Transcript Weather
WEATHER • Meteorology is the study of the atmosphere and its phenomena • Aristotle wrote a book on natural philosophy (340 BC) entitled “Meteorologica” – Sum knowledge of weather/climate at time – Meteors were all things that fell from the sky or were seen in the air – “meteoros” : Greek word meaning “high in air” WEATHER VS. CLIMATE Weather is NOW and is comprised of: a) air temperature b) air pressure c) humidity d) clouds e) precipitation f) visibility g) wind Climate represents long-term (e.g. 30 yr) averages of weather. • Invention of weather instruments – 1500’s Galileo invented water thermometer – 1643 Torricelli invented mercury barometer – 1667 Hooke invented anemometer – 1719 Fahrenheit developed temp scale based on boiling/freezing water – 1735 Hadley explained how the earth’s rotation influences winds in tropics – 1742 Celsius developed the centigrade temp scale • 1787 Charles discovered relationship between temp and volume of air • 1835 Coriolis used math to demonstrate the effect that the earth’s rotation has on atmosphere • 1869 first isobars were placed on map • 1920 concepts of air masses and weather fronts were formulated in Norway • 1940’s upper air balloons 3-D view of atmosphere • 1950’s high speed computers • 1960 Tiros 1 first weather satellite Air Pressure = the weight of the air above you from where you are to the top of the atmosphere. Air is compressible • Half of all air molecules are below 18,000 feet of altitude; 3/4 are below 36,000 feet, etc. • At sea level air exerts a force of about 14 pounds per square inch Air pressure is measured with a barometer Air Pressure in the News - Cold air has higher pressure • Cold air is more dense, so it sinks down and pushes harder. – Barometer goes? • Hot air expands, becomes less dense, rises, and exerts less pressure. – Barometer goes? Wind = movement of air • Wind goes from an area of high pressure toward an area of lower pressure. • Wind direction is described by where the air comes from - a west wind blows from west toward the east. Local WindsWhich way would the wind blow? What time of day is shown in this picture? More Local WindsWhich way would the wind blow? What time of day is shown in this picture? How do meteorologists predict wind direction? Coriolis Effect • Because the Earth rotates on its axis, things moving from North to South or from South to North are deflected. • In the Northern Hemisphere, the deflection is to the right. • In the Southern Hemisphere, the deflection is to the left. Winds Around Pressure Systems Let’s take one more look at that map. A Low Pressure System Over Iceland Why don’t high pressure systems show up on satellite photographs? The summer Monsoon • During July, August and half of September, precipitation comes from thunderstorms associated with the summer monsoon • Convection occurs in moist southerly flow • Because of its high intensity/short duration, most summer rainfall runs off quickly and/or evaporates – Less replenishment of soil moisture and recharging of aquifers than winter precip Winds shift to a southerly or southeasterly direction. This produces a change in moisture. Together with daytime heating causes the Arizona monsoon. The recipe for monsoon precipitation in Arizona Middle-level flow Westerlies well to the north High pressure belt moves north of AZ, bringing midlevel easterly flow to AZ Low-level surge of warm, moist air H H Convection breaks out over AZ Mid-level Gulf moisture enters AZ from the east Physical Ingredients for a Monsoon • Land-Sea differences: land and water have different heating capacities & water can store more heat because it is a fluid and can mix heat down for future release • Planetary rotation: introduces swirl and much stronger winds • Moisture: water vapor, collected over oceans through evaporation, condenses over land and marginal seas releasing vast quantities of heat • Orography: acts as elevated heat source which intensifies flow & also conducts flow. Winter Rains Air circulation pattern is called the Pacific/North America or PNA pattern. It is characterized by: • Warm, dry weather in the western half of the country (dominated by a ridge of high pressure). • Cold, wet weather in the eastern half of the country (dominated by a trough of low pressure). Trough: Cold / Wet East Ridge: Warm/Dry West