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Extreme Temperatures and Humidity AOSS 102: Extreme Weather Lecture 8 25 September 2015 Regional Controls on Temperature •  La@tude & Seasons •  Eleva@on •  Land use and heat capaci@es •  Ocean currents & Land-­‐sea distribu@ons Urban Heat Island Specific Heat Capacity The specific heat capacity cp of a solid or liquid is defined as the heat required to raise a unit mass of substance by one degree of temperature. Heat supplied (
) =
mass (m)
x specific heat capacity (cp)
x change in temperature (
)
Thought experiment: Heat supplied, assume constant Heat capacity determines how much the material heats up (assume mass m is constant) Specific Heat Capacities cp of Materials Values are valid for constant pressure condi@ons (isobaric). Mul$ple Choice
Image Quiz
Regional Controls on Temperature •  La@tude & Seasons •  Eleva@on •  Land use and heat capaci@es •  Ocean currents & Land-­‐sea distribu@ons Inland loca@ons (here Winnipeg at 50°N) in the middle of the con@nent have large annual temperature ranges This loca@on is exposed to very cold Arc@c air in the winters that can travel southward. Monthly-­‐mean temperature Fig. 3.14, p. 77
Loca@ons near the ocean (here Bergen, Norway, at 60°N) have moderate annual temperature ranges. Why is this? The ocean serves as a heat buffer (rela@vely cold sea surface temperatures cool the air) in the summer and releases its stored summer heat during the winters, thereby warming the loca@ons near it. Fig. 3.16, p. 79
Loca@ons near the ocean (here San Francisco at 37°N) have moderate annual temperature ranges. But why is San Francisco so cold in comparison to Bergen, despite the fact that it lies south of Bergen? The cold, upwelling deep-­‐
water ocean currents near San Francisco suppress large seasonal varia@ons of the sea surface temperature, which leads to cool summers and mild winters. Fig. 3.17, p. 79
Average Posi@on and Extent of Major Surface Ocean Currents red: warm, blue: cold Fig. 3.15, p. 78
Average Air Temperature Near Sea Level – July (F) Caused by cold California current Fig. 3.13, p. 76
Average Air Temperature Near Sea Level – January (F) Caused by warm Gulf stream Fig. 3.12, p. 76
Wind Chill: Cold Temperatures and Wind Measures of Humidity Vapor pressure (e) expresses the amount of water vapor in terms of the “par@al pressure” that the water vapor molecules exert. Satura=on vapor pressure (es) expresses the vapor pressure at which an air parcel will be saturated. Vapor Pressure: Clausius-­‐Clapeyron Graph
Super-­‐saturated: RH > 100% Saturated: RH = 100%, satura@on vapor pressure es Sub-­‐saturated: RH < 100% Image Quiz
Absolute Humidity & Rela@ve Humidity •  Absolute humidity describes the mass of water vapor in a fixed volume of air, or the water vapor density. Mass of H2Ov
Absolute Humidity = Volume of Air Parcel
•  Rela@ve Humidity: Ra@o between the actual amount of water vapor in the air to the amount of water vapor required for satura@on (forma@on of water droplets at a given temperature and pressure). Rela@ve Humidity versus Temperature If we do not change the amount of moisture in the atmosphere, rela@ve humidity will increase as air cools, and rela@ve humidity will decrease as air warms. Dew Point Temperature •  Dew point temperature (or simply dew point) represents the temperature to which the air must be cooled in order for satura@on to occur. •  If the temperature and dew point temperature of an air parcel are iden@cal, then the air is saturated (100% rela@ve humidity). Temperature and dew points are iden@cal in cloudy or foggy condi@ons: 100% rela@ve humidity. Image Quiz
Average surface dew-­‐point temperatures (F) in January Cold, dry winds from Canada keep the Plains dry Winter
©2012 P. Samson -­‐ University of Michigan Average surface dew-­‐point temperatures (F) in July Mountains shield region from significant amounts of moisture from the west Rather dry Warm and humid: Muggy Caused by warm moist air Summer
from the Gulf of Mexico ©2012 P. Samson -­‐ University of Michigan Dew Point Extremes Extremes occur near large bodies of very warm water. Cause discomfort since body can not cool effec@vely: leads to high wet bulb temperature.