Precipitation: Rain, Snow and Hail

Introduction

Clouds contain huge amounts of water, yet many clouds drift past without a single drop falling. For water to return to the ground, the tiny droplets in a cloud must grow large enough to fall through the rising air that holds them up. When they do, water arrives as rain, drizzle, snow, sleet or hail — together called precipitation . Precipitation is the step of the water cycle that brings fresh water back to land, refilling rivers, lakes, soils and aquifers.

Core explanation

Why cloud droplets do not fall. A cloud droplet is about 0.01 mm across. It is so light that air resistance and gentle updraughts keep it suspended, and even in still air it would fall only about a centimetre per second. A typical raindrop is about 2 mm across — around a million times the volume of a cloud droplet. So the key question is how droplets grow.

Growth by coalescence (warm clouds). In clouds warmer than 0 °C, droplets of different sizes move at different speeds, collide and merge. This joining is called coalescence . Larger drops fall faster, sweep up more small droplets and grow quickly. When they are too heavy for the updraughts, they fall as rain . Very small drops falling from low clouds form drizzle .

Growth through ice crystals (cold clouds). In many clouds, especially outside the tropics, the upper parts are below 0 °C. Surprisingly, tiny droplets can stay liquid well below 0 °C (they are supercooled ). Where ice crystals are present, water vapour deposits on them more readily than on the droplets, so the ice crystals grow at the droplets' expense. The crystals stick together into snowflakes .

What reaches the ground depends on the air below.

- Snow — the air all the way down is cold enough for the flakes not to melt. - Rain — snowflakes fall through warmer air and melt. Much rain in Britain starts as snow high up. - Sleet — flakes partly melt, falling as a mixture of rain and snow (in some countries sleet means small ice pellets formed when raindrops refreeze). - Hail — forms in tall thunderclouds with very strong updraughts. Ice pellets are carried up and down repeatedly, collecting a new layer of frozen water each trip, like an onion. When they become too heavy, they fall as hailstones , sometimes several centimetres across.

Measuring precipitation. A rain gauge collects precipitation in a funnel over a known area. The amount is recorded as a depth in millimetres, as if the water had spread over flat ground without soaking in or running off.

Precipitation is fresh. Because it comes from evaporated and condensed water, precipitation is almost pure. It does, however, dissolve some carbon dioxide from the air, which makes natural rain slightly acidic, with a pH of about 5.6.

Step-by-step reasoning

To explain how a snowflake can become a raindrop:

1. Ice crystals grow in the cold upper part of a cloud and join into snowflakes. 2. The snowflakes become too heavy for the updraughts and fall. 3. They pass through air warmer than 0 °C nearer the ground. 4. They melt and reach the ground as rain.

Visual explanation

Picture a tall cloud drawn with a horizontal line marking 0 °C. Above the line, six-sided crystals grow and clump into flakes. Below the line, the flakes turn into round drops. In a separate thundercloud, a curved arrow loops a small ice ball up and down, adding a ring each time.

Real-world analogy

A raindrop grows like a snowball rolling downhill: the bigger it gets, the more it collects on the way. Small cloud droplets are like loose snow that never gathers until one larger ball starts rolling and sweeping them up.

Real-world example

Mountain ranges often create a wet side and a dry side. In the Lake District, moist westerly winds rise over the fells and drop large amounts of precipitation — some places receive more than 3000 mm a year — while areas further east, in the "rain shadow", are much drier.

Why?

Why is hail most common in summer thunderstorms rather than in winter? Hail needs very strong updraughts to hold ice pellets in the cloud long enough to grow. Strong updraughts come from intense convection, which happens when the ground is heated strongly on hot days.

Common misconception

"Hail is just frozen rain." Hailstones grow in layers while being lifted and dropped many times inside a thundercloud; they are much larger and more complex than simply frozen raindrops.

Worked example

Question: A rain gauge funnel has an area of 100 cm² and collects 50 cm³ of water in a day. What was the rainfall in millimetres?

Reasoning: Depth = volume ÷ area = 50 cm³ ÷ 100 cm² = 0.5 cm. Since 1 cm = 10 mm, 0.5 cm = 5 mm.

Answer: 5 mm of rain.

Quick check

1. What name is given to the merging of colliding cloud droplets? Answer: Coalescence.

Exam focus

Be able to explain why droplets must grow before they fall, and to describe the formation of rain, snow and hail. Link precipitation to the water cycle: it returns fresh water to the land after condensation in clouds.

Advanced insight

Cloud seeding attempts to encourage precipitation by adding particles, such as silver iodide, that act as ice nuclei in supercooled clouds. Ice crystals then form and grow at the expense of the droplets. Its effectiveness is still debated, because it is hard to prove how much rain would have fallen anyway.

Summary

Precipitation is water falling from clouds as rain, drizzle, sleet, snow or hail. Tiny cloud droplets must grow before they fall: by coalescence in warm clouds, or through ice crystals growing in cold clouds. The temperature of the air below decides whether snow melts into rain. Hail grows in layers in strong thunderstorm updraughts. Precipitation returns fresh water to the land.

Practice questions

1. Why do cloud droplets not fall to the ground straight away? Answer: They are so small and light that air resistance and updraughts keep them suspended. 2. Describe how raindrops form in a warm cloud. Answer: Droplets collide and coalesce; larger drops fall faster, collect more droplets and eventually become heavy enough to fall as rain. 3. Explain the difference between how snow and rain reach the ground from the same cold cloud. Answer: If the air below stays below 0 °C, snowflakes arrive as snow; if they fall through warmer air they melt and arrive as rain. 4. Explain how hailstones become layered. Answer: Strong updraughts carry ice pellets up and down through the cloud many times, and each trip adds a new layer of frozen water. 5. Why is natural rain slightly acidic? Answer: It dissolves carbon dioxide from the air, forming a weak acid, giving a pH of about 5.6.