often high (rivers, estuaries, marsh land) and
towns (usually on lower land) increase air pollution. Both tend to make fog or cloud more
likely. Smog (smoke - fog) occur with bad pollution (as in Athens, LA, Mexico City, pre-war
London) Low-lying cloud in valleys seems like fog at ground level 2 The Frost Hollow effect
tends to operate when the ground surface cools, usually overnight when cloud cover is limited.
It is common in mountains where snow and ice cover maintain cold surfaces for long periods,
reflecting insolation and may cause cold winds down slope (eg Mistral in S.France). 3 4
4. Thermal imaging of Atlanta shows the correlation of warmer temperatures and the built
up area. The centre is warmest, outside the city is coolest. Bodies of water help reduce the
effect. The location of the CBD and tarmac roads may be clearly seen. The effect is to warm
major urban areas by 2-3°C by day and night more than rural areas. URBAN HEAT ISLAND
Thermal images of Atlanta show radiant energy being absorbed during the day (above) and
retained during the day (below). The roads can be seen as tarmac absorbs radiation most
effectively. ATLANTA’s heat island
5. RUSH HOUR TRAFFIC THROUGH A HAZE OF FUMES MOTOR EXHAUSTS DARK AND
DRY TARMAC SURFACES DOMESTIC HEATING FACTORY & OTHER POLLUTION SMOG
RESULTS FROM POLLUTION URBAN HEAT ISLAND - REASONS Human heat sources
(domestic heating, cars, factories) all warm the air. Pollution by exhausts, factories and other
dusts absorb radiation and prevent heat loss during the night. Dark surfaces have a low
albedo. Dry surfaces reduce latent heat loss by evaporation In humid conditions, this may
result in smog (a mixture of fog and smoke) which was common in pre-war London and still is
in LA, Rome, Athens, Mexico City etc where surrounding hills prevent the escape of polluted
air.
6. Ice is common on exposed dark surfaces, as they lose heat rapidly overnight. Black ice is
a hazard on roads.and pavements A permanent haze hangs over Mexico City Denver’S
‘Brown cloud’ Clouds above Tripoli aided by fires URBAN CLIMATES The albedo of various
surfaces in urban areas tends to be different to rural areas; tarmac is dark; glass is lighter.
Reduced snow and ice cover reduces albedo. Warmer cities reduce snow cover and frost
frequency, advancing plant growth. Cities designed on the grid system channel any wind
along streets that contiue for many kms (wind canyons). Other cities reduce wind speed by
ground level friction. Increased pollution by traffic and other combustion tends to reduce
sunshine, espcially in winter when the sun is at a low angle, passing through ,more
atmosphere. Air pollutants increase condensation and cloud development and so rainfall
intensity and amount.
7. Temperature decreases with height by 0.6°C per 100m. This can result in permanent
snowcaps on mountains above forests where snow is seldom seen (here, on Cotopaxi
volcano in Mexico near the equator, the snowline is at 5000m). A hill farm in Snowdonia An
orchard on a slope below woodland in Devon ALTITUDE • In Britain, upland areas such as
Snowdonia which range from 0-1000m above sea level, produce climates ranging from
temperate maritime to the almost Arctic. • Lower temperatures cause greater soil saturation;
higher altitude also tends to increase precipitation (and making it more likely to be snow rather
than rain). At t higher altitudes, the growing season is shorter, frosts are more frequent and
harder while winters are longer. • Agriculture is strongly affected. Some arable crops are
possible at low level, on valley floors. Higher up, pasture becomes is enclosed. Above this,
open moorland is used for sheep to roam, but is covered mainly with heather and other hardy
plants. Besides temperature, wind speed, evaporation rates, and humidity are also affected. •
Elsewhere, sensitive crops (fruit orchards, right - or vineyards) can only exist below the cold
and windy upper slopes due to frost frequency in early spring. The lowest points may also be
unsuitable due to the frost hollow effect.
8. In the northern hemisphere, a southerly aspect gives effectively a higher angle of sun in
the sky, and longer days. • In the southern hemisphere, a northerly aspect is warmer • The
growing season is longer (by about a month for each 1°C higher in annual average
temperature), • Frosts are less frequent, less severe • Maximum temperatures are higher.
Sensitive crops may nly be possible on south-facing slopes in Europe (eg vines below); the
opposing slope is pasture alone. VINES ASPECT Isolated snow patches are likely to remain
in spring on north facing slopes (in Britain) where the sun takes longer to melt the snow. The
right hand slope (above) is facing the sun, keeping it free of snow for longer. In some arid
environments, shade is important, reducing temperature, humidity and evaporation rates.
Shaded areas, especially if north facing, remain damper with reduced temperatures,
evaporation and humidity. This also affects vegetation, soil moisture which may, in turn, affect
frosts and temperature variations
9. Trees reduce temperature during the day, but retain heat during the night. Temperatures
are thus more even (less extreme) • Wind speed is reduced • Evaporation are lower,
especially in the day but also at night. Locally, air becomes saturated (and is not blown away)
due to transpiration; this reduces evaporation. • Humidity levels remain high and constant due
to transpiration and low evaporation rates. Mosses are common on the forest floor
VEGETATION - WOODLAND Shade can be welcome in the desert, but on the forest floor, the
lack of sunlight is a serious deterrent to other plants. Thick undergrowth occurs only in
clearings or where old trees fall British forest floors with moss and marsh at ground level
Rainforest transpiration also increases cloud and rainfall To reduce windspeed in orchards
(evaporation, frost and blossom loss) windbreaks are planted. They may reduce soil loss in
arable fields.
10. Other plants (freshwater reeds, right) may also reduce windspeed or water current, retain
sediment and allow other vegetation to colonise. VEGETATION - OTHER PLANTS •
Vegetation on sand (marram grass, above) not only anchors the moving sand with its roots
but also: • Reduces wind speed which stabilises sand (ripples show wind) • Increases
humidity locally (cms) • Keeps temperature more even • Reduces frosts, evaporation. • Dune
systems grow as a result Temperatures are more extreme where vegetation is absent; ice
forms (left) on a bare rock surface due to rapid radiation loss overnight. By reducing light
penetration to the forest floor (right) , trees are prevent the growth of competing species.
AR203 TROPICAL DESIGN MODULE 4:SUN PATH
Topics
Introduction to the sun's radiation and the Annual Sun Path Diagram
Interpretation and analysis of the Sun Path Diagram
Computation of the Sun Angles at different Times and Dates
Generating Computer Modelling and the likes of the Sun Path Diagram
SOLAR RADIATION
Solar radiation is the radiant energy emitted by the sun from nuclear fusion reaction that
creates electromagnetic energy.
The spectrum of solar radiation is close to that of a black body with a temperature of about
5800 K. About half of the radiation is in the visible short-wave part of the electromagnetic
spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of
the spectrum.
Solar radiation as a climatic parameter
Solar radiation is the main driver of climate, since it influences temperature and gives rise to
regional winds.
The temperature at a given latitude depends on the angle of incidence of solar rays to the
ground:
•
•
lt is highest at the equator and lowest at the poles.
The higher the angle of incidence (and thus the lower the latitude) the more energy reaches
the ground and the higher the air temperature.
SOLAR RADIATION
The Earth's orbit around the Sun is an elliptical path which causes the Earth's
distance from the to vary over a year. The variation in the distance from the sun causes the
amount of solar radiation received by the Earth to vary by 6% annually.
EFFECTS OF SUN ON THE CLIMATE
The temperature of the air as well as that of the land is mainly a result of the amount of solar
radiation absorbed by land or the water, which heats or cools
the air above it.
Winter occurs because:
There is a reduced amount of daylight during this time of the year. The sun is lower in the sky
and its intensity is spread over a larger surface
SOLAR RADIATION ON A SURFACE
When calculating the solar radiation incident on a surface, one can refer to two parameters,
irradiance and irradiation.
•
•
Irradiance is the instantaneous solar power incident on the surface
Irradiation is the cumulative energy captured from the surface in a given period (day,
month, year)
In general, global irradiation increases from dawn until noon and then decreases until sunset,
but its values are greatly affected by cloud cover and possible shading obstructions.
SOLAR RADIATION ON A SURFACE
Direct irradiation - comes straight from the sun, is influenced by the spatial disposition of the
surface
Diffuse irradiation - depends on the spatial disposition of the surface, and more precisely on
how this ’’sees’’ the sky dome.
Reflected irradiation - depends on the mutual spatial disposition of the absorbing and the
reflective surface, on the incident radiation onto the reflecting surface and on the albedo of the
reflecting surface.
SOLAR GEOMETRY
The earth moves along an elliptical orbital trajectory around the sun in a little more than 365
days, and also rotates around its own axis, which is inclined by about 67° to the plane of the
orbit. It takes about 24 hours to perform a complete 360° revolution.
Seasonal climate change is the result of the different ways in which the sun's rays hit the
various regions of the earth during the year. This is due to the inclination of the plane of the
equator, thus to the inclination of earth's axis.
Solar declination
The tilt of earth's axis with respect to the plane of the orbit is constant but the angle formed
between the line joining the center of the earth with the center of the sun and the equatorial
plane changes day by day, or, it is better to say, instant by instant. This angle is called the solar
declination δ,
•
is equal to zero at the spring and autumn equinoxes, and
•
is +23.45° at the summer solstice and -23.45° at the winter solstice.
The angle of solar declination varies continuously, very slowly, and can be assumed that its
value is approximately constant in a single day.
SOLSTICES AND EQUINOX
After the spring equinox, the sun still continues to follow a higher path through the sky with the
sunshine duration growing longer. On the 21sf June, the sunshine duration is the longest and
the sun traces the highest path through the sky and directly above the Tropic of Cancer, called
the summer solstice.
This day the sun rises not exactly in the east but north of east and sets north of west. After the
summer solstice, the sun follows a lower path through the sky each day until it reaches the
point where the sunshine lasts exactly 12 hours. This day is called the autumn (fall) equinox.
SOLSTICES AND EQUINOX
On 21st December, the sunshine duration is the shortest of the year and the sun traces the
lowest path in the southern sky, called the winter solstice. The sun rises not exactly in the east
but south of east and sets south of west.
Each day after the winter solstice, the sun begins to rise closer to the east and set closer to the
west until it rises exactly in the east and sets exactly in the west. On this day, about 21st March,
the sunshine lasts for 12 hours and is called the spring equinox.
SOLAR GEOMETRY
Knowledge of solar geometry is very important for architectural design and energy efficiency
strategies, since solar energy greatly influences the energy performance of buildings. When the
sun is low on the horizon, it is more difficult to control its effect and the rays can penetrate
deeply through the windows. The contribution of light could certainly be useful, but the
associated thermal loads can result in heavy energy consumption or in conditions of discomfort.
SUN PATH
Sun path refers to the apparent significant seasonal and hourly positional changes of the Sun
(and length of daylight) as the Earth rotates and orbits.
The duration of sunshine is determined by the length of time when the sun is above the horizon
and varies throughout the year as the Earth-Sun geometric relationship changes
SUN PATH
As a consequence of the earth's movements around the sun, in the course of the year, an
observer on earth perceives different solar paths, which are characterized by variable heights
and lengths, depending on the time of year and latitude.
The latitude is represented by the angle between the equatorial plane and the radius from the
earth's center to its surface at the specific location and ranges from 0° at the Equator to 90°
(North or South) at the poles. Generally, in the calculations, the northern latitudes are
considered positive and the southern ones negative.
SUN PATH
In order to make the study of the solar geometry more intuitive, it is convenient to refer to ‘the
apparent movement of the sun, assuming, that it moves on the inner surface of a sort of dome
(the sky dome), having as its base the horizon line of the site.
In this way (and it is consistent with our perception), the sun rises in the east, climbs in the sky
with a trajectory depending on the hemisphere, the latitude angle Φ and on the day of the
year, and sets in the west.
Solar Position Angles
The position of the sun in the sky at any given moment can be determined by two values:
•
•
the solar altitude; and
the solar azimuth
The boundary between the visible and invisible portions of the celestial sphere is called the
horizon. The poles of the horizon, those points directly overhead and underneath are called the
zenith and the nadir.
ANGULAR SOLAR COORDINATES OF THE SUN
Solar altitude (β) – angle between the direction of the geometric center of the sun's apparent
disk and the horizontal horizon plane
Solar Azimuth (a) - the angle measured on the horizontal plane, from the south-pointing10
coordinate axis to the projection of the line of sight to the sun on the ground.
Generally, it is assumed that azimuth angle is zero when the sun is exactly in the south, has
positive values eastward and negative westward.
SUN PATH DIAGRAM
Sun path diagrams are representations on a flat surface of the sun's path across the sky.
They are a convenient way to represent the annual changes in the sun path through the sky on
a single 2D diagram
Polar diagrams – polar sun charts are obtained by projecting the solar paths onto a horizontal
plane, on which the four cardinal axes are represented. These charts have a common base,
represented by a series of concentric circles and radial straight lines that branch out from the
center.
SUN PATH DIAGRAM
Sun path diagrams are a convenient way to represent the annual changes in the sun path
through the sky on a single 2D diagram
Polar diagrams – polar sun charts are obtained by projecting the solar paths onto a horizontal
plane, on which the four cardinal axes are represented. These charts have a common base,
represented by a series of concentric circles and radial straight lines that branch out from the
center.
Cartesian diagram – similar with the polar diagrams when it comes to perspective but this time,
the azimuthal values are plotted along the x-axis, and the altitude values are plotted along the
y-axis for different parts of the day throughout the year.
AZIMUTH ANGLES
run around the edge of the diagram in 15° increments. A point's azimuth from the reference
position is measured in a clockwise direction from True North on the horizontal plane. True
North on the stereographic diagram is the positive Y axis (straight up) and is marked with an N.
ALTITUDE ANGLES
are represented as concentric circular dotted lines that run from the center of the diagram out,
in 10° increments from 90° to 0. A point's altitude from the reference position is measured from
the horizontal plane up.
DATE LINES
represent the path of the sun through the sky on one particular day of the year. They start on
the eastern side of the graph and run to the western side. There are twelve of these lines
shown, for the 1st day of each month.
HOUR LINES
represent the position of the sun at a specific hour of the day, throughout the year. They are
shown as figure-8 type lines (Analemma) that intersect the date lines. The intersection points
between the date and hour lines give the position of the sun. Half of each hour line is shown as
dotted, to indicate that this is during the latter six months of the year
Reading a Polar Sun Path Diagram
1 Locate the required date line
2 Find the intersection point
3. Draw a line
4. Read the azimuth as an angle
5. Trace a concentric circle
6. interpolate
SUN PATH DIAGRAM
Sun path diagrams are used to evaluate how the sun affects the design context.
For instance, once the sun's position (that corresponds to a point on the chart) at a given
monthly average day and hour has been found, it is possible to draw an imaginary line, ideally
representing the sun's rays, from this point to the building