EES215

Lecture 19

 

Weather terms:

  • Dew Point:  Temperature to which a parcel of air needs to be cooled in order to reach saturation
  • Windchill Equivalent Temperature:  Not an actual temperature but an expression that relates wind speed to the temperature we perceive. Table 1

Air masses are characterized by Temperature, Pressure and Humidity. Where air masses come together, gradients in these parameters exist and cause changes in the weather.

  • Cold front:  cold, dry air replaces warm, moist air
  • Warm front:  warm moist air replaces cold, dry air

At the fronts, formation of clouds.
 

Classification of clouds:  based on altitude and moisture content
Distribution of clouds  Fig. 1

Precipitation:  as air moves upward, cooling occurs (usually adiabatically), less water can be kept in solution à condensation. Because condensation nucleus usually is necessary, water often reaches supercooled state, especially with respect to ice. Typically, condensation leads directly to the formation of ice crystals. When ice crystals become too large to be carried in suspension, they start falling à precipitation. If surface temperature is above 4oC, ice crystals melt à rain.

Thunderstorms  formed by charge separation in large cumulonimbus clouds.  Fig. 2
Formation of these clouds needs large temperature difference between land surface and high regions à most likely in mid-afternoon in Summer and early Fall.

Hail:  updraft in large cumulonimbus cloud is sufficient to prevent ice crystals to fall à large crystals form and fall only once they overcome the force of the updraft. Rapid release of large ice crystals, formation of ice streak by moving cloud.  Fig. 3

Special case:  Tornado:

Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere.  Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical.  An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.  Fig. 4 Tornadoes produce very strong winds over a narrow band (~100 m wide) and leave a path of destruction over distances of 1 to 10 km.  Typical funnel shape of a tornado: Fig. 5.

Tornadoes occur mostly over flat land, typically during the summer months, prime areas for tornadoes are the plain states and Florida. 

Hurricanes:  Hurricane is the name used in the USA for a tropical cyclone; other names for similar storms are cyclones (Australia; New Zealand) or Taifun (East Asia).  They occur in the areas north and south of the equator over regions with large pools of warm water (Fig. 6).  The storms are fueled by latent heat released by condensation of water vapor (Latent heat of water ~2.5 kJ/kg).  Release of latent heat warms air à upward draft à.decreased pressure near surface à increased inflow of air.

Start of tropical cyclone (hurricane); large quantity of warm, moist air present during late summer by warm ocean water which provides heat and moisture à tropical cyclones form over warm waters of southern N. Atlantic and N. Pacific; path reflects predominant wind direction and deflection by Coriolis Force.

Path of Hurricane Katrina (Aug./Sept. 2005) (Fig. 7).  Hurricane Katrina on Aug. 28, just before landfall (Fig. 8). 

Fig. 9: Windspeeds (CWS), maximum gusts (MXGT1) and pressure (BARO1) for Katrina.