Understanding Weather Phenomena and Climate Patterns
The Environment: Concept and Parameters
We refer to the set of weather phenomena that characterize the weather situation and weather in a particular place on Earth as the environment.
Climate should not be confused with weather (temperature, humidity, cloudiness, precipitation, and wind) at a specific time. Climate results from interactions involving latitude, altitude, continentality, and wind patterns. Understanding climate requires interpreting climographs, which represent different climates of land areas, and learning concepts like precipitation and fronts.
A) Formation of Precipitation
Precipitation is water falling to the surface in liquid or solid form. Clouds must form before precipitation occurs. Clouds can form in three ways: thermal convection, orographic ascent, and frontal convection.
Cloud-Thermal Convection
These clouds form in atmospheric instability when warm, humid air rises to the condensation level, creating small, shallow clouds. If there is enough heat and moisture, multiple shallow clouds can form a vertical tower-shaped cloud called a cumulonimbus. Significant temperature differences exist between the base and frozen top of these clouds. This contrast creates strong upward thermal currents, lifting tiny water droplets from the cloud base, causing them to collide and merge, forming larger drops that fall as rain.
Orographic Clouds
When moist air encounters a mountain, it is forced to rise, reaching the condensation level. This typically forms horizontally developed clouds called strata, creating rain clouds in contact with the slope, resulting in horizontal precipitation. After passing the mountain’s peak, the cloud loses most of its water, which converts to vapor, and descends as warm air on the opposite side, creating a dry rain shadow zone.
Frontal Convection Clouds
A front is the contact area between two air masses of different temperature and humidity, creating a significant thermal contrast. These air masses behave as isolated systems, colliding instead of mixing. The energy released at the front due to temperature differences causes rain or winds.
Types of Frontal Storms:
- Cold Fronts: Form when a cold air mass moves by wind and contacts warmer air. The faster, denser cold air wedges under the warm air, forcing it to rise and forming a storm or depression with vertical development clouds (cumulonimbus).
- Warm Fronts: Form when a warm air mass moves to meet cooler air. The less dense warm air rises, but more slowly than in a cold front, creating horizontally developed nimbus clouds (lower) and altostratus (upper). These cover the sky with a gray, unattractive layer, bringing persistent weak rain and snowfall. Cirrus clouds form in higher layers, indicating good weather.
- Occluded Fronts: Occur when a cold and warm front overlap, with one (usually warm) losing contact with the ground (occlusion), leaving the other (usually cold) in contact with the surface.
B) Types of Rainfall
Rainfall includes rain, sleet, and snow.
- Rain: Liquid precipitation. Light rain is called drizzle, while heavy rain over a large area from nimbus clouds is called persistent rain.
- Torrential Rains: Rainfall exceeding 200 liters per square meter in 24 hours, posing a flood risk. Warning systems (white, yellow, red) are based on rainfall amount over 12 hours.
Rules to follow before storm risk:
- Crouch down, but do not raise your arms or open an umbrella.
- Stay away from metal objects (antennas, cables, tools) as they are good conductors.
- Do not run with wet shoes or clothes, or swim, as moisture conducts electricity.
- Snow and Sleet: Snow forms when ice crystals in cumulonimbus clouds collide and form hexagonal crystals. Snow is dangerous in mountainous areas due to avalanche risk. Wind chill (wind over 50 km/h, snow, and temperatures below -7°C) is another risk. Hail forms in spring/summer when ice crystals fall from the cloud’s peak to the middle area. Large hail can have multiple ice layers, posing risks to agriculture and causing damage or death by impact.
The Weather at These Latitudes
The climate in temperate areas of the Northern Hemisphere is determined by the polar front and jet streams. The front separates cold polar air from warm tropical air.
- Polar Jet Stream: A high-altitude, fast-moving wind current encircling the Earth.
- Polar Front: A boundary separating cold northern air from warm southern air, consisting of warm, cold, and occluded fronts.
The circumpolar vortex is a wave of storms forming the polar front. Its position (zonal index) determines weather situations A, B, or C.
- Situation A: The vortex is a closed circle near the North Pole, occurring during warm seasons.
- Situation B: The ITCZ, subtropical anticyclones, and subpolar storms move south, causing Rossby waves (up and down winds). Descending waves create storms.
- Situation C: If situation B persists, meanders expand and break, sending subpolar storms south, causing rain.
Storm Wave Formation:
- Polar winds create an upward ripple.
- Westerlies push back, forming another upward ripple, creating a storm between them.
- Occluded fronts form, causing heavy rainfall.
- Fronts dissipate, and the polar front recovers its rectilinear form.
- Blocking Anticyclones: Stationary anticyclones that prevent rain, causing droughts.
The Climate of Spain
Spain’s climate is influenced by the Azores subtropical anticyclone. In summer, it moves north, blocking storms and causing stormy rains from thermal convection. Winds from the Sahara Desert can cause heatwaves. In winter, the anticyclone moves south, but a continental anticyclone forms, causing droughts with fog or frost, diverting rain towards the Cantabrian coast and northern Europe. Rainfall occurs when the continental anticyclone dissipates, allowing frontal storms.
- Cold Drop: Cold air from higher latitudes spirals down into warmer air, causing convective storms.
- Tornadoes: Rotating columns of wind and dust from the ground to a cumulonimbus base, formed by surface heating and wind shear.
The Climate of Low Latitudes
A) Monsoons
Large-scale sea breezes with yearly land-sea wind alternations. In winter, a cold anticyclone forms over Asia, expelling cold, dry air. In summer, the anticyclone dissipates, and the ITCZ moves over Asia, causing monsoon rains in India and Southwest Asia.
B) Tropical Cyclones, Hurricanes, or Typhoons
Storms rotating around a low-pressure center (eye). Rotation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Tropical storms have winds below 120 km/h. Intense sunlight warms seawater, causing evaporation and convection, forming large vertical storm clouds. Cyclonic swells can flood coasts. Hurricanes weaken over land but can regain strength over water. Dangers include wind speed, torrential rain floods, and cyclonic swells.
The Naming of Hurricanes
Since 1953, the WMO uses alphabetical lists of names, initially female, but including male names since 1978.
Hurricanes and Climate Change
Some studies suggest increased hurricane frequency and intensity due to rising sea surface temperatures. Others find no significant change in frequency. The influence of climate change on hurricane activity is not yet fully understood.