question: why does 20° in pools feel so much colder than 20° in a lake?
comment: Probably because there's a greater contrast in temperature between the pool and the air than a lake and its surrounding air. You feel the contrast more so it seems colder.
passages: BULLET::::- In lakes exposed to geothermal activity, the temperature of the deeper water may be warmer than the surface water. This will usually lead to convection currents.
  
  BULLET::::- Water at near-freezing temperatures is less dense than slightly warmer water - maximum density of water is at about 4°C - so when near freezing, water may be slightly warmer at depth than at the surface.
   Heat transfers very well into water, and body heat is therefore lost extremely quickly in water compared to air, even in merely 'cool' swimming waters around 70F (~20C). A water temperature of can lead to death in as little as one hour, and water temperatures hovering at freezing can lead to death in as little as 15 minutes. This is because cold water can have other lethal effects on the body, so hypothermia is not usually a reason for drowning or the clinical cause of death for those who drown in cold water.
   Section::::Important abiotic factors.:Temperature.
  
  Temperature is an important abiotic factor in lentic ecosystems because most of the biota are poikilothermic, where internal body temperatures are defined by the surrounding system. Water can be heated or cooled through radiation at the surface and conduction to or from the air and surrounding substrate. Shallow ponds often have a continuous temperature gradient from warmer waters at the surface to cooler waters at the bottom. In addition, temperature fluctuations can vary greatly in these systems, both diurnally and seasonally.
   Summer afternoon temperatures reach well into the nineties at Lake Bruin. Night temperatures in the summer drop into the seventies. During winter, high temperatures are in the fifties. On winter nights, the mercury may dip into the thirties.
   A number of materials contract on heating within certain temperature ranges; this is usually called negative thermal expansion, rather than "thermal contraction". For example, the coefficient of thermal expansion of water drops to zero as it is cooled to 3.983 °C and then becomes negative below this temperature; this means that water has a maximum density at this temperature, and this leads to bodies of water maintaining this temperature at their lower depths during extended periods of sub-zero weather. Also, fairly pure silicon has a negative coefficient of thermal expansion for temperatures between about 18 and 120 kelvins.
   Although a temporary seasonal feature, the thermal bar plays an important role in lake ecology by restricting mixing between coastal and offshore waters. This role is particularly evident during the spring runoff period when the retention of coastal waters may benefit aquatic organisms by providing warmer water temperatures and elevated nutrient concentrations, or may threaten coastal environments by retaining pollutants in coastal waters.
  
  Section::::See also.
  
  BULLET::::- Water (molecule)#Density of water and ice
  
  BULLET::::- Limnology
  
  BULLET::::- Water
  
  BULLET::::- Aquatic ecology
  
  Section::::Additional references.
   Temperature-controlled warm-water therapy pools are used to perform aquatic bodywork. For example, Watsu requires a warm-water therapy pool that is approximately chest deep (depending on height of the therapist) and temperature-controlled to about 35 °C (95 °F).
  
  Section::::Facilities, equipment, and supplies.:Dry-water massage tables.
   Floating pool covering systems and separate STCs are used for pool heating.
  
  Pool covering systems, whether solid sheets or floating disks, act as insulation and reduce heat loss. Much heat loss occurs through evaporation, and using a cover slows evaporation.
   In the 1980s Monarch Spas developed the dual-zone "swim spa" so that pumps and other equipment needed for the pool could also be used to power a separate spa. Today, the advantage of the modern "dual-zone" system is that the two pools can be at different temperatures using different chemicals - the Hot Tub (using bromine), is hot enough for relaxation and massage, while the swim zone is cool enough for strenuous exercise (using chlorine).
  
  Section::::Volume-driven machines.
   Unglazed or formed collectors are similar to flat-plate collectors, except they are not thermally insulated nor physically protected by a glass panel. Consequently, these types of collectors are much less efficient when water temperature exceeds ambient air temperatures. For pool heating applications, the water to be heated is often colder than the ambient roof temperature, at which point the lack of thermal insulation allows additional heat to be drawn from the surrounding environment.
  
  Section::::Components.:Collector.:Evacuated tube.
   These splash pools are often isolated from the moderating influence of the ocean and therefore the pools can vary dramatically in environmental factors such as salinity and temperature over the course of hours or days. "T. californicus" has the ability to thrive under these variable environmental conditions (factors that limit predators such as fish to lower pools in the intertidal zone). Temperature in the pools that this copepod inhabits tend to track air temperatures more closely than ocean temperatures and salinities in pools can change as pools evaporate, receive freshwater inputs from rain, or saltwater from wave actions.
   (Illustrated by a still lake, where the surface water can be comfortably warm for swimming but deeper layers be so cold as to represent a danger to swimmers, the same effect as gives rise to notices in London's city docks warning 'Danger Cold Deep Water).
   At the lake surface, the thermal bar may be visible as a foam line between the stratified water shoreward of the thermal bar and unstratified water on the offshore side. At this convergence, waters mix and sink when they reach the temperature of maximum density, roughly 4 degrees Celsius for freshwater, a process known as cabbeling.
   Similar to light zonation, thermal stratification or thermal zonation is a way of grouping parts of the water body within an aquatic system based on the temperature of different lake layers. The less turbid the water, the more light is able to penetrate, and thus heat is conveyed deeper in the water. Heating declines exponentially with depth in the water column, so the water will be warmest near the surface but progressively cooler as moving downwards. There are three main sections that define thermal stratification in a lake. The epilimnion is closest to the water surface and absorbs long- and shortwave radiation to warm the water surface. During cooler months, wind shear can contribute to cooling of the water surface. The thermocline is an area within the water column where water temperatures rapidly decrease. The bottom layer is the hypolimnion, which tends to have the coldest water because its depth restricts sunlight from reaching it. In temperate lakes, fall-season cooling of surface water results in turnover of the water column, where the thermocline is disrupted, and the lake temperature profile becomes more uniform.
   The water temperature in the lakes are always cold, ranging from . This is due to the lakes being large, making it difficult for the sun to influence its temperature owing to their large heat capacity. Smaller lakes can freeze in the winter, particularly those located in the higher elevations.
  
  Section::::Transport.
   Due to the unusual relationship between water's temperature and its density, lakes form layers called thermoclines, layers of drastically varying temperature relative to depth. Fresh water is most dense at about 4 degrees Celsius (39.2 °F) at sea level. When the temperature of the water at the surface of a lake reaches the same temperature as deeper water, as it does during the cooler months in temperate climates, the water in the lake can mix, bringing oxygen-starved water up from the depths and bringing oxygen down to decomposing sediments. Deep temperate lakes can maintain a reservoir of cold water year-round, which allows some cities to tap that reservoir for deep lake water cooling.
   Unlike triathlons, which allow swimmers to wear wetsuits when the water is below a certain temperature (the standard is at the surface or up to for unofficial events.), most open water swim races either do not permit the use of wetsuits (usually defined as anything covering the body above the waist or below the knees), or put wetsuit-clad swimmers in a separate category and/or make them ineligible for race awards. This varies by locales and times of the year, where water temperatures are substantially below comfortable.
  
  Section::::Insulation.
   At night the top metre of surface water loses heat to the cold desert air but insulates the lower layer. The insulated lower layer continues to gain solar energy each day and to accumulate heavy brine from above. Solar Lake develops some of the highest temperature solar-heated waters of any lake: certain layers can reach as high as 60 °C, temperatures matched only by some geothermal lakes. Due to the strong layering during certain times, the thermal gradients can be as high as 18 ºCm.
   The water temperature at the springs is usually below 10 °C (50 °F). Within the rivers and lakes, the water temperature rises up to 20 °C (68 °F). The following example for different water temperatures measured on July 7, 1954 demonstrates large temperature differences within the lakes: Within lake Kozjak at the depth of 4 m a temperature of 18.9 °C (66 °F) has been measured. At a depth of 20 m, 5 °C (41 °F) have been measured. At 44 m depth, virtually at the bottom of the lake, a temperature of 4.1 °C (39 °F) has been measured.
  
  Section::::The dissolution and creation of rock.
   During the process of lake stratification, shallow areas generally become stratified before deeper areas. In large lakes this condition may persist for weeks, during which a temperature front known as a thermal bar forms between the stratified and unstratified areas of the lake. The thermal bar generally forms parallel to shore and moves toward the lake center as deeper areas of the lake stratify. While thermal bars can form in both fall and spring, most studies of the thermal bar have investigated aspects of the feature in the spring, when the lake is warming up and the summer thermocline is beginning to form. 
   BULLET::::- Thermal conductivity of water is higher than that of air. As water conducts heat 20 times more than air, divers in cold water must insulate their bodies with diving suits to avoid hypothermia. Gases used in diving have very different thermal conductivity; trimix conducts heat more than air and argon conducts less heat than air hence the reason many trimix divers inflate their drysuits with argon.
   A pool or unglazed collector is a simple form of flat-plate collector without a transparent cover. Typically polypropylene or EPDM rubber or silicone rubber is used as an absorber. Used for pool heating it can work quite well when the desired output temperature is near the ambient temperature (that is, when it is warm outside). As the ambient temperature gets cooler, these collectors become less effective.
  
  Section::::Solar thermal collectors heating water.:Evacuated flat plate collectors.:Bowl collectors.
   Water's sizable enthalpy of vaporization is why one's skin can be burned so quickly as steam condenses on it (heading from red to green in "Fig. 7 "above). In the opposite direction, this is why one's skin feels cool as liquid water on it evaporates (a process that occurs at a sub-ambient wet-bulb temperature that is dependent on relative humidity). Water's highly energetic enthalpy of vaporization is also an important factor underlying why "solar pool covers" (floating, insulated blankets that cover swimming pools when not in use) are so effective at reducing heating costs: they prevent evaporation. For instance, the evaporation of just 20 mm of water from a 1.29-meter-deep pool chills its water 8.4 degrees Celsius (15.1 °F).
   Liquid covers are also an option. They use a microscopically thin layer of liquid (such as cetyl alcohol) that sits on the water surface and reduces evaporation, which is one of the major sources of heat loss as well as water loss. Unlike other covers, the pool can be used while the liquid cover is in place, and the nontoxic material is safe for people as well as pumping / filtering systems. The liquid must be replenished regularly (monthly or more), and may not be effective in windy areas (since the wind will disperse the thin layer).
  
  BULLET::::- Safety covers
   The water temperature of the lake ranges from about during the summer months to under during the winter months. In the summer, at a depth of , the water temperature is about cooler than at the surface. Surface water temperature is usually about during spring and fall months.
  
raw_presuppositions: 20° in pools should not feel so much colder than 20° in a lake. 20° in pools should not feel so much colder than 20° in a lake.
raw_corrections: Since there is a greater contrast in temperature between the pool and its surrounding air than a lake and its surrounding air, one feels the contrast more making it seem colder. Since there is a greater contrast in temperature between the pool and its surrounding air than a lake and its surrounding air, one feels the contrast more making it seem colder.