Climate Variations: Urban Centers and Seasonal Changes
Winter Climate
Wintertime typically corresponds to a dry, cold season with clear skies. This is due to the influence of anticyclones. The Azores anticyclone, usually positioned to the south, retreats. However, a seasonal anticyclone often forms at the heart of the Plateau, causing very low temperatures, below 0°C, resulting in frost and morning fog. The Iberian Peninsula may also be affected by other anticyclones, such as those from Central Europe or Siberia. The concentrated cold air in the lower areas leads to temperature inversions in plains and valleys. If a river is present, fog often occurs, dissipating throughout the day. If the cold is very intense, sharp frosts and rime appear.
While this occurs in the interior of the peninsula, the northern coast usually experiences rain due to storms associated with the Polar Front and the Icelandic Low. This rainy weather is restricted to the Cantabrian coast because of the peninsula’s relief. Along the rest of the coast, the sea acts as a temperature regulator, resulting in milder temperatures and less fog. In large cities, under anticyclonic conditions, air pollutants are suspended in the lower layers, causing pollution.
Cold waves also occur occasionally during winter, usually lasting about three days. They happen when a high-pressure system is present in central Europe or Scandinavia, combined with a low-pressure system in the Mediterranean. This triggers a rush of cold, dry air from the northeast, with clear skies and heavy frosts. It affects the entire peninsula except for the Andalusian coast, which is shielded from these cold winds by the Penibaetic mountain system.
Summer Climate
Summertime is normally hot and dry. During this season, the Azores anticyclone brings air over the peninsula, even to the Cantabrian area. This stability is only interrupted by the formation of a thermal low-pressure system, which produces convective rains due to the atmosphere overheating from excessive heat. This can generate summer storms. Heat waves can also occur with the arrival of continental air from Africa.
Autumn and Spring Climate
The characteristic weather during autumn and spring is variable, alternating between rainy and clear days. This is due to storms associated with the Polar Front moving from the Atlantic to the mainland, producing precipitation. These may affect some regions more than others. Except for the Cantabrian coast, the rest of the peninsula receives the most rainfall during the equinoctial seasons. Sometimes, especially in autumn, a “cold drop” phenomenon can occur. This happens when a low-pressure system forms in the Mediterranean due to the area overheating during the summer, and a cell of cold air descends from the Jet Stream at high altitude. Rainfall caused by a cold drop is very violent and intense, often causing catastrophes such as river overflows, flooding of crop fields and homes, and loss of life.
The Urban Climate
Unlike other climates, an urban climate creates a “heat island” due to human intervention. This results from activities in large cities, air pollution, skyscrapers, traffic, heating, etc. This climate is considered a microclimate due to the size of the affected area, not its impact. In many cases, it concerns the majority of a region’s inhabitants concentrated in one or more municipalities.
Since its origins, the city has always had different physical conditions of life, a climate distinct from rural areas. However, since industrialization, the contrasts have heightened to the point of being perfectly measurable in the following areas: fog, humidity, wind, rainfall, and temperature.
- Smog is not only a horizon of water droplets in suspension that reduces visibility, but it also contains other pollutants such as smoke, harmful aerosol gases, and so on. It can even occur without moisture, and this type of fog is called “smog” (smoke and fog), an internationally accepted word synonymous with a thick, heavily polluted atmosphere. This is common in large cities during winter, with calm winds and under the influence of an anticyclone. The effects on those with respiratory ailments can be lethal.
- Humidity: It seems that there are still no conclusive studies on the values of relative humidity in the city and its surroundings. The causes of variations in humidity in urban areas lie in the absence of large bodies of water and vegetation within city limits, despite the large volumes of steam supplied by combustion from heating and traffic.
- The wind speed in the city is 10-20% weaker than in the countryside due to constant friction with buildings and the presence of perpendicular streets.
- Urban precipitation is somewhat higher than in nearby areas, by 5-10%, up to 30%. Among the causes of this phenomenon are:
- Convective updrafts originating in the heat island.
- The presence of numerous condensation nuclei that feed mists and clouds.
- The provision of abundant water vapor from heating and industrial uses.
In summary, there is evidence that large cities receive more rainfall than their surroundings, experience more rainy days, and have more frequent summer storms (some accompanied by hail). Conversely, snow is less common due to the heat that envelops the city. The city’s annual temperature is somewhat higher (1-3°C in outlying areas), but this value is not as relevant. The real differences are established during the night: 4-6°C are common, and differences of up to 11°C have been recorded. The center has been called a “heat island” (or artificial oasis) because it behaves as such. Many factors contribute to the development of this phenomenon:
- Due to atmospheric depletion, the city receives less radiation than the countryside, but this same contaminated layer acts as a protective barrier during the night, preventing heat from escaping.
- Throughout the day, buildings and streets accumulate heat, which they radiate at night, a phenomenon unknown in the countryside. Inner cities like Madrid have a microclimate mainly due to the barrier formed by their buildings against the elements.
- When it rains, water stays very little time on urban surfaces, as they are mostly impermeable. As it quickly sinks, only a very small part of the liquid can evaporate. This process consumes 600 calories per gram in the atmosphere, leading to significant cooling of the heat island.
The heat island reaches its maximum development during anticyclonic conditions and calm winds. If a cyclone or rain-laden winds arrive, the contours are cleared. Finally, the presence of prominent relief or a wide river can break the continuity of the heat island, lowering temperatures, just as large buildings raise them. Hence, the thermal maximum is identified with the nucleus, being less dense at the periphery.