1 2 3 4 5 6 7 8 9 10 11 12 SEASONAL AND DAILY TEMPERATURES Chapter 3 Earth revolves in elliptical path around sun every 365 days. Earth rotates counterclockwise or eastward every 24 hours. Earth closest to Sun (147 million km) in January, farthest from Sun (152 million lm) in July. Distance does not have a major effect on the seasons. The amount of energy that reaches the Earths surface is influence by the distance from the Sun, the solar angle, and the length of daylight. When the Earth tilts toward the sun in summer, higher solar angles and longer days equate to high temperatures. Seasons in the Northern Hemisphere Summer solstice: June 21, Sun directly above Tropic of Cancer, Northern Hemisphere days greater than 12 hours Winter solstice: December 21, Sun directly above Tropic of Capricorn, Northern Hemisphere days less than 12 hours Autumnal and Spring Equinox: September 22, Marc 20, Sun directly above Equator, all locations have a 12 hour day Special Topic: First day of winter December 21 is the astronomical first day of winter, sun passes over the Tropic of Capricorn; not based on temperature. Indian Summer Middle of Autumn, especially in eastern 2/3 of US: Large high-pressure area stalls near southeast coast. Clockwise 1
13 14 15 16 17 18 19 20 Middle of Autumn, especially in eastern 2/3 of US: Large high-pressure area stalls near southeast coast. Clockwise flow of air brings warm air from Gulf of Mexico. Seasons in the Southern Hemisphere Opposite timing of Northern Hemisphere Closer to sun in summer but not significant difference from north due to: Greater amount of water absorbing heat Shorter season Local temperature variations Southern exposure: warmer, drier locations because they are facing south. Implications for Vegetation Viniculture Ski slopes Landscaping Architecture Local temperature variations Environmental Issues: Solar Heating In order to collect enough energy from solar power to heat a house, the roof should be perpendicular to the winter sun. For the mid-latitudes the roof slant should be 45-50 Each day like a tiny season with a cycle of heating and cooling Daytime heating Air poor conductor so initial heating only effects air next to ground As energy builds convection begins and heats higher portions of the atmosphere After atmosphere heats from convection high temperature 3-5PM; lag in temperature Properties of soil: its moisture content and vegetative cover, affect the rate of conduction from Earth to atmosphere. 2
21 22 23 24 25 26 27 Properties of soil: its moisture content and vegetative cover, affect the rate of conduction from Earth to atmosphere. Dry desert soil produces the hottest air above it: none of the solar energy is used to evaporate water, it just heats the soil/ sand. Average maximum July temperature: Atlanta: 87 o F Phoenix: 105 o F Nighttime cooling As sun lowers, the lower solar angle causes insolation to be spread across a larger area Radiational cooling as infrared energy is emitted by the Earth s surface Radiation inversion: air near ground much cooler than air above Nighttime cooling Thermal belt: Coldest air settles into valleys, hillsides are warmer thermal belts Protecting crops from cold Cover: Prevents radiation cooling Orchard Heaters (Smudge pots): Set up convection currents close to the ground. Fans: Mix cold ground air with warmer air above. During a freeze, sprinklers can coat branches and buds with a layer of ice. This keeps them at freezing temperature, which is better than the colder temp. of the air. The controls of temperature Latitude: solar angle and day length Land & water Specific heat makes water bodies a heat sink: Eastern shores of the Great Lakes freeze later than the west. Ocean currents: warm and cold currents 3
28 29 30 31 32 33 34 35 36 37 Ocean currents: warm and cold currents Elevation: cooling and increase range Average Temperature Maps Isotherms: Lines of equal temperature Average Temperature Maps Isotherms are closer in the winter: more variations in solar radiation by latitude. Air temperature data Daily, monthly, yearly temperature Daily Range: maximum minus minimum Large ranges occur in dry regions: Reno, NV = 45 o F Small ranges occur in humid regions or near water: Charleston, SC = 18 o F Smaller ranges occur in cities because of urban heat islands Other temperature data: Mean Daily, Annual Range, Mean Annual Air temperature data Special topic: What s normal? Climate normal is the 30 year average for a given temperature variable. Air temperature data The use of temperature data Heating degree-day: people heat when temperature below 65 F If mean daily temperature is 55 F, there is 10 heating degree-days on this day. Cooling degree-day: people cool when temperature above 65 F Growing degree-day: temperature above base temperature for specific crop Air temperature and human comfort Sensible Temperature: Temperature perceived by humans. 4
38 39 40 41 42 43 Sensible Temperature: Temperature perceived by humans. Varies with wind and humidity. Body heats through metabolism Wind-chill index Moving air removes warm thermal layer and increases evaporative cooling. Frostbite and Hypothermia Body cools through emitting infrared energy and evaporation of perspiration Air temperature and human comfort Observation: 1000 degrees Thin air at the top of the atmosphere does not have enough molecules to create a high temperature as measured by a thermometer. Measuring air temperature Thermometers: liquid-in-glass, maximum, minimum, electrical resistance, bimetallic ASOS Stations: Automated Surface Observing Systems (over 900 in use) Meteorological Assimilation Data Ingest System (MADIS) Thermistors Infrared sensors Measuring air temperature Observation: Thermometers in the shade Radiant energy from the Sun in direct sunlight increases the temperature recorded by a sensor. True air temperature measured in the shade. Homework for Chapter 2 Chapter 3 Questions for Review, p. 85 #1, 3, 6, 10-12, 14, 16, 18, 20-22, 27 Chapter 3 Questions for Thought, p. 86 #1, 4, 8, 13 Project for Chapter 3 Project #2: Build a table comparing the average monthly temperatures for Seward with actual monthly temperatures measured by 5
for Seward with actual monthly temperatures measured by our weather station for each month for the years 2004-2010. Calculate monthly averages for the seven years we have measured. Make a graph showing all data and comment on what you have found. Mean monthly temperatures for Seward: http:// www.weather.com/weather/wxclimatology/monthly/68434 6