UNIT VI: METEOROLOGY Review Book pp. 113-138 This ppt has been modified (Aug.2012) from one created by Mr. Elliott from Fort Plain HS. Objectives #1 & 2 1. What is weather and how can it be described? 2. Explain the atmospheric variables: temperature, dewpoint, relative humidity, air pressure and wind. Meteorology: The study of our atmosphere, weather and forecasting. meteorologist Weather: Short-term conditions of our atmosphere. Most weather takes place in the troposphere and is driven by the unequal heating of the Earth by the sun. Unequal heating creates density differences in the atmosphere resulting in weather! P. 14 in ESRT’s Four Basic Weather Variables Temperature Air Pressure Weather Wind Humidity Weather Variables Temperature: The average kinetic energy of molecules in a substance. (measured with thermometer in oC, oF or K) Warming Cooler Hotter Temperature is not the same as heat! Heat: The total kinetic energy of all the atoms in a substance. (measured in calories or joules) Example: A teaspoon of boiling water has a higher temperature (is hotter) than a bathtub full of lukewarm water. However, the bathtub water has more heat energy because _____________________________the total volume of moving water ________________________________________________________.molecules is much greater ( a lot of mild temperature vs a little hot) The air in Earth’s atmosphere is heated by the sun raising its temperature. Warm air rises and evaporates water while cool air sinks and condenses forming precipitation! Temperature is affected by: Intensity and duration of sunlight, different surface materials (land vs. water), altitude, cloud cover & cold/warm fronts. Additional Temp Notes Highest temp Usually 2-3pm Lowest temp Usually ½ hour before sunrise. Coldest conditions Clear and high pressure. Warmest conditions Partly cloudy and low pressure. Objective #3 Relate dew point, air temperature, saturation and relative humidity and use the charts on ESRT p.12 to find them. Humidity: The actual amount of water vapor in the air. Dewpoint: The temperature at which water vapor in the air saturates the air and begins to condense. Which temperature to the left is the dew-point temperature? Without changing the 30 °C temperature, is When the air there anything else that temperature reaches the could change to raise dewpoint, the air is the 28% relative holding as much water humidity to 100%? as is possible and is said to be saturated. Humidity: The actual amount of water vapor in the air. Look at the graph at the right to see the relationship between the amount of water the air can hold and temperature! Warmer the air, the more water the air can hold. Cold air has the capacity to hold less water. Fact: There’s about 3,100 cubic miles of water vapor in the troposphere right now! (some as clouds, some as precipitation) Relative Humidity: The actual amount of water vapor in the air compared to the maximum amount possible at a given temperature.Both dewpoint and relative When the humidity are found in ESRT p.12. R.H. is 50%, the air is holding half of the water vapor it is capable of holding. Lowest R.H.=0% 100 % means “Saturated” Highest R.H.=100% Air Temp= Dewpoint Temp Note: Dew-point temperature is also called the “dew point” because dew or frost forms on exposed surfaces when the outside temperature falls to the dew-point. Water Vapor Two things can raise the relative humidity to 100%: 1. Cooling the air down to the dew point 2. Adding more water vapor to the air (↑ dew point) So as the difference between the dew point and air temperature decreases…. The r.h. increases and there is a greater probability of water vapor condensing to form precipitation. Measuring Dew point and Relative Humidity Sling Psychrometer: Instrument used to find the relative humidity and dew point. Wet Bulb Thermometer Dry Bulb Thermometer Slinging cools the wet bulb thermometer (evaporation cools) while the dry bulb just measures the regular air temperature. The amount of evaporation depends on the moisture content of the air. The lower the moisture content of the air, the more evaporation will occur from the wet bulb and the cooler the wet bulb temperature will be. The greater the difference between the wet and dry bulb temperatures, the drier the air. What’s true of the relative humidity, even after slinging, if the two thermometers are equal in temperature? It’s 100% or “saturated” ESRT p.12 ESRT p.12 Barometric or Air Pressure: The force exerted by the weight of air above and around us pushing down. Lowest pressure (less air) Highest pressure (more air) Fact: Earth’s atmosphere has a mass of 5,000,000,000 tons! Barometer: Instrument that measures air pressure. Units are usually in millibars (mb) or inches (in) of mercury. Mercurial barometer Aneroid barometer Home-made barometers are easy to make! ESRT p.13 Standard Atm. Pressure: The average air pressure when measured at sea level is: ______one atmosphere __________1013.2 mb of Hg __________29.92 in of Hg Isobars: Lines connecting points of equal air pressure on a weather map Pressure Map Wind: The horizontal flow of air Winds are named for the direction that they come from! Anemometer: measures wind speed Wind vane: measures wind direction What direction does a north wind blow?__________south Wind moving southeast is called a ____________northwest wind. Air’s ability to hold water vapor vs. temp. The amount of water that air can hold is directly related to the temperature of the air. So the warmer the air, the more water that the air can hold. So warm air has a greater water holding capacity than cool air. Warm air is less dense and has more space. For every 10oC increase, air can hold about twice as much water vapor. Important weather rule: Warm air has a higher potential to hold water vapor than cold air! This is exactly why some summer days can feel so uncomfortably humid! The warm air truly contains WAY more water vapor in it than could an equal amount of cold air. Cool Fact: When air is saturated at 100% relative humidity it can’t get any more saturated! In other words, if you try to evaporate more water vapor into already saturated air it immediately condenses the extra back out into a liquid. As the air temperature changes, so does the dew point and relative humidity. As air is cooled to the temperature at which it becomes saturated (dew point) the relative humidity approaches 100%. Cooling the air causes it to contract and become more dense, decreasing the space available between air particles for water vapor. As the difference between the dew point and air temperature decreases, the r.h. increases and there is a greater probability of water vapor condensing to form precipitation. Variations in pressure may be caused by three factors….. Temperature Altitude Moisture (Humidity) Cold air has higher Warm air has lower pressure pressure Air Pressure vs. Temperature As air temperature rises its molecules of N2, O2, CO2, and water vapor move farther apart. Since the atmosphere is not like a container holding these gases in, they speed up and fly farther apart. The end result is warm air is less dense than cool air and has less mass per unit volume. Being less dense, warm air exerts less pressure and is lighter. (less weight of air pressing down). In contrast, cooling air contracts increasing its density and so is heavier and exerts more pressure on Earth’s surface. ppt Increase temperature – decrease pressure PRESSURE T What type of relationship is this? Higher pressureHigher Air molecules Air Pressure vs. Altitude The force of gravity pulls most air molecules close to the Earth. This results in more air molecules and more air pressure near Earth’s surface and less air molecules and less air pressure far above the Earth. Water bottle pic Notice how pressure What type of decreases with altitude? relationship? Air molecule (28-32 amu) More dense Water vapor molecule (18 amu) Less dense Air Pressure vs. Humidity Fact: 1 water vapor molecule weighs less than an average molecule of air! This means air at 100% relative humidity has lots of the lighter water vapor molecules and less of the heavier air molecules. The result: Moist (humid) air is lighter, less dense, and lower in air pressure than dry air, which is heavier, more dense, and higher in air pressure. So what type of relationship? Must know .Winds move from high pressure to low factoid: pressure Since temperature and pressure are inversely related, uneven heating of the Earth’s surface can create temperature and pressure variations that make the winds blow. Monsoons-seasonal winds Monsoons bring either very dry or very wet weather depending upon the season and their direction of flow. Seasonal temperature differences in large land masses such as Asia cause a disruption in the wind belts. Asia Winter Monsoons Winter, Asia is cold. ↓Temp, ↑Pressure Winds blow off land (↑Pressure) to low pressure over water Bring dry air to India, Thailand, etc. “Dry Winter Monsoons” Asia Summer Monsoons Summer, Asia is hot! ↑Temp, ↓Pressure Wind blows from water (↑pressure) to low pressure land. Winds off ocean bring moisture. “Wet summer monsoons” This monsoonal effect occurs in other places as well, but not to as great an extent. A similar daily effect may be experienced along the immediate coast. These daily effects are called land and sea breezes. Image taken from www.valleywx.com on 9/8/12. Sea Breeze: Winds that blow from the cool water to the land. (daytime) Sea breeze Land Breeze: Winds that blow from the cool land to the water (night).