Weather Elements in Spain: A Comprehensive Guide
Weather Elements in Spain
1. Cloud Cover and Insolation
Insolation is the amount of solar radiation received by the Earth’s surface. Due to its latitude, Spain receives over 2,000 hours of sunshine per year. However, there are significant contrasts between the Cantabrian coast, which receives less sunshine, and the southeastern peninsula and islands, which receive high insolation.
Cloudiness refers to the extent to which the sky is covered by clouds. The cloudiest area in Spain is the Cantabrian coast. The Guadalquivir Valley, the southern Atlantic coast, and parts of the Canary Islands experience the highest number of cloudless days.
2. Air Temperature
Temperature is the degree of heat in the air, measured in degrees Celsius (ºC) with a thermometer. Isotherms are lines on maps connecting points with equal temperatures.
Average temperatures in Spain vary substantially due to several factors:
- Latitude
- Altitude
- Distance to the sea
Two key variables related to temperature are:
a) Annual Temperature Range: The difference between the average temperature of the warmest and coldest months. Inland areas experience the greatest temperature variation, while the islands and the Cantabrian coast experience smaller variations.
b) Frost: Occurs when air temperature drops below 0ºC. Water vapor in the atmosphere freezes and deposits as ice on surfaces.
- Radiation Frost: Occurs on clear nights when the ground cools, transferring low temperatures to the air in contact with it.
- Advection Frost: Caused by the arrival of a cold air mass. Frosts are less frequent on the Spanish coast. The northern sub-plateau and the Ebro Valley experience colder temperatures due to temperature inversions.
Thermal Inversion
A thermal inversion is an anomalous atmospheric situation where air temperature is cooler at the surface than at higher altitudes. It typically occurs in winter during anticyclonic conditions, where topography favors air stagnation for several days in valleys and intermountain basins.
How it occurs: Nighttime cooling of the ground transfers to the air immediately above it, making it colder than the air higher up.
Consequences: Temperature inversions often cause low fog and dew at dawn.
Other Inversions: Inversions can also occur when a cold air mass is introduced beneath a warm one.
3. Humidity, Fog, and Haze
a) Air Humidity: The amount of water vapor in the air. It depends on proximity to the sea and temperature, decreasing as temperature increases. The Spanish coast and the northern sub-plateau have an average relative humidity above 70% per year (although the sub-plateau experiences significant variations, with maximum humidity in winter and minimum in summer).
b) Fog: A suspension of tiny water droplets in the lower atmosphere, limiting visibility to less than one kilometer. It occurs when air in the lower atmosphere cools and condenses.
- Radiation Fog: Results from heat loss from the ground at night, typical in winter.
- Advection Fog: Occurs due to the arrival of warm, moist air over cold ground, or the arrival of cold air over warmer, humid surfaces (sea, reservoir, or river).
c) Haze: A suspension of fine dust particles in the lower atmosphere, reducing visibility. In Spain, it forms during dry summers with no cyclonic activity, when dry soils release particles that are lifted and suspended by upward air movements caused by strong ground heating.
4. Pressure and Wind
a) Atmospheric Pressure: Depends on the characteristics of the air masses affecting the peninsula. High pressures dominate in winter and summer, although low pressures can occur in summer due to ground heating. Low pressures dominate in autumn and spring.
b) Winds: Horizontal movements of air relative to the Earth’s surface, caused by pressure differences. Westerly winds dominate the peninsula. The Canary Islands experience dominant trade winds with a northeasterly component.
Local winds also exist, such as north, east, and south winds. Pressure differences also cause sea and mountain breezes, which change direction between day and night.
5. Precipitation
Precipitation: Water falling to the Earth’s surface from clouds, in liquid or solid form. It is measured in millimeters (mm) or liters per square meter with a rain gauge. Isohyets are lines on maps connecting points with equal precipitation.
Precipitation occurs when rising air masses cool, causing condensation of water vapor in the atmosphere.
Types of Rainfall:
- Orographic: Due to mountainous terrain.
- Convective: Due to ground heating.
- Frontal: Due to the contact between two air masses with different characteristics (one warm and one cold). The cold air mass wedges beneath the warm air, forcing it to rise and cool.
Spain’s rainfall is modest in annual volume, variable, seasonal, and unevenly distributed. This is due to several factors:
- Latitude and Location: Determine the prevalence and annual succession of storms and anticyclones.
- Exposure to the Sea: In areas isolated from maritime influence, winter cold favors cyclone formation, while summer heat promotes air condensation and storm formation with low rainfall.
- Relief: Precipitation increases with altitude and decreases in areas enclosed by mountains.
6. Evaporation, Evapotranspiration, and Aridity
a) Evaporation: The process by which water turns to vapor at ambient temperature. Evaporation is stronger at higher temperatures. In Spain, it is stronger in the south than in the north, and more intense in summer and during the middle of the day.
b) Evapotranspiration: The loss of moisture from the Earth’s surface due to solar radiation and transpiration from plants and soil. Actual evapotranspiration is the amount that actually occurs. Potential evapotranspiration (PET) is the amount that would occur if there were sufficient water.
c) Aridity: The insufficiency of water in the soil and atmosphere. It depends on the relationship between precipitation and temperature, increasing with higher temperatures and lower precipitation.
- Gaussen Index (2T(ºC) – P(mm)): Measures monthly aridity. A month is dry when monthly rainfall is less than or equal to twice the monthly average temperature.
- Martonne Index (P / T + 10): Measures the general aridity of an area.