NZ Weather Forum

Hi all

So what is daily global irradiance? And how does it differ from sunshine hours? For example, in the attached Taumaranui has a higher daily global irradiance than Auckland, even though it has substantially less sunshine hours. In fact, Auckland appears to have the same daily solar radiation as Invercargill!

Rather confused.

Hope someone can clarify the difference between the two.

Thanks
Jeremy

As I understand it, and someone correct me if I'm wrong, basically sunshine is simply a measure of relatively unobstructed direct sunlight over time, regardless of strength (obviously it will be stronger at midday than towards dusk or dawn). Solar radiation however is the actual amount of total energy received from the sun. Invercargill's longer summer daylight hours would balance out its shorter winter daylight hours.

Interesting word.
I've heard of the word irradiation, a noun from the word irradiate, which means to shine upon or subject to radiation, but not irradiance

If you can say transmittance, you can say irradiance !
Very interesting map, thanks for bringing it up igaquo1. Further explanations would be great but I'm sure we can find something on the worldwidewebance.

And Nev, following your hypothesis we should see Stewart Island with a much higher than Invercargill then.

absolute humidity might be "watering down" the suns power in certain areas

Lacertae wrote:...And Nev, following your hypothesis we should see Stewart Island with a much higher [?] than Invercargill then.

Stewart Island would see longer daylight hours (as in longer days) during summer, but not necessarily more 'sunshine'. Just to clarify, 'sunshine' is usually measured by a very discernible shadow cast by the sun within a certain tolerance, i.e. no strong shadow, no sunshine.

However, my very basic understanding is that, 'solar radiation' and 'irradiance' are a measure of the strength of the sun's rays, which can vary depending on cloud cover, angle, etc. Perhaps the more technically minded could clarify or elaborate?

the units is the clue
MJ/m2/day
MJ is energy, i.e energy per metre squared per day
so its a measure of the actual solar energy received at the ground surface
how clear the air is could have an influence
but also though patchy mid level CU increases the solar received, as it reflects the sunlight ...so you get 2 bites of the same cherry
so it might reflect that as well, the map
there is a trend from north (being higher) to south (being lower)
and west to east there is a trend (west being lower, east being higher)
as you would expect

I wonder if UV light reaching the ground follows the same pattern as the irradiance? The UV warnings are always higher the further north you are, as in theory the higher sun angle should pose the greater risk

^ Yes, but you need consider factors such as in the amount UV filtering ozone in the upper atmosphere. My brother visiting from Cairns got sunburnt here in October, despite the Qld tan.

in the tropics, the sun is more overhead
so it does not reflect of things onto you as much (e.g water, concrete)
thats one factor...and the fact its shining down on top of you more...i.e your shoulders would get burnt for sure...

the attached map suggests UV drops away with the distance from the equator with the ozone hole not being much of a factor. yet i have also heard anecdotally that it is easier to burn here with a harsher light and less "haze" than the tropics

Edit: " Because of differences in ozone, sun-earth separation and pollution, the peak UVI in NZ is approximately 40% greater than at comparable latitudes in the Northern Hemisphere."
http://www.niwa.co.nz/our-services/onli ... -and-ozone

"The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter."
http://en.wikipedia.org/wiki/Solar_irradiation

the earth is closer to the sun in the orbit for the southern hemisphere summer..thats one factor...but more oceans in the southern hemisphere moderates that (for temperature) (same in winter)

Nev wrote:^ Yes, but you need consider factors such as in the amount UV filtering ozone in the upper atmosphere. My brother visiting from Cairns got sunburnt here in October, despite the Qld tan.

My experience is I can spend much more time in the sun in the tropics than I can in NZ summer light before getting burnt, I put it down to a much cleaner atmosphere here.

If its any help, here at our weather station we have a number of solar instruments.

One is a Campbell Stokes recorder - it records bright sunshine onto a card. It only records when the sun is very bright and unobstructed (i.e no clouds). This I believe is sunshine hours.

We also have another sensor that looks at the sky MINUS the sun. That is, how bright it is, with the sun specifically blocked out (so as to not overload the device).

Another looks at the whole sky, everything. As I understand it, this would be measuring irradiance. How much sunlight in total that position recorded. There's a second of this style sensor that looks electronically at the bright sunshine as well - its basically calibrated to stop recording when the sun is obstructed in any way. This is the electronic counterpart to the Campbell Stokes recorder.

There's also a solar radiation recorder too, measuing the radiation from the sun.

So for the Auckland anomaly, could it be caused by a relatively higher incidence of summer cloudiness around the middle of the day (when the radiation is the strongest), caused by the sea breeze convergence zone? whereas for Taumaranui, a lot of the cloud would relate to the thick early morning fogs which frequent the river valley, but because the sun is weak then, it doesn't count a huge amount against the total solar radiation for the day??

yes, it is along those lines, I would say

any other thoughts?

Nice map in the original post. It is plain to see that the south of Wellington is more sunny than the north.