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Temperature

Temperature. Prof. John Toohey-Morales, CCM St. Thomas University Miami Gardens, Florida. Daytime Warming. Sun’s radiant energy heats the ground Ground warms the air by conduction Air rises (thermals) to warm the air above by convection

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Temperature

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  1. Temperature Prof. John Toohey-Morales, CCM St. Thomas University Miami Gardens, Florida

  2. Daytime Warming • Sun’s radiant energy heats the ground • Ground warms the air by conduction • Air rises (thermals) to warm the air above by convection • Calm days: Thermals are small so the air at the surface is much hotter • Windy days: Mechanical stirring causes forced convection and warm air is mixed upwards

  3. Daytime Warming (continued)

  4. Lag in Temperature • Noon is when the Sun’s rays are the most intense, but NOT the peak of outgoing IR • Earth continues to absorb solar radiation past noon • Maximum temperature occurs as outgoing IR energy and incoming solar energy balance

  5. Hottest Places • Deserts because of sand, low humidity, clear sky and lack of vegetation • 136°F (58°C) El Azizia, Libya • 134°F (57°C) Death Valley, California • Phoenix, AZ average July hi temp is 105°F • Atlanta, GA is at the same latitude as Phoenix but July average hi temp is 87°F • Humidity, haze, clouds make days cooler

  6. Nighttime Cooling • During radiational cooling ground cools faster than air • Air in contact with ground gets colder before the air above: nocturnal inversion • Ideal conditions include long, clear nights, calm winds and dry air

  7. Coolest Time and Places • IR radiation continues as sun rises and air may still cool further as sunlight is too dim / dew evaporates • Dense, cold air drains into valleys, making them colder than surroundings • Record cold temperatures • Minus 129°F (-89°C) in Vostok, Antarctica • Minus 80°F (-62°C) in Prospect Creek AK

  8. Protecting Agriculture • Crops can be severely damaged by cold • Wind Machines are fans used to mix cold ground air with warmer air above • Sprinklers can coat buds & fruit with water that can turn to ice. The latent heat from freezing keeps the ice temperature at 32°F, protecting the fruit at a higher temperature than the subfreezing air.

  9. Temperature Controls • Latitude (sunshine), land & water distribution (specific heat), ocean currents, elevation

  10. Temperature Data • Diurnal Range: Difference between maximum & minimum daily temperature • HIGH: deserts • LOW: humid regions, water, urban heat island • Mean Daily Temperature: Average of the high and low temp for a 24-hour period • “Normal” temperature is a 30-year average • Annual Range: Difference between average temperature of the warmest and coldest months • HIGH: mid-high latitudes LOW: near the equator

  11. A Tale of Two Cities • Mean Annual Temp: Average of the 12 monthly mean temps • San Francisco, CA & Richmond, VA have the same mean annual temperature (57°F), but at 21°F Richmond’s average January low is much lower than the coldest ever recorded in San Francisco

  12. Sensible Temperature • Defined as the temperature we perceive • Constant exchange of heat between the body’s skin and the environment • Body radiates, conducts and initiates convection so that a thin layer of warm air molecules forms close to the skin, protecting it from the surrounding cold air • In calm air, a cold day feels “warmer”

  13. Wind Chill Index (WCI) • When the wind blows, the insulating layer of air near the skin is swept away & heat is rapidly removed by cold air • WCI: How cold the wind makes us feel

  14. Moisture and The Way We Feel • Cold, rainy, drizzly, foggy days feel colder than “dry” days because water on the skin conducts heat away better than air • Hot, humid days feel hotter than hot / dry days because perspiration can’t be readily evaporated due to near-saturation of the surrounding air (less evaporational cooling)

  15. Measuring Temperature • It is impossible to measure air temperature accurately in direct sunlight because the thermometer absorbs radiant energy from the sun in addition to energy from the air molecules (needs shelter) • Liquid-in-glass (mercury or alcohol) & electrical thermometers are calibrated to insure uniform readings

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