question: Why do our noses run when it’s cold?
comment: It's your body putting out fluids to keep the air you're breathing at a safe level of humidity and moisture for your lungs. If the air is too dry, your lungs can get damaged and prone to infection. When it's cold, the air gets drier, so your nose may start running to help keep the air moist. Of course - colds are also more common in cold weather. Your nasal passages get dried out and damaged, which means viruses can get into your body more easily, causing colds. Additionally, you spend more time inside with the windows closed around other people who are more susceptible to colds. So, you may find you just get more colds - causing runny noses - in the winter as well.
passages: During cold, dry seasons, the mucus lining nasal passages tends to dry out, meaning that mucous membranes must work harder, producing more mucus to keep the cavity lined. As a result, the nasal cavity can fill up with mucus. At the same time, when air is exhaled, water vapor in breath condenses as the warm air meets the colder outside temperature near the nostrils. This causes an excess amount of water to build up inside nasal cavities. In these cases, the excess fluid usually spills out externally through the nostrils.
  
  Section::::Respiratory system.:Lower respiratory tract.
   Section::::Sense of direction.
  
  The wet nose of dogs is useful for the perception of direction. The sensitive cold receptors in the skin detect the place where the nose is cooled the most and this is the direction a particular smell that the animal just picked up comes from.
  
  Section::::Structure in air-breathing forms.
   In the lungs a temperature of 37 °C and 100% relative humidity (RH) is the ideal condition for the ciliary activity. If the conditions are too warm or cold, the cilia beat slower and at some point not at all. During normal nasal inspiration, air of 22 °C and 40% RH is conditioned into air of 32 °C and 99% RH at the level of the trachea.
   We use our nose and throat as a regenerative heat exchanger when we breathe. The cooler air coming in is warmed, so that it reaches the lungs as warm air. On the way back out, this warmed air deposits much of its heat back onto the sides of the nasal passages, so that these passages are then ready to warm the next batch of air coming in. Some animals, including humans, have curled sheets of bone inside the nose called nasal turbinates to increase the surface area for heat exchange.
  
  Section::::Types of regenerators.:Cryogenics.
   Section::::Organisation of the barrel fields.:Barrels of the major facial whiskers.
   Nasal hair
  
  Nasal hair or nose hair, or vibrissae, is the hair in the nose. Adult humans have hair in the interior nasal passage. Nasal hair functions include filtering foreign particles from entering the nasal cavity and collecting moisture. In support of the first function, the results of a 2010 study indicated that increased nasal hair density decreases development of asthma in those who have seasonal rhinitis, possibly due to an increased capacity of the faces to filter out pollen and other allergens.
   Section::::Structure.:Blood supply.
  
  There is a rich blood supply to the nasal cavity. In some animals, such as dogs, the capillary beds flowing through the nasal cavity help cool the blood flow to the brain.
  
  Blood supply comes from branches of both the internal and external carotid artery, including branches of the facial artery and maxillary artery. The named arteries of the nose are:
  
  BULLET::::- Sphenopalatine artery and greater palatine artery, branches of the maxillary artery.
  
  BULLET::::- Anterior ethmoidal artery and posterior ethmoidal artery, branches of the ophthalmic artery
   Rhinorrhea is especially common during winter months and certain low temperature seasons. Cold-induced rhinorrhea occurs due to a combination of thermodynamics and the body's natural reactions to cold weather stimuli. One of the purposes of nasal mucus is to warm inhaled air to body temperature as it enters the body. In order for this to happen, the nasal cavities must be constantly coated with liquid mucus. During cold, dry seasons, the mucus lining nasal passages tends to dry out, meaning that mucous membranes must work harder, producing more mucus to keep the cavity lined. As a result, the nasal cavity can fill up with mucus. At the same time, when air is exhaled, water vapor in breath condenses as the warm air meets the colder outside temperature near the nostrils. This causes an excess amount of water to build up inside nasal cavities. In these cases, the excess fluid usually spills out externally through the nostrils.
   Excessive moisture as tears collected in the lacrimal sac travel down the nasolacrimal ducts where they drain into the inferior meatus in the nasal cavity.
  
  Section::::Structure.:Nose shape.
  
  The shape of the nose varies widely due to differences in the nasal bone shapes and formation of the bridge of the nose. Some nose shapes were classified for surgeries by Eden Warwick in "Nasology" 1848:
   Dogs and other canids also possess a very well-developed set of nasal turbinates, an elaborate set of bones and associated soft-tissue structures (including arteries and veins) in the nasal cavities. These turbinates allow for heat exchange between small arteries and veins on their maxilloturbinate surfaces (the surfaces of turbinates positioned on maxilla bone) in a counter-current heat-exchange system. Dogs are capable of prolonged chases, in contrast to the ambush predation of cats, and these complex turbinates play an important role in enabling this (cats only possess a much smaller and less-developed set of nasal turbinates). This same complex turbinate structure helps conserve water in arid environments. The water conservation and thermoregulatory capabilities of these well-developed turbinates in dogs may have been crucial adaptations that allowed dogs (including both domestic dogs and their wild prehistoric ancestors) to survive in the harsh Arctic environment and other cold areas of northern Eurasia and North America, which are both very dry and very cold.
   Section::::Distribution.
   Clive Finlayson of the Gibraltar Museum said the large Neanderthal noses were an adaption to the cold, Todd C. Rae of the American Museum of Natural History said primate and arctic animal studies have shown sinus size reduction in areas of extreme cold rather than enlargement in accordance with Allen's rule. Therefore, Todd C. Rae concludes that the design of the large and Neanderthal nose was evolved for the hotter climate of the Middle East and Africa and remained unchanged when they entered Europe 
   A rapidly dilating arteriolar circulation to these bones may lead to a sharp increase in the pressure within, in response to acute cooling of the body core. The pain from this pressure is often referred to as "brain freeze", and is frequently associated with the rapid consumption of ice cream. The shallowness of the venous blood supply of the mucosa contributes to the ease with which nosebleed can occur.
  
  Section::::Structure.
   In humans, the conchae divide the nasal airway into 4 groove-like air passages, and are responsible for forcing inhaled air to flow in a steady, regular pattern around the largest possible surface area of nasal mucosa. As a ciliated mucous membrane with shallow blood supply, the nasal mucosa cleans and warms the inhaled air in preparation for the lungs.
   Section::::Research.:Infection.:Signs of disease.
  
  The signs observed with WNS include unusual winter behavior like abnormally frequent or abnormally long arousal from torpor (temporary hibernation), flying, loss of body fat, damage and scarring of the wing membranes, and death.
  
  As early as 2011 it was hypothesized that prematurely expending the fat reserves for winter survival may be a cause for death.
  
  Section::::Research.:Infection.:Pathophysiology.
  
  Until December 2014 the cause for the abnormal behavior was unclear, as no physiological data linking altered behavior to hypothesized increased energy demands existed. 
   There is a nasal valve area in the cavity responsible for providing resistance to the flow of air. This enables an increased time for warming and moistening the air. This area is of two nasal valves. The internal nasal valve is the narrowest part of the airway in the middle third of the cavity. The larger external nasal valve is located in the alar wall.
  
  Section::::Structure.:Internal nose.:Nasal vestibule.
   Section::::Nerve supply.:Parasympathetic supply.
   Miquel Hernández of the Department of Animal Biology at the University of Barcelona said the "high and narrow nose of Eskimos" and "Neanderthals" is an "adaption to a cold and dry environment", since it contributes to warming and moisturizing the air and the "recovery of heat and moisture from expired air".
  
  Section::::Neoteny.
   Reindeer have specialised counter-current vascular heat exchange in their nasal passages. Temperature gradient along the nasal mucosa is under physiological control. Incoming cold air is warmed by body heat before entering the lungs and water is condensed from the expired air and captured before the reindeer's breath is exhaled, then used to moisten dry incoming air and possibly be absorbed into the blood through the mucous membranes. Like moose, caribou have specialised noses featuring nasal turbinate bones that dramatically increase the surface area within the nostrils.
  
  Section::::Physical characteristics.:Hooves.
   Section::::Whisker barrel neurophysiology.
   The conchae comprise most of the mucosal tissue of the nose and are required for functional respiration. They are enriched with airflow pressure and temperature-sensing nerve receptors (linked to the trigeminal nerve route, the fifth cranial nerve), allowing for tremendous erectile capabilities of nasal congestion and decongestion, in response to the weather conditions and changing needs of the body. In addition, the erectile tissue undergoes an often unnoticed cycle of partial congestion and decongestion called the nasal cycle. The flow of blood to the nasal mucosa in particular the venous plexus of the conchae is regulated by the pterygopalatine ganglion and heats or cools the air in the nose.
   Hunting reaction
  
  The hunting reaction or hunting response is a process of alternating vasoconstriction and vasodilation in extremities exposed to cold. The term Lewis reaction is used too, named after Thomas Lewis, who first described the effect in 1930.
   These polar stratospheric clouds form during winter, in the extreme cold. Polar winters are dark, consisting of three months without solar radiation (sunlight). The lack of sunlight contributes to a decrease in temperature and the polar vortex traps and chills air. Temperatures hover around or below −80 °C. These low temperatures form cloud particles. There are three types of PSC clouds—nitric acid trihydrate clouds, slowly cooling water-ice clouds, and rapid cooling water-ice (nacreous) clouds—provide surfaces for chemical reactions whose products will, in the spring lead to ozone destruction.
   One study into the learning ability of dogs compared to wolves indicated that dogs have a better sense of smell than wolves when locating hidden food, but there has yet been no experimental data to support this view.
  
  The wet nose, or rhinarium, is essential for determining the direction of the air current containing the smell. Cold receptors in the skin are sensitive to the cooling of the skin by evaporation of the moisture by air currents.
  
  Section::::Senses.:Taste.
   The nose is the first organ of the upper respiratory tract in the respiratory system, and its main function is the supply and conditioning of inhaled air to the rest of the respiratory tract and the lungs. Another function is to filter the air by removing particulates. Nasal hair in the nostrils traps large particles preventing their entry into the lungs.
  