Rain

What Exactly Is Rain?

Rain, that gentle yet powerful force of nature, is a form of precipitation where water droplets condensed from atmospheric vapor fall under gravity. Have you ever wondered how rain forms and why it’s so crucial for our planet? It all starts with moisture moving along temperature and moisture contrasts in weather fronts, creating clouds like cumulonimbus that can organize into narrow rainbands.

The Formation of Rain

In mountainous areas, heavy precipitation occurs due to upslope flow. On the leeward side, desert climates exist, while urban heat islands lead to increased rainfall downwind of cities. Global warming is causing changes in precipitation patterns, with some regions experiencing more rain and others facing droughts.

Understanding Rainfall Parameters

The globally averaged annual precipitation over land is 715 mm, but over the whole Earth, it’s much higher at 990 mm. Climate classification systems use average annual rainfall to differentiate between climate regimes. Rainfall can be measured using rain gauges and estimated by weather radar.

The Science Behind Raindrops

Water vapor in the air becomes saturated when cooled, leading to condensation that forms clouds. Cooling mechanisms include adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling. Water vapor is added through wind convergence, precipitation, evaporation from surfaces, transpiration, and lifting air over mountains.

Clouds and Raindrops

Elevated weather fronts create clouds with upward motion, forming stable cloud decks like stratus. Coalescence occurs when water droplets fuse to form larger droplets in clouds above freezing. In clouds below freezing, ice crystals fall due to temperature and mass requirements. Raindrop sizes range from 0.1-9 mm, developing an oblate shape as they increase in size.

Terminal Velocity of Raindrops

Raindrops impact at their terminal velocity, which is greater for larger drops due to their mass-to-drag ratio. Imagine a raindrop falling from the sky; it’s like a tiny parachute, but with its own unique journey. The final droplet size distribution follows an exponential distribution, with the number of droplets per unit volume decreasing exponentially with diameter. Parameters of rainfall are temperature-dependent and scale with the rate of rainfall.

Rainfall Patterns and Their Impact

The distribution of raindrop sizes tends to fit averaged rainfall but can deviate for small droplets and different conditions. Rainbands in advance of warm occluded fronts and warm fronts are associated with weak upward motion, and tend to be wide and stratiform in nature.

Urban Influence on Rainfall

Rainfall rates downwind of cities increase between 48% and 116%. Monthly rainfall is about 28% greater between 32 and 64 km downwind of cities compared to upwind. Cities can induce a total precipitation increase of 51%. Increasing temperatures tend to increase evaporation, leading to more precipitation.

Global Precipitation Trends

Precipitation has increased over land north of 30°N but declined in the tropics since the 1970s. Eastern regions have become wetter, while the Sahel and parts of southern Asia have become drier. Heavy precipitation events have increased worldwide, as well as droughts in the tropics and subtropics.

Acidity and Rainfall

The acidity of rain varies depending on its origin. Rain that is derived from the Atlantic Ocean typically has a pH of 5.0–5.6; rain that comes across the continental from the west has a pH of 3.8–4.8; and local thunderstorms can have a pH as low as 2.0.

Acid Rain Formation

Sulfuric acid and nitric acid are the two strong acids primarily responsible for making rain acidic. Sulfuric acid is derived from natural sources such as volcanoes, and wetlands; and anthropogenic sources such as the combustion of fossil fuels, and mining where H2S is present. Nitric acid is produced by natural sources such as lightning, soil bacteria, and natural fires; while also produced anthropogenically by the combustion of fossil fuels and from power plants.

Acid Rain Trends

The concentrations of nitric and sulfuric acid have decreased in presence of rainwater over the past 20 years, which may be due to the significant increase in ammonium (most likely as ammonia from livestock production), which acts as a buffer in acid rain and raises the pH.

Climate Classification

The Köppen climate classification depends on average monthly values of temperature and precipitation. The most commonly used form of the Köppen classification has five primary types labeled A through E: tropical, dry, mild mid-latitude, cold mid-latitude, and polar.

Climate Subtypes

The five primary classifications of climate can be further divided into secondary classifications such as rain forest, monsoon, tropical savanna, humid subtropical, humid continental, oceanic climate, Mediterranean climate, steppe, subarctic climate, tundra, polar ice cap, and desert.

Measuring Rainfall

Rainfall is measured in units of length per unit time, typically in millimeters per hour or inches per hour. Rain gauges include standard rain gauges, wedge gauges, tipping bucket rain gauges, weighing rain gauges, and homemade can-based gauges. Precipitation measurements can be submitted through the Internet through networks such as CoCoRAHS or GLOBE.

Weather Radar

Weather radar is used to assess precipitation over large basins for hydrological purposes. For instance, river flood control, sewer management and dam construction are all areas where planners use rainfall accumulation data. Radar-derived rainfall estimates complement surface station data which can be used for calibration.

Rainfall Intensity

Rainfall intensity is classified according to the rate of precipitation, which depends on the considered time. Categories include light rain (< 2.5 mm/h), moderate rain (2.5-7.6 mm/h), heavy rain (> 7.6 mm/h), and violent rain (> 50 mm/h).

Return Periods of Rainfall

The average time between occurrences of an event with a specified intensity and duration is called the return period. The return period is often expressed as an n-year event, such as a 10-year storm or a 100-year storm. As with all probability events, it is possible, though improbable, to have multiple 100-year storms in a single year.

Quantitative Precipitation Forecast (QPF)

The Quantitative Precipitation Forecast (QPF) predicts the expected amount of liquid precipitation accumulated over a specified time period over a specified area. QPFs consider terrain and synoptic hours when forecasting, and can be generated on a quantitative or qualitative basis.

Impact of Rainfall

Rainfall has a significant impact on agriculture, requiring varying amounts of rainfall to survive. Too much or too little rain can be harmful, causing drought or erosion. In areas with wet and dry seasons, soil nutrients diminish and erosion increases during the wet season.

Cultural Significance

Global climatology shows that approximately 505,000 km³ of water falls as precipitation globally each year. Deserts are defined as areas with less than 250 mm (10 in) average annual precipitation per year.

Polar Deserts

Rain rarely falls when surface temperatures are below freezing. Frigid climates see little rainfall and are known as polar deserts. Antarctica is the world’s driest continent due to no rain.

Conclusion

In conclusion, rain is not just a simple phenomenon but a complex interplay of atmospheric conditions that shape our planet in countless ways. From its formation and measurement to its impact on climate and culture, understanding the intricacies of rain can help us better appreciate this essential natural resource. Whether it’s soothing or challenging, rain remains an integral part of our lives, reminding us of nature’s power and beauty.