4: Forces, Pressure, and Fronts Flashcards
what is PGF
- pressure gradient force: high to low pressure
- pressure gradient arises due to the difference in pressure between two points
Coriolis Force
In Northern hemisphere: Coriolis force deflects moving objects to the right
In Southern hemisphere: the Coriolis force deflects moving objects to the left
What is Coriolis Force dependent on
- Dependent on latitude
- zero at the equator
- maximum at the poles - Dependent on speed
- greater for faster speeds
- zero if the object is stationary
Frictional force
roughness of the surface slows down wind speed —> this produces eddies and turbulence
- lower wind speed near surface
-obstructions can also lower wind speed
Geostrophic winds
winds that result from an exact balance between the pressure gradient force and the coriolis force
- winds blow parallel to the isobars or contour lines above the level of friction
how are winds named
from their origin. Ex: east to west wind is called westerly
upper level winds
in the upper atmosphere, winds flow parallel to the contour lines in west to east direction
-contours decreases in height from south to north
- strong temperature gradient = strong height gradient = strong wind
relationship between geostrophic and gradient winds with contour lines
- when contours are close together there wind and gradient is strong
- geostrophic: straight
- gradient: curved
- when wind follows contour lines then there is a balance between the pressure gradient force and the Coriolis force
Surface winds
- frictional drag at the surface slows the wind speed
- slower wind speed means weaker Coriolis force therefore it no longer balances the pressure gradient force
- wind flows across the isobars towards the low pressure, in low pressure systems it is toward the center and in high pressure systems it is outside of the system (outwards)
Vertical air motions in low and high pressure systems and what image
- image 20
Low pressure system/surface low: inward wind flow to the low pressure center (convergence) = rising air = upper level divergence (left) —> results in vertical cloud development and precipitation
High pressure system/surface high: outward wind flow because center is hight pressure (divergence) = sinking air (sucks it down) = upper level convergence —> results in clear sky and warm surface temperature
Source Regions of Air Masses in NA and what image
image 21
* Maritime: origiantes over the oceans; humid
* Continental: originates over continents; dry and tropical
- Cold
- Maritime polar (Pacific) air masses
- Arctic air masses
- Continental polar air masses
- Maritime polar (Atlantic) air masses
- Warm
- Maritime tropical (Pacific) air masses
- Continental tropical air masses
- Maritime tropical (Gulf) air masses
- Maritime tropical (Atlantic) air masses
Characteristics of North American Air Masses
try to recite image 22
What is a stationary front and what image
- cold dense air and warm air separated by topography
- No movement
- Cold dense cP air butts up against the north-south Rocky Mountains
- Milder mP on the western side of the Rockies
- Surface wind blow parallel to front boundary in opposite direction
- Do not produce significant precpitation
- image 25
Cold and warm fronts and what images
Cold fronts: have cold air advancing and displacing warm air
Warm fronts: have cold air retreating and warm air overriding it
image 23 (top is cold) (bottom is warm)
image 24
What happens as a warm or cold front passes
- Wind direction shifts clockwise
- Temperature and dew point temperature are likely to change
- Uplift of less dense air mass (warm) results in clouds, precipitation, and a drop in surface pressure
What are fronts characterized by on weather charts
characterized by a kink in contours
What weather elements does a cold front affect and how does it work and what image
- image 26
- Cold, dry, stable, polar air replacing warm, moist, conditionally stable subtropical air
- Sharp temperature fluctuation over a short distance
- Change in the moisture characteristics
- Change in wind direction
- Clouds and precipitation pattern
Characteristics of a cold front
- Td and T will drop
- Winds will veer clockwise (WNW to SSW)
- Surface prssure is lowest under the front (in front of triangle)
- Because of lifting, pressure falls and produces precipitation
Type of Clouds of a Cold Front and ppt and what image
- warm air is lifted ahead of the cold front
- clouds and precipitation form just ahead of the front
- primarily cumuliform clouds, sometimes stratiform
- heavy precipitation and thunderstorms
- image 27
How does warm front operate
- leading edge of warm, moist, subtropical air replaces retreating cold polar air
- warm front moves at a slower speed
- warm air overrides cold air creating frontal inversion (warm air on top of cold air) and cold air gradually retreats
- image 28
Characteristics of a warm front
- T and Td will increase
- Winds will veer clockwise (SSW to ESE)
- Surface pressure lowest under the front
Typse of clouds and ppt of a warm front and what image
- clouds and precipitation form well ahead of the front
- primarily stratiform clouds and sometimes cumuliform
- extensive cloud cover
- light, steady precipitation
- image 29
What is an occluded fornt
- When cold fornt catches and overtakes the warm front
- Represented by a purple line with triangles and semicircles
- two main types of occlusions: cold type and warm type (most observed)
what is a cold type occlusion and what image
- cold front rapidly approaches the warm fornt and overtakes it
- Warm air mass lifts off the ground
- image 30
Warm type occlusion and what image
- Cold cP air ahead of warm front
- Cool mP air following warm front
- Cold front approaches the warm front
- Mild, lighter mP air behind the cold front could not life the colder, heavier cP air off the ground so it will go over the warm air mass
- image 31
Weather conditions associated with occluded fronts
memorize image 32
* Temperature
* Pressure
* Clouds
* Precipitation
* Visibility
* Dew point
Clouds before, during, and after passing in occluded fronts in the winter in NA
Before: Ci, Cs, As, NS
During: Ns, sometimes Tcu and Cb
After: Ns, As or scattered Cu
Criteria to locate fronts
- Large change in T over short distance aka packed isotherms
- Large change it Td over short distance
- Sudden change in wind direction and pressure
- Cloud and precipitation pattern
- Front located along troughline, along kink or bend
