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Weather

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Temperature Temperature is the average motion of the molecules of a substance (measured with a thermometer) There are 3 different temperature scales: Fahrenheit used to record surface temp in US Celsius used overseas & upper air obs. Kelvin used in scientific formulas

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Humidity Humidity is the amount of water vapor in the air. Theres several ways to measure humidity, but the most useful for meteorologists is the dew point Dew point temperature is the temperature at which the air would become saturated (and dew would begin forming) High relative humidity occurs when the temperature is close to the dew point

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Pressure Atmospheric pressure is the force per unit area exerted against a surface by the weight of the air above it. (measured by barometer)

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Pressure If the amount of air molecules above a surface increases, pressure increases too If the amount of air molecules above a surface decreases, pressure decreases too High pressure is usually associated with fair wx and Low pressure with stormy wx. There are several different pressure units

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Variables influencing the atmosphere pressure: High Pressure Region where air has piled up Warmer and dryer than surrounding air Low Pressure Region where air has stretched out Tends to rise and cool, vapor condenses into clouds

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Pressure These units include: Inches of Mercury (Hg) Pascals (Pa) Millibars (mb) Meteorologists use millibars most often

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Pressure Conversions Standard sea level pressure is the average pressure on Earth at sea level. Standard sea level pressure is =29.92 in Hg=1 atm=1013.25 mb =101325 Pa=101325 N/m^2

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Winds around Highs &Lows Because of the Coriolis force (discussed in a later course) winds in the Northern Hem: blow counterclockwise (or cyclonically) around Low Pressure And blow clockwise (or anti- cyclonically) around High Pressure

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Winds around Highs and Lows If the wind is at your back in the Northern Hemisphere, low pressure is always on your left! (Remember Low pressure = Left!)

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Wind Wind has 2 different measurements (speed and direction) Wind speed is measured with an anemometer Wind direction is measured with a vane Vane Anemometer

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Variables influencing the atmosphere winds: Due to differential heating of the earths surface Warm air rises Cooler air flows in

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Units of Wind Wind speed can be measured in mph, knots, or meters per second 1 knot = 1.15 miles per hour 1 m/s = 2.24 miles per hour Wind Direction tells us FROM WHERE the wind is blowing Ex: A SW wind means wind is FROM SW

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Wind Direction Wind direction is measured in degrees (like a compass, not like the degrees in math!) 0=North, 90=East, 180=South, 270=West Meteorology 90 180 270 0 or 360 N

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Latitude and Longitude Latitude and Longitude lines divide up the Earth into a usable coordinate system Lines from N to S = meridians of Longitude Longitude measures how far E or W a location is from the Prime Meridian (through Greenwich, England) Values of Longitude go from 180W to 180E

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Latitude Lines from E to W (parallel to equator) are meridians of Latitude Latitude measures how far N or S a location is from the equator. Values of Latitude go from 90S to 90N Norman, OK is located at 35.22N latitude 97.45W longitude

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The global energy balance also varies with longitude, priamrily due to the differences of land, water and elevation

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Global Air Circulation Patterns

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Ocean Currents What causes them? Wind drag. Friction with the wind drags the ocean surface to produce a current. The semi- permanent position of the subtropical high over the north and south Pacific and the north and south Atlantic help drive the anticyclonic gyre of water in each basin. Thermohaline circulation. Density variations in water arise from differences in temperature (warmer=> less dense) and salinity (more salty=>more dense). More dense water tends to sink relative to less dense water, giving rise to vertical circulations known as thermohaline circulations. Interactions of ocean currents with bottom topography and lateral boundaries Other ocean weather, i.e. ocean waves, vorticies, etc.

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How do ocean currents affect weather and climate? Oceans store and transport heat. The high heat capacity of water makes it an excellent mechanism to store the suns energy and transport it from one place to another Oceans store liquid water and pump vapor into the air as a key link in the global water and energy cycle. Ocean storage and release of heat is a key forcing mechanism for weather Oceans absorb (release) atmospheric gasses, such as oxygen and CO2. Absorption of CO2 by oceans is the most important sink of CO2 from the atmosphere Amount of CO2 that the oceans can hold is inversely proportional to temperature, i.e. colder water holds more CO2 and warmer water (think global warming here) holds less. Oceans are a major source of oxygen for the atmosphere due to photosynthesis of microscopic plants in the ocean.

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Basic Ocean Circulations

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Thermohaline Circulation

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What is a front? At a frontal boundary, some or all of the following changes occur: Temperature change Change in dewpoint Wind Shift Pressure trough

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Cold Fronts A cold front is a boundary where cold, dry air is sliding under warm, moist air. A typical temperature change with a cold front is about 20F but varies greatly

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Fronts = a clash of air masses Cold Front digs in underneath a warm air mass Warm air rises, mixes with colder air, & condenses into clouds

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More on Cold Fronts Ahead of a cold front, winds are usually south to southeastrly winds. Behind a cold front, winds usually from the north or northwest. A cold front is denoted on a surface map like this Filled triangles point towards direction of movement

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3-D Picture of Cold Front Notice the warm air is lifted up and over the cold air and forms clouds. Warm air is less dense than cold air so it rises where they meet.

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Warm Fronts A warm front is a boundary where warm, moist air is advancing against cooler, drier air. Temperature change averages 15F but varies.

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More on warm fronts Ahead of a warm front, winds are usually east or northeasterly winds. Behind it, south to southwesterly winds prevail. A warm front looks like this where the half circles point toward the direction its moving.

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Stationary Front Boundary between warm, moist air and cold dry air in which neither air mass is advancing. Indicated by alternating blue triangles and red half circles. Often happens when front loses its punch or is blocked by high pressure.

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Occluded front A occluded front, or occlusion, occurs when a warm air mass and a cool air mass come in contact with each other, making boundaries at both the ground as well as aloft. Often associated with showers or snow(in winter) and is the beginning of the end to a storm. Indicated by triangles and half circles on the same side.

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What are isolines? Isolines (also called isopleths)- lines that connect equal values of a variable In meteorology, we frequently use: - isobars (pressure) - isotherms (temperature) - isotachs (wind speed)

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Isolines Isolines make it easy to see different values on a map Allow us to interpolate values at a point Can pick out different features like fronts, jetstreams, drylines, pressure fields, etc. How can I draw an isoline?

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Tropical Meteorology Anything between about 30N and 30S in latitude Or between Tropic of Cancer (23.5N) and Tropic of Capricorn(23.5S) Fuzzy definition Radiation budget > 0 in tropics Job of tropics = export heat poleward

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Tropical Meteorology Very difficult compared to mid-latitudes Cant just measure pressure and get an approximate wind field Geostophic Balance - doesnt happen in in tropics Effects of Earths rotation not a big player (Coriolis force is small) Try to measure convergence (winds coming together and divergence- winds pulling apart) to deduce areas of upward motion

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Tropical Cyclone Classification TCs form from a wave or disturbance Trop disturbance = any area of disturbed weather ex: stalled out front Trop wave = elongated area of low pressure (just a trough). Go from East to West (direction of winds at these latitudes). Trop depression = closed low (trough pinches off) Trop storm = 39-73mph, hurricane=74+

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Strength Versus Intensity Intensity = core region (center to 100 km) An increase in intensity = stronger max winds or lower minimum surface pressure Intensity can change quickly Strength = outer part of core Associated with area weighted average wind speed outside of core

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Eyewalls & Rainbands Region of clouds/intense rain/strongest winds that seperates sinking air in eye from rest of storm Typically not vertical (45 deg slope = common) Rainbands = stronger areas of rain with higher clouds between them Sometimes, tornadic supercells will form in rainbands as storm hits land (b/c of added friction)

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Severe Storm A thunderstorm must have one or more of the following to be considered a severe storm: (NWS classification) Winds 58 mph (50 knots) or more Hail 3/4 in diameter or larger Tornado

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Supercell Thunderstorms A supercell thunderstorm is a t.s. with a deep rotating updraft (mesocyclone) Updraft elements usually merge into the main rotating updraft and then accelerate rapidly Flanking updrafts "feed" the supercell updraft, rather than compete with it Small percentage of all t.s.s are supercells but they cause the majority of damage

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Diagram of a Supercell

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A Look from the SE

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El Nino Original definition: Annual warming of water off of the Coast of Peru around Christmas. El Nino referred to the Christ child. Since early 1980s new definition: Warm water occurring off of the Peru and reaching out to the dateline that tends to occurr periodically every 2-5 years.

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El Nino Sea Surface Temperature Anomaly El Nino

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La Nina New term coined recently to refer to the opposite of an El Nino when the Equatorial Pacific is colder than normal and the SOI is negative

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Cross-section showing change in ocean temperatures for 1997 El Nino development

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ENSO sea surface height anomaly

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La Nina sea surface height anomaly

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USA impacts of El Nino and La Nina: temperature

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USA impacts of El Nino and La Nina: precipitation