Special features of the coastal climate around the Cap de Creus

_{from Dipl.Met. Paul Bohr}

1. Temperature differences between water and country
The coastal climate is strongly influenced by the variation of the rate of warming of land and sea through solar radiation. Water has a considerably higher heat capacity than land: More heat energy is required for a 1° centigrade warming of gram water than for the same warming of a gram of land. Thus, the same application of heat from the sun would increase the temperature of water less than the increase of the temperature of land. In addition, the solar energy provided to the earth remains near the surface and warms it up faster because the earth does not conduct heat very well. Conversely, water temperature increases more slowly because the temperature increase is soon distributed quite easily to deeper water. Water, as deep as 100 m is impacted by solar heat.

Similar is the different nocturnal and winter cooling of land and sea through the infrared heat radiation into space. It proceeds from the thin land surface considerably faster and more strongly than in the turbulence and thicker water layer, which means that water cools down more slowly than the land.

One recognizes these facts easily by the middle month or year values of the temperature of a meteorological land station and a neighbouring sea station, as shown schematically by illustration 1. One recognizes clearly the raised temperature amplitude between the year maximum and the year minimum on the land opposite to the values of the sea and also the fast warming and cooling of the land as compared to the sea.

	The land surface reaches its temperature maximum in July and is at its minimum in January, in each case a month later than the highest the sun level in the summer and the lowest sun level in the winter, while the water temperature limps along about two months later, after the Sun’s highest and lowest levels.
illustration 1(Schematic yearly diagram of the month values of the temperature on land and on the sea)

2. Land and sea wind

Liquid media as air moves in direction of their density slope from cold to warm. With calm or gentle breezes in high-pressure areas this thermal circulation, due to the temperature differences between country and sea, is easy to see. During warm weather, in the morning for example, the air temperature over land is higher than the air temperature over the sea, the cooler sea air temperature moves itself in the direction of the coast and brings cooling. The air flowing off at the ocean surface is replaced by dropping mountain air. Over the land the sea air is warmed up, and gains thermal height. The thermal circuit is completed as the warm air flows down from the heights back to the sea. The ascending air, as it gets higher, has lower pressure and expands. The expansion of the air reduces its inner energy, which means its temperature. For a 100 m ascension the air temperature cools approximately 1° C. Similarly, falling air has increasing pressure and warms up through compression around 1 degree C per 100 m. Through cooling the temperature of the air approaches its point of condensation, that is, the temperature at which the moisture contained in it is saturated and condenses to form cloud droplets. Then clouds appear over the coast. The dropping air over the sea gets warmer and becomes drier. Available clouds are diminished.

At night, when the air over the land cools down and drops below the temperature of the sea, the circulation turns and the land air advances to the sea, is warmed up there, ascends, forms possibly clouds, returns from the heights to the land, drops, and the air, as it descends, becomes in this case warmer and drier. The sky over the land is then clear.

3. Influence of cold air convection on the Mediterranean Sea

On the back of low-pressure areas and at the front of high-pressure areas the air supply is from the north. Mostly cold polar air is led here from the north. The impact is most obvious, especially in the winter, with the temperature variation of the relatively warmer Mediterranean Sea. The cold air flows over the warm water and the results are strong thermal up currents and cloud formation over the sea, and thence showers. That is why the Mediterranean Sea has most of its rainfall during the cold season.

4. Influence of warm air convection.

If hot air flows over colder water, it is cooled from below. If the temperature falls below the dew point, fog formation occurs above the ocean surface. Fog is likely when the air is warm, usually from the south, or in the spring or summer when the air is from the mainland, from central Spain, moves over the cool sea. Because the central Spain air is often very dry, the cooling does not, as often, fall below the point of condensation, but approaches to it. Relative humidity increases and the air becomes bleaker.

5. Influence of the friction differences

An additional influence is caused by the flow of air over the relatively rough earth’s surface compared to the smoother sea surface. The uneven surfaces tend to slow down the airflow speed; the smooth surface of the sea does not impact the air speed. That is why air speed over the land, at the same pressure, is up to 50% slower than over the sea. In addition the direction of flow that runs parallel to the lines of the same air pressure - the isobars - is distracted by the friction to the left to the deeper pressure, the more, the more strong the friction is. This diversion can reach up to 45 degrees over very rough land, yet over the sea by not more than 20 °C. Because the friction influence decreases with the height the wind flows up toward to the frictionless level at the right and increases in this case up to this height.

As the illustration shows, the friction leads an offshore wind from the country to the sea to a coast divergence (3.a.), which decreases by the friction upwards. There, more air flows over the water because of the greater speed than air coming from the country with its lower speed. This mass drain is compensated by the air dropping from the height. The dropping air warms up per 100 m around 1° C and, as it warms, becomes drier. Available clouds dissolve.

In reverse, less air flows over the coast at onshore winds from the sea to the land, than flows to the land from the sea. Over the coast a mass jam happens which decreases also upwards. This mass influx in ground level is compensated by ascending air upwards. The ascending air chills itself around 1° C per 100 m and if it is under the point of condensation, clouds form and already existing clouds are intensified. Of course, rainfall becomes more likely if there are sufficient clouds.

Also in this case of the coast convergence through friction the shelving coast of the Costa Brava supports the cloud and rainfalls at onshore winds. These are the reasons that our area does not always have blue sky, but we enjoy a varied climatic spectrum.

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