Understanding Weather Elements: Insolation, Clouds, and More
Weather Elements
a) Insolation
Insolation refers to the amount of incoming solar radiation reaching the Earth’s surface. In Spain, insolation is at its maximum during the summer and at its minimum during the winter.
b) Cloud Cover
Cloud cover describes the atmospheric conditions where the sky is covered to a greater or lesser degree by clouds. Cloud cover is typically at its minimum in summer and at its maximum in winter. Cloud height is measured using a ceilometer.
c) Temperature
Temperature is the degree of heat in the air, measured in degrees Celsius using a thermometer. On maps, temperature is represented by isotherms (smoothed by the sea and decreasing with height). Key temperature concepts include:
- Annual temperature range: The difference between the average temperature of the warmest month and the coldest month.
- Frost: Occurs when the air temperature falls below 0°C.
d) Air Humidity
Air humidity is the amount of water vapor present in the air due to evaporation. Humidity decreases as temperature increases. It is measured using a hygrometer.
e) Fog
Fog is a suspension of tiny water droplets in the lower layer of the atmosphere, reducing visibility to less than a mile. It occurs when moist air condenses in the bottom layer of the atmosphere. There are two main types of fog:
- Irradiation fog: Forms when air in contact with the ground cools.
- Advection fog: Forms when warm, moist air moves over a cold surface.
f) Haze
Haze is a phenomenon occurring in the lower layers of the atmosphere due to the presence of a large number of fine dust particles. In Spain, haze often forms during summer anticyclonic conditions.
g) Atmospheric Pressure
Atmospheric pressure is the weight of the air above a unit area, measured in millibars (mb). Generally, over 1016 mb indicates an anticyclone (high pressure system), while values less than or equal to 1016 mb indicate storms. Atmospheric pressure is measured using a barometer and represented on weather maps with isobars.
h) Wind
Wind is the horizontal movement of air relative to the land surface. It occurs due to differences in pressure, moving from areas of high pressure to areas of low pressure. An anemometer measures wind intensity, and a wind vane indicates wind direction.
i) Rainfall
Rainfall is water falling from clouds in either solid or liquid form, measured with a rain gauge in millimeters or liters per square meter. Rainfall is caused by the elevation and cooling of air to below the dew point (the temperature at which water vapor condenses and precipitates) due to:
- Terrain (orographic lift, leading to the foehn effect)
- Convection (heating of the soil)
- Frontal systems (contact between cold and warm air masses)
A storm is an intense, localized weather event accompanied by thunder, lightning, and heavy rain over a short period. It is a type of convection storm.
j) Evaporation
Evaporation is the physical process by which water transforms into vapor at room temperature.
k) Evapotranspiration
Evapotranspiration is the loss of moisture from the Earth’s surface due to insolation and transpiration from plants and soil.
l) Aridity
Aridity is the relationship between temperature and humidity in a given area. It increases with higher temperatures and lower rainfall. Two indices are used to calculate aridity:
- Gaussen index: Measures monthly aridity. A month is considered dry when 2T (°C) > P (mm), i.e., when double its average temperature is greater than or equal to its total monthly rainfall.
- Lautensach index: A month is considered dry when it has less than 30 mm of monthly precipitation.