review from exam 2
How is wind direction defined?
Wind direction is defined by the direction from which the wind is blowing. That is, if the wind is blowing from the north, it is a northern wind.
What is an inversion?
A layer at which temperature increases with height. We use temperature to divide the atmosphere into vertical layers Layer in which the temperature increases with height They are, from bottom to top, the troposphere (weather), tropopause (marks top of troposphere), stratosphere (inversion layer - temp increases with height, or ozone layer), stratopause (marks top of stratosphere), mesosphere ("middle atmosphere" - temp decreases with height), mesopause (marks top of mesosphere), and thermosphere (temp increases with height).
What is advection?
Advection - Transfer of heat (or moisture) through horizontal movements of air sea or lake breeze
What is a mid-latitude cyclone?
Also called an extratropical cyclone An area of surface low pressure Require upper-level lows/trough (at 300-500 mb level) to form Any cyclone is an area of low pressure Center of cyclone is center of low Lifetime: Several days to a week+ Scale: Several hundreds to 1000+ miles across Possible weather: Thunderstorms and tornadoes Blizzards and ice storms Widespread snow and rain Warm and cold fronts
What is an air mass? How are they classified?
An air mass is a large body of air with similar temperature and moisture concentrations in the horizontal Air masses form over flat, homogeneous regions of the earth's surface The air acquires the characteristics of the underlying surface Blue shaded area (below) is an Arctic air mass Note fronts and low pressure systems separate areas of warm and cold air Counter-clockwise flow around low pressure systems (cyclones) moves air masses Air masses are modified by the surfaces over which they move What is the center of an air mass? Centers of air masses are surface high pressure systems Air mass boundaries are along surface troughs of low pressure These are called fronts Do not confuse these with upper-air troughs (found on 300/500 mb maps) How are air masses classified? Classified based on moisture and temperature Two-letter classification First letter refers to moisture, second letter refers to temperature Moisture: Continental (c): Dry (forms over land) Maritime (m): Moist (forms over water) Temperature: Tropical (T): Warm/hot Polar (P): Cool/cold Arctic (A): Frigid cP = Continental Polar Dry and cold cT = Continental Tropical Dry and hot mP = Maritime Polar Moist and cool mT = Maritime Tropical Moist and warm cA = Continental Arctic Dry and very cold
How are fronts represented on weather maps?
Cold air is more dense than warm air! Fronts are identified by the wind direction (relative to the front) in the COLD AIR! Cold front: Winds in cold air blow toward front Stationary front: Winds in cold air blow parallel to front Warm front: Winds in cold air blow away from front Region south of warm front and head of cold front is called warm sector Temperatures are warm; within warm air mass
What is a constant pressure surface?
Constant height maps Example: Surface maps Map has same elevation everywhere Pressure varies Constant pressure maps Upper-air maps Map has same pressure everywhere Height varies constant pressure surface: Assume uniform surface pressure and temperature (for now) This means that constant pressure surfaces parallel to constant height surfaces (flat)
What is a cyclone? An anticyclone?
Cyclones: Low pressure systems NH: Flow counter-clockwise SH: Flow clockwise Cyclonic flow Anticyclones: High pressure systems NH: Flow clockwise SH: Flow counter-clockwise Anticyclonic flow
What is dewpoint? Why is it useful?
Dewpoint Temperature: The temperature to which air must be cooled to achieve saturation. Higher Dewpoint: More moisture in the air. Dewpoint is useful: because you can estimate low temperature by looking at afternoon dewpoint We can saturate the air in two ways: Add water to the air (while keeping temperature constant) Cool the air (while keeping moisture constant) If we cool the air while keeping moisture constant, the temperature at which saturation occurs is called the dewpoint temperature A higher dewpoint always means there is more moisture in the air, regardless of the actual air temperature Dewpoint of 50°F means there is enough water vapor in air to saturate air with a temperature of 50°F Dewpoint of 70°F means there is enough water vapor in air to saturate air with a temperature of 70°F warm air contains more moisture at saturation than cold air does. dewpoint os always less or equal to the temperature Moist air is less dense than dry air
What is convergence? Divergence?
Divergence aloft causes surface pressures to fall Air converges into low pressure systems at the surface Surface winds flow counter-clockwise and inward in NH Air converges, rises, cools, and water vapor condenses, forming clouds and precipitation Convergence aloft causes surface pressures to rise Air diverges from high pressure systems at the surface Surface winds flow clockwise and outward in NH Air sinks, warms, and dries, inhibiting cloud formation Usually opposite between surface and aloft Upper-air divergence generally above surface convergence and vice versa In, up, and out (lows) In, down, and out (highs) Net divergence: DIV > CONV Net convergence: CONV > DIV How to diagnose regions of convergence and divergence aloft??? Wind speeds matter!! Can regions of slower and faster flow aloft cause convergence or divergence? YES! Traffic jam example: http://www.youtube.com/watch?v=DKJIkYarp5E When faster flow slows down, convergence results When slower flow speeds up, divergence results Where can we find slower and faster flow aloft?
What is a front?
Fronts are boundaries between air masses of different densities Warm air less dense than cold air Moist air less dense than dry air Do NOT confuse fronts and air masses Fronts are generally strongest at the surface and weaken with height Temperature gradients are found on the cold side of fronts
How are low and high heights related to temperature and weather?
How are low and high heights related to temperatures below that pressure surface? High heights are indicative of warm air below that pressure level Warm air is less dense than cold air As you heat air, it expands! Low heights are indicative of cold air below that pressure level High heights on a constant pressure surface are analogous to high pressures on a constant height surface Low heights on a constant pressure surface are analogous to low pressures on a constant height surface If you only think about this in terms of "pressure always decreases with height," you will likely get it WRONG! High heights are related to warm air below that pressure level Analogous to high pressures on constant height surface Remember: A high is a high! Low heights are related to cold air below that pressure level Analogous to low pressures on a constant height surface Remember: A low is a low! Forces that are in the atmosphere.. Pressure gradient force (PGF) Coriolis force Apparent force due to rotation of earth Friction Near-surface flow only Gravity Vertical motions only Newton's Second Law says that a force is required to accelerate anything
What is latent heat?
Latent heat released via condensation powers many weather systems, The heat energy used to change the state of an object without changing it's temperature.
How do winds blow around high and low pressure systems in the Northern Hemisphere?
Low Pressure Systems: Winds circulate COUNTERCLOCKWISE and INWARD. High Pressure Systems: Winds circulate CLOCKWISE and OUTWARD.
What is pressure and how does it change with height?
Masses of warm and cold air meet at fronts (see next slide) Cold front - Cold air advances Warm front - Cold air retreats Examine pressure data to find areas of high and low pressure Fronts connect to low pressure centers Low pressure (L): Counter-clockwise circulation and clouds/rain High pressure (H): Clockwise circulation and clear skies Assume uniform surface pressure and temperature (for now) This means that constant pressure surfaces parallel to constant height surfaces (flat) Now heat left side, cool right side Warm air expands "Pushes" pressure surface upward Cold air contracts Pressure surface moves downward High heights are indicative of warm air below that pressure level Warm air is less dense than cold air As you heat air, it expands! Low heights are indicative of cold air below that pressure level High heights are related to warm air below that pressure level Analogous to high pressures on constant height surface Remember: A high is a high! Low heights are related to cold air below that pressure level Analogous to low pressures on a constant height surface Remember: A low is a low! Pressure = Force/Area Pressure is related to the weight of the atmosphere above you Greater mass of air above a high pressure system than above a low pressure system Pressure is force/area. Atmospheric pressure ALWAYS decreases with height. 1 mb with every 10 m columns Columns of air above lower elevations ALWAYS weigh more than columns above
How are convergence and divergence related to the development of surface highs and lows?
Net divergence (net loss of air molecules) decreases surface pressure Divergence aloft greater than convergence at surface (lows) Divergence at surface greater than convergence aloft (highs) Subtracts molecules from air column Lows get lower (strengthen) Highs get lower (weaken) Net convergence (net gain of air molucules) increases surface pressure Adds weight to air column Convergence at surface greater than divergence aloft (lows) Convergence aloft greater than divergence at surface (highs) Lows get higher (weaken) Highs get higher (strengthen) How do convergence and divergence influence surface pressure? Strengthening Low: Pressure drops Net divergence (DIV Aloft > CONV Sfc) Weakening Low: Pressure rises Net convergence (CONV Sfc > DIV Aloft) Strengthening High: Pressure rises Net convergence (CONV Aloft > DIV Sfc) Weakening High: Pressure drops Net divergence (DIV Sfc > CONV Aloft) How do mid-latitude cyclones (low pressure systems) form? Convergence and divergence are key! Net divergence - Net decrease of air molecules over a point on the surface Surface pressure decreases Regions of net divergence are favored for surface low pressure system formation
How do you determine the surface wind direction from isobars?
Northern Hemisphere Lows: Winds spiral inward, counter-clockwise rotation Highs: Winds spiral outward, clockwise rotation Southern Hemisphere Lows: Winds spiral inward, clockwise rotation Highs: Winds spiral outward, counter-clockwise rotation Surface winds ALWAYS cross isobars at an angle from high to low pressure SURFACE WINDS DO NOT BLOW PARALLEL TO ISOBARS!!!!
How does the pressure gradient force influence the wind speed?
PGF causes wind to blow Stronger PGF = Stronger Winds
What is relative humidity? What are the two ways to change it?
Relative Humidity: Ratio of water vapor in air to that required for saturation. Increase Relative Humidity: Add moisture to air. Cool the air. Decrease Relative Humidity: Remove moisture from air. Warm the air. RH: Depends on temperature. Humidity (in general) is a measure of water vapor in the atmosphere Does not only mean relative humidity There are many ways to measure atmospheric We will discuss three ways to measure water vapor in the atmosphere: Vapor pressure Relative humidity Dewpoint temperature Ratio of water vapor in air (VP) to that required for saturation (SVP) RH does NOT indicate actual amount of water vapor in air Higher RH does NOT necessarily mean there is more moisture in the air Higher RH means air is closer to saturation called relative humidity because it is relative to the temperature (SVP) at saturation, RH =100% (vp=svp) RH=vp/svp x 100% Increase relative humidity: Add moisture to the air (increase VP) Cool the air (decrease SVP) Decrease relative humidity: Remove moisture from the air (decrease VP) Warm the air (increase SVP)
Review soundings
Right line is always temperature Left line is always dewpoint Why? Recall that T ≥ Td What is a pressure level? A layer is bounded by two pressure levels Example: 850-500 mb
What is the difference between satellites and radar?
Satellites: View of clouds from space. Radar: View precipitation from ground. Doppler radar - Can sense motion of precipitation particles toward/away from radar
What is saturation? What mathematical relationships are true at saturation?
Saturation: Evaporation and Condensation are equal Assume that air in jar is initially unsaturated and jar remains at constant temperature Initially, more water will evaporate from water surface than condense back onto it Evaporation > Condensation Water vapor concentration in jar increases this increases condensation rate there is more water vapor in the air to condense evaporation rate remains the same Saturation occurs when evaporation and condensation rates are equal Air is NEVER so "full of water" that there is no room for any more AIR IS NOT A SPONGE!
What is a dryline?
Separate mT air (warm/moist from Gulf of Mexico, to east) from cT air (hot/dry from deserts, to west) Typically found in KS/OK/TX during spring and can trigger strong to severe thunderstorms
What is a jet stream? Where are jet streams found?
The jet stream is a river of fast moving air in the upper atmosphere Exists near tropopause level About 10 km (6 miles) above sea level 300-200 mb Flow is geostrophic (parallel to height contours) Flow is not the same speed everywhere along jet stream axis
How do you identify ridges and troughs?
Trough - Valley; low heights (pressures) Ridge - High heights (pressures) Do NOT just look at shape of contour lines!!!
Why is vertical motion important?
Vertical motions usually small Steady rain: cm/s (about 1 mile/day) Faster rising air = More interesting weather Strong upward-directed PGF balanced by gravity Called hydrostatic balance Works well most of the time Does not apply to thunderstorms, tornadoes
How do mid-latitude cyclones influence the weather?
Why are they important? Bring cold air southward and warm air northward Cool the Tropics, warm Polar Regions Provide precipitation to many regions of world Produce much of the severe weather across North America
how do clouds form?
Why is rising air crucial to cloud formation? Most clouds form in RISING air Why is rising air important? As air rises, it expands and cools Amount of water vapor in air does not change as air rises As temperature decreases, relative humidity increases Air eventually rises high enough (cools enough) to become saturated (RH = 100%) What causes air to rise? Surface heating/convection (thermals; see right) Uplift by topography (mountains) Surface convergence (after exam) Fronts (after exam)
What is the pressure gradient force? In what direction does it act?
review of pressure: Pressure = Force/Area Pressure is related to the weight of the atmosphere above you Greater mass of air above a high pressure system than above a low pressure system Air molecules want to flow from where there is greater pressure to where there is less pressure Extreme case: A vacuum! Increasing the pressure gradient yields greater acceleration and faster flow Pressure gradient = change in pressure / distance Units = mb/km Tightly packed isobars - Strong pressure gradient Big change over small distance Widely spaced isobars - Weak pressure gradient Small change over big distance PGF directed from higher to lower pressure (or higher heights to lower heights on constant pressure surface) Acts perpendicular to isobars (or height contours) Video: http://www.youtube.com/watch?v=t_qATQTSkWY PGF causes wind to blow Stronger PGF yields stronger winds
Where is warm air and cold air found with respect to a surface low?
warm air to the east cold air to the south/ southwest