Written By : Neville Koop, Managing Director. Bula Fiji, trust you have had an enjoyable week. The long weekend weather is going to be great so enjoy! A few people have been asking me when will the cool weather arrive, “why is it still so hot” they ask. So why don’t we take a look at what controls our temperature.
To start with, let’s check out the fundamentals. The earth gets all of its heat from the sun. The amount of power that arrives from the sun is properly defined as the amount of solar energy that falls per second on an area of 1m2 above the earth’s atmosphere that is at right angles to the Sun’s rays. Its average value is about 1400 Wm-2. This number is sometimes known as the SOLAR CONSTANT. As you may recall from our discussion on the sun a few weeks ago, the sun it isn’t precisely constant and can vary fractionally due to sun spots and solar flares, but for the purposes of this article we can assume it’s there and is pumping out a constant stream of energy in the form of short wave electromagnetic radiation.
Now the amount of energy entering an area of 1m2 in the earth’s atmosphere (i.e. 1400 Wm-2) is not the same as the power that arrives on 1 m2 of the Earth’s surface. Scattering and absorption in the atmosphere results in less than half of this arrives at the Earth’s surface.
The diagram above explains that not all incoming solar energy is reaching the Earth’s surface. Approximately 6% of incoming solar energy is reflected by atmosphere, 20% reflected by clouds and 4% reflected from earth’s surface. Furthermore, approximately 16% of incoming solar energy is absorbed by atmosphere and 3% absorbed by clouds. This results in only 51% of original incoming solar energy to reach the Earth’s surface. This explains how the amount of solar energy that arrives to the Earth’s surface depends greatly on the weather conditions.
Perhaps even more importantly, the latitude of the earth greatly affects incoming solar radiation. Different parts of the Earth’s surface (regions at different latitude) will receive different amounts of solar radiation. The influence of latitude on the amount of solar radiation reaching the earth’s surface is described in the image below.
Average input of solar radiation to the earth’s surface directly related to the latitude. Heating is most intense when the sun is directly overhead, so that incoming solar radiation strikes perpendicular to the earth’s surface. The higher latitudes are cooler than the tropics because the same quantity of solar radiation is dispersed over a greater surface area on the earth’s surface.
Think of the fixed incoming solar radiation (i.e. the Solar Constant) as a simple electric heater (with a fixed amount of heat it can release). In a small room this heater will warm it nicely. This small room is like being on the equator. Moving north or south away from the equator increases the area to be warmed, like increasing the size of the room. As the room gets bigger, the same energy goes to heating a larger space, and so the overall effect is less warm than in the small room (at the equator). By the time you get to the poles the area warmed by the solar constant is largest, and so the room is coldest.
The amount of solar radiation received by Earth’s surface will also vary with the seasons since this will affect how spreads out the rays have become. The earth tilts on its axis as it orbits the sun. The changing of the earth’s tilt is what gives us our seasons. The effect of the earth’s tilt on weather is shown in the image below. Right now the tild of the earth is very close to that depicted in the top two images – June 22nd is the northern summer solstice and our winter solstice in the southern hemisphere. By September 22nd we will be near the equinox with the earth vertical on its axis (middle), then on 22nd December we reach the northern winter solstice, and our summer solstice in the southern hemisphere (bottom).
During summer time, the Northern part of the Earth receives more intense sunlight since the Northern parts of the Earth tilts toward to the sun. Comparing to intensity of sunlight that the Southern part receives, the Northern part receives more intense sunlight than that of the Southern part because the same quantity of solar radiation is dispersed over lesser surface area. This applies oppositely when the season changes to winter. Notice here that regions lay in the equatorial regions constantly receives solar radiation and intensity over time.
At any particular location the solar radiation flux is determined by the time of the day, the season, and the geographical latitude. Furthermore, weather provides an unpredictable element. Under clear skies the radiant energy will be mostly direct, with only about 15% being diffuse, while under overcast conditions, obviously 100% of the radiant energy is diffuse. Fiji is fortunate that it lies in the tropical regions and so its solar radiant energy flux is much more direct and constant than most countries, with peak maxima rising as high as 1.4 kW m-2. Notice here that while countries on the Northern or the Southern side of the Earth has sinuous period in receiving solar radiations due to seasonal and regional variations, the countries that lie on the equator receive consistent solar radiations throughout the year.
So Fiji enjoys plenty of incoming solar radiation. Thus we are a warm country. But the temperature of Fiji has one major moderating feature which limits the extremes of temperatures we can expect. This is the Pacific Ocean.
With the atmosphere around Fiji in direct contact with the ocean surface the temperature response of the air is governed by the sea temperatures surrounding Fiji. The relatively small land mass allows for only small fluctuations in temperatures during the day and as a result Fiji’s air temperature on any given day is determined by the temperature of the surrounding sea. For a good first guess take the sea temperature and add 5 degrees for maximum in daytime and minus 5 degrees for minimum at night time. Places that are elevated will be cooler.
For the past few months sea temperatures around Fiji have been much warmer than usual in response to the La Nina event that occurred through the latter half of 2010 and the first few months of 2011. Right now, sea surface temperatures close to Fiji remain a degree or so above average, around 26 degrees Celsius.
This daytime highs will be around 30 or 31 degrees, and overnight down to 20 or 21 degrees. Insummer when ocean temperatures rise to around 28 degrees we see warmer temperatures, say 33 or 34 in the day time and 24 25 at night.
Water has a great capacity to store heat. Unlike land, which warms tens of degrees in a few hours when subject to incoming solar radiation, water, and hence the world’s oceans, warm only slowly, fractions of a degree per week. Similarly when incoming solar radiation ceases (i.e. the sun sets) the earth quickly cools, while the ocean barely changes overnight.
Cooling of sea temperatures by radiation takes a long time, a good rule of thumb is that thermal response of water lags behind the heating cycle by about 1/4 pi radians. In simple terms, if a year (i.e. a complete cycle of the sun) is 2 pi radians, then 1/4 pi would be about 1 and a half months. This is why mid February is the hottest time of the year, even though maximum heating occurs in late December, and conversely August is the coldest even though the minimum in heating is on the winter solstice in late June.
So dig out those extra blankets and coats, it won’t be long until you need them.