TemperatureTemperature

Prof. John Toohey-Morales, Prof. John Toohey-Morales, CCMCCM

St. Thomas UniversitySt. Thomas University

Miami Gardens, FloridaMiami Gardens, Florida

Daytime WarmingDaytime Warming

• Sun’s radiant energy heats the groundSun’s radiant energy heats the ground

• Ground warms the air by conductionGround warms the air by conduction

• Air rises (thermals) to warm the air above Air rises (thermals) to warm the air above by convectionby convection– Calm days: Thermals are small so the air at Calm days: Thermals are small so the air at

the surface is much hotterthe surface is much hotter– Windy days: Mechanical stirring causes Windy days: Mechanical stirring causes

forced convection and warm air is mixed forced convection and warm air is mixed upwardsupwards

Daytime Warming Daytime Warming (continued)(continued)

Lag in TemperatureLag in Temperature

• Noon is when the Sun’s Noon is when the Sun’s rays are the most rays are the most intense, but NOT the intense, but NOT the peak of outgoing IRpeak of outgoing IR

• Earth continues to Earth continues to absorb solar radiation absorb solar radiation past noonpast noon

• Maximum temperature Maximum temperature occurs as outgoing IR occurs as outgoing IR energy and incoming energy and incoming solar energy balancesolar energy balance

Hottest PlacesHottest Places

• Deserts because of sand, low humidity, Deserts because of sand, low humidity, clear sky and lack of vegetationclear sky and lack of vegetation– 136136°F (58°C) El Azizia, Libya°F (58°C) El Azizia, Libya– 134°F (57°C) Death Valley, California134°F (57°C) Death Valley, California– Phoenix, AZ average July hi temp is 105°FPhoenix, AZ average July hi temp is 105°F

• Atlanta, GA is at the same latitude as Atlanta, GA is at the same latitude as Phoenix but July average hi temp is 87°FPhoenix but July average hi temp is 87°F– Humidity, haze, clouds make days coolerHumidity, haze, clouds make days cooler

Nighttime CoolingNighttime Cooling

• During radiational During radiational cooling ground cools cooling ground cools faster than airfaster than air

• Air in contact with Air in contact with ground gets colder ground gets colder before the air above: before the air above: nocturnal inversionnocturnal inversion

• Ideal conditions Ideal conditions include long, clear include long, clear nights, calm winds nights, calm winds and dry airand dry air

Coolest Time and PlacesCoolest Time and Places

• IR radiation continues as IR radiation continues as sun rises and air may still sun rises and air may still cool further as sunlight is cool further as sunlight is too dim / dew evaporatestoo dim / dew evaporates

• Dense, cold air drains into Dense, cold air drains into valleys, making them valleys, making them colder than surroundingscolder than surroundings

• Record cold temperaturesRecord cold temperatures– Minus 129Minus 129°F (-89°C) in °F (-89°C) in

Vostok, AntarcticaVostok, Antarctica– Minus 80°F (-62°C) in Minus 80°F (-62°C) in

Prospect Creek AKProspect Creek AK

Protecting AgricultureProtecting Agriculture

• Crops can be severely damaged by coldCrops can be severely damaged by cold• Wind Machines are fans used to mix Wind Machines are fans used to mix

cold ground air with warmer air abovecold ground air with warmer air above• Sprinklers can coat buds & fruit with Sprinklers can coat buds & fruit with

water that can turn to ice. The latent water that can turn to ice. The latent heat from freezing keeps the ice heat from freezing keeps the ice temperature at 32temperature at 32°F, protecting the °F, protecting the fruit at a higher temperature than the fruit at a higher temperature than the subfreezing air. subfreezing air.

Temperature ControlsTemperature Controls• Latitude (sunshine), land & water distribution Latitude (sunshine), land & water distribution

((specific heatspecific heat), ocean currents, elevation), ocean currents, elevation

Temperature DataTemperature Data• Diurnal Range: Difference between maximum Diurnal Range: Difference between maximum

& minimum daily temperature& minimum daily temperature– HIGH: desertsHIGH: deserts– LOW: humid regions, water, urban heat islandLOW: humid regions, water, urban heat island

• Mean Daily Temperature: Average of the high Mean Daily Temperature: Average of the high and low temp for a 24-hour periodand low temp for a 24-hour period– ““Normal” temperature is a 30-year averageNormal” temperature is a 30-year average

• Annual Range: Difference between average Annual Range: Difference between average temperature of the warmest and coldest temperature of the warmest and coldest monthsmonths– HIGH: mid-high latitudesHIGH: mid-high latitudes LOW: near the LOW: near the

equatorequator

A Tale of Two CitiesA Tale of Two Cities

• Mean Annual Temp: Mean Annual Temp: Average of the 12 Average of the 12 monthly mean tempsmonthly mean temps

• San Francisco, CA & San Francisco, CA & Richmond, VA have the Richmond, VA have the same mean annual same mean annual temperature (57temperature (57°F), °F), but at 21°F Richmond’s but at 21°F Richmond’s average January low is average January low is much lower than the much lower than the coldest ever recorded coldest ever recorded in San Franciscoin San Francisco

Sensible TemperatureSensible Temperature

• Defined as the temperature we perceiveDefined as the temperature we perceive• Constant exchange of heat between the Constant exchange of heat between the

body’s skin and the environmentbody’s skin and the environment• Body radiates, conducts and initiates Body radiates, conducts and initiates

convection so that a thin layer of warm convection so that a thin layer of warm air molecules forms close to the skin, air molecules forms close to the skin, protecting it from the surrounding cold protecting it from the surrounding cold airair

• In calm air, a cold day feels “warmer”In calm air, a cold day feels “warmer”

Wind Chill Index (WCI)Wind Chill Index (WCI)• When the wind blows, the insulating layer When the wind blows, the insulating layer

of air near the skin is swept away & heat is of air near the skin is swept away & heat is rapidly removed by cold airrapidly removed by cold air

• WCI: How cold the wind makes us feelWCI: How cold the wind makes us feel

Moisture and The Way We Moisture and The Way We FeelFeel• Cold, rainy, drizzly, foggy days feel Cold, rainy, drizzly, foggy days feel

colder than “dry” days because water on colder than “dry” days because water on the skin conducts heat away better than the skin conducts heat away better than airair

• Hot, humid days feel hotter than hot / Hot, humid days feel hotter than hot / dry days because perspiration can’t be dry days because perspiration can’t be readily evaporated due to near-readily evaporated due to near-saturation of the surrounding air (less saturation of the surrounding air (less evaporational cooling)evaporational cooling)

Measuring TemperatureMeasuring Temperature

• It is impossible to measure air It is impossible to measure air temperature accurately in direct sunlight temperature accurately in direct sunlight because the thermometer absorbs because the thermometer absorbs radiant energy from the sun in addition radiant energy from the sun in addition to energy from the air molecules (needs to energy from the air molecules (needs shelter)shelter)

• Liquid-in-glass (mercury or alcohol) & Liquid-in-glass (mercury or alcohol) & electrical thermometers are calibrated electrical thermometers are calibrated to insure uniform readingsto insure uniform readings