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Natal knots

Question: Do midwives actually tie a knot in the umbilical cord after birth? If not, what surgical procedure do they perform? And what happened in the days before modern medicine standardised the process?

Answer: Umbilical cords consist of three blood vessels and a surrounding medium called Wharton鈥檚 jelly, the whole lot enclosed by a sheath. This makes the cord too thick to be tied in a knot. Today, where medical supplies are readily available, a plastic clamp is used to compress the cord and cut off the blood supply. Then the cord is cut just above the clamp with scissors.

A clean piece of string or anything that can be tied tightly around the cord, such as a strip of leather or strong grass, would serve just as well if clamps aren鈥檛 available. A knife, a piece of sharp flint, or even sharp and determined teeth would do instead of the scissors.

The clamp is taken off the cord after three days, and the cord itself rots through a process of dry gangrene and falls off between five and ten days after birth.

Sarah Carter

Brighton, East Sussex

Answer: To my knowledge, tying an actual knot in the umbilicus after birth has never been practised, certainly not in my experience as a midwifery sister.

These days, at delivery, the midwife places two artery forceps on the umbilical cord. She makes a cut between the two forceps using sterile surgical scissors, to separate the placenta. A plastic cord clip is then attached to the cord on the baby鈥檚 side of the remaining forceps, around 2.5 centimetres from the belly button. The final piece of umbilical cord shrivels up and falls off within a few days.

In the past, a couple of different methods have been used. In the 1960s, during my training, midwives adopted the same procedure but used a sterile rubber band instead of the clip. Before this, midwives would tie the umbilical cord with lengths of string.

Mary Cole,

Midwife, Colchester General Hospital

Answer: When our daughter was born nearly nine years ago, her umbilical cord was sealed off with a small plastic clamp. After a few days the cord shrivelled up and dropped off of its own accord. We then found that the clamp was ideal for holding our muesli bag closed. It lasted for another few years until it eventually broke and we were forced to have another child. His clamp is still going strong.

Rob Ives

Maryport, Cumbria

Heavy or light

Question: What causes different types of rain? Sometimes it comes down in 鈥渟tair rods鈥濃�攍engthened droplets that fall at great speed and bounce high after hitting the ground. Other times there鈥檚 just a misty drizzle that blows aimlessly in the breeze. How can rain fall so heavily that it can cause physical pain, or so lightly that it is just a soaking mist. And how do you get the types in between?

Answer: Elongated stair rods are an illusion. Large drops actually tend to be flattened by air resistance. When they land they are called in Afrikaans (and I understand, in Welsh) 鈥渙ld women with clubs鈥�. The circular sheet of splashing water suggests a wide skirt and the centrally rebounding droplet a cudgel.

Droplet size is the main factor in creating different kinds of rain. This depends on conditions at the time of formation, particularly humidity, temperature and the number of airborne nuclei such as dust particles. For example, moderate numbers of nuclei in moist updrafts tend to promote drops that grow large, because there is plenty of water for them and they cannot come down before they grow heavy enough to fall faster than they are lifted. When nuclei are crowded they compete with each other and can only form small droplets that may evaporate before they reach the ground.

In still air, big drops fall fast and hard. Drops that are about 1 centimetre in diameter reach speeds of around 30 kilometres per hour, at which point their own slipstream tears them into smaller droplets, unless they are partly frozen. This limits raindrop size. However, a large number of falling raindrops can create a downdraft, increasing the downward velocity a drop can achieve without splitting, while strong horizontal winds can more than double the speed of impact.

And remember that kinetic energy rises with the square of the velocity.

Jon Richfield

Somerset West, South Africa

Answer: Rainfall intensity depends mainly on the depth of the cloud and the strength of the updrafts. Rapidly rising air produces fast condensation of water droplets and large amounts of rain, mostly when the cloud extends high enough for ice crystals to form among supercooled water droplets.

Shallow clouds with weak updrafts only give drizzle, which rarely falls faster than 3 metres per second. Large raindrops can reach a terminal velocity of about 10 metres per second. Their fall speed increases with size until the diameter approaches 6 millimetres, at which point wind resistance flattens the base, increases the drag and prevents further acceleration.

However, if the rain is caught in a 鈥渄ownburst鈥� where an air column is descending at 20 metres per second or more, the rain hits the ground harder. Downbursts are often associated with cumulonimbus clouds that contain almost vertical air currents. The weight of precipitation in the cloud may be enough to trigger a downburst.

Rain from deep flat cloud layers is usually caused by slow diagonal ascent along a sloping frontal surface. Such rain is persistent but seldom heavy. This can change if prolonged lifting makes the layer unstable. Then massive turrets containing strong updrafts grow vertically out of the layer. These can produce heavy downpours from a cloud mass which had previously only given moderate rain.

Tom Bradbury

Stroud, Gloucestershire

This week鈥檚 questions

The sandman cometh: My colleagues and I have been wondering, during those rare moments of reflection, what the scientific term is for the yellowish crystalline substance sometimes found encrusted on eyelids when you wake up. Some people call it 鈥渟and鈥� or 鈥渟leep鈥�, but does it have a medical name? What is its composition and why does it form?

Simon Smith

Cardiff

Mr Blobby: This is a bit of a girly question, but what exactly is cellulite? There鈥檚 loads of information on the Web about creams or remedies that promise to miraculously make it disappear, but not much about what it really is.

Cathy Turner

London

Ey up, sing reet: Do birds have regional accents? Do blackbirds on mainland Europe sing the same song as those living in Britain? If they do sound the same then why don鈥檛 we?

David Wright

Exmouth, Devon