Measurement of Radiation

- Solar radiation

- Long wave radiation

- Net radiation

- Exposure of radiation sensors

Solar (short wave) radiation

Visible ~ 0.4 –0.7 um

Integral under curve ~ 1400 W/m2, perpendicular to the beam. This is the solar “constant”.

UV < 0.4 “Near” IR > 0.7Vis

Absorption by the atmosphere removes about 20% of the solar radiation before it reaches the Earth’s surface,

on a clear day.

Top of atmosphere

At Earth’s surface

Fig. from Strangeways

Effect of Sun angle on surface solar power

Power is reduced when beam is spread over a larger area as Sun’s elevation angle is decreased.

Reduction in power is proportional to ratio of x/y.

X

ya

b

a = elevation angleb = zenith angle

x/y = sin (a) = cos (b)

To estimate max solar power on horizontal surface, start with the 1400 W/m2 value at the top of the atmosphere, and correct for :- Absorption by the atmosphere [ x 0.8]- “Spread out” due to sun angle [ x sin (elevation angle) ]

Estimate max solar power for : 25N, 45N, & 65N latitudes…

25N 1120 W/m2

45N 1050 W/m2

65N 860 W/m2

Latitude Max elevation

25 90 deg.35 8045 7055 6065 5075 40

A good radiation sensor should show this response when a parallel light beam is pointed at the sensor from different angles.

Response is linked to the zenith angle rather than the elevation angle and called “cosine response”.

Solar Radiometers

<<< A high-quality solar radiation sensor has a black disk exposed under glass domes for wind and rain protection.

The less costly sensor, below, uses a photo-detector similar to a camera light meter. It is less accurate, but suitable for an operational weather station.

These sensors are called pyranometers.- fire (of the Sun) meters

Thermocouples connected in series (a thermopile) are used to make the temperature difference measurement in pyranometers.

In a high-quality radiation sensor, a black disk is protected from wind and rain by glass domes so heating of the disk is only a function of the radiation.

The temperature difference between the exposed disk and the shaded sensor body is proportional to the radiation.

Fig. from Strangeways

- very non-linear and doesn’t cover the full solar spectrum.- calibrated for solar spectrum, so OK for weather stations- not calibrated for other radiation sources – e.g. growth rooms or reflected solar radiation measurements- ideal for photosynthesis measurements (“quantum” sensor)

Photo-detector Pyranometer

The “Bright Sunshine” recorder (Campbell-Stoakes)

Solar radiation is focused onto the green cardboard strip by the glass ball (an all-directional magnifying glass). A line is charred into the strip when there are no clouds. The strip is marked off in hours… so “hours of bright sun” are recorded. The strip is replaced every day.

Terrestrial (long wave) radiation

All “far” IR

Atmosphere absorbs most of the long wave radiation emitted by the Earth’s surface, and sends much of it back to the surface.

32 Escape to space

400 333

368

32 Escape to space

400 333

368

32 Escape to space

400 333

368

Estimating long wave radiation from surface & atmosphere Earth’s surface: Lout = S x (T4), T is air temp in degrees K

Use S = 5.67 x 10-8 to get Lout in W/m2

Atmosphere: Lin = (Emission efficiency) x S x (T4)Emission effic. ~ 0.8 when clear, ~1.0 when overcast

Check that these “recipes” for surface and sky long wave radiation give numbers similar to the diagram on the right.

The Earth’s average temperature is about 15C, and average atmospheric emission efficiency is about 0.85.

Pyranometer - glass dome transmits short wave, but not long wave, radiation.

Change the dome to silvered silicon (below), which transmits long wave, but reflects short wave radiation, and you have a….

...Pyrgeometer >>>(fire of the earth meter)

What is Net Radiation * ?

Solar in Reflected solar Sky LW Surface LW

Net Radn = Solar in + Sky LW – Reflected solar – Surface LW

= K in + L in - K out - L out

(albedo) x K in

* Very important variable in studies of evaporation from water bodies, crops and forests.

A “net pyrradiometer” (net radiometer)

Incoming radiation is sensed by the warming of the black plate. An identical plate on the bottom side senses outgoing radiation. Output = Top – Bottom, so… Net radiation = Kin + Lin – (Kout + Lout)

Some models cover the plates with polyethylene domes, which transmit both short and long wave radiation. Why add a dome?

How could we use thermocouples to give a signal proportional to the temperature difference between the two black plates?

This signal would give us the net radiation directly.

1. Estimate the net radiationEstimate the maximum value of net radiation you might expect.

Net radn = K in + L in – (albedo) x K in – L out

How about an estimate of the minimum value? (Hint: think about nighttime.)

2. Estimate the maximum and minimum signals, after reading the calibration factor from a net radiometer.

(If no radiometer is available, use “ 10 W/m2 per mV”)

How to estimate the signal size for a net radiometer…

This sensor measures all 4 components of net radiation separately. It provides signals for each component, or an overall net radiation signal.

K in

K out

L in

L out

Exposure of radiation sensors

1. Radiometer looking up

- For weather station, clear view of horizon is required to avoid shadows on the sensor and allow all direct and diffuse radiation from the sky to be recorded.

- Or if monitoring a partly shaded area, keep radiometer close to surface so timing of shadows on plot and radiometer match.

Exposure of radiation sensors

2. Radiometer looking down

- radiometer’s view includes plot plus some of surrounding land out to the horizon.

- View factor = percent of received radiation that comes from plot

- 90% view factor achieved when ratio of radiometer height to plot diameter is about 1:6 . For 95%view factor, ratio is about 1:9.

Plot

Radiometer height vs. “seen diameter” for various view factors.

He

igh

t of

rad

iom

ete

r (f

ee

t)

Diameter of seen area (feet)

10

60 90

Exposure of radiation sensors

Suppose you were making net radiation measurements in connection with a study of evaporative losses from the lake behind a dam.- what combination of sensor mounting height and distance from the shore would you choose?

Fig. 3.28 [3.23]

Absorption of short and long wave radiation by gases in the atmosphere