chapter 9

Weather and Climate, Chapter 9
Weather Patterns

I Fronts

Polar-Front Theory
Types and characteristics

II Midlatitude Cyclones

Cyclogenesis

Midlatitude cyclones (storms, see Fig. 9-2 for an excellent example of one storm) are low pressure systems that are

about a 1000 km in diameter
travel (generally!) from west to east across the globe
last from a few days to about a week or so

(hence these are synoptic scale features!). We look at fronts first because midlatitude cyclones develop in conjunction with fronts

I Fronts ** Figs. 9-3, 9-4, 9-6, 9-8, 9-9 **

Fronts are the boundary between two air masses that exhibit different properties (mostly temperature and density). They appear as a line in a weather map because even if these fronts are NOT lines they are very narrow compared to the size of the air masses that they separate (something like 15 to a 100 km compared to the size of air masses which are a 1000 km or more)

Since air masses have different temperatures and/or humidities, the frontal zone is an area with steep temperature gradients. Also, typically there is a steep humidity gradient.

Types of fronts

Warm front when a warmer air mass is moving into the area currently occupied by a colder air mass
Cold front when a colder air mass is moving into the area currently occupied by a warmer air mass
Stationary front a front that is not moving. This does not mean that the air masses are not moving!
Occluded front when a cold front overtakes a warm front
Drylines separation between air masses that have different density/hunidity

Characteristics

Warm fronts - red line with red semicircles in weather maps - These have gentle slopes stretching hundreds of miles (slope: ratio of height to horizontal distance). Typical warm front slopes are 1:200 (Question: what does this mean?) therefore the uplift is gradual, resulting in more stratus-type clouds, with rain over a fairly wide area. As a warm front approaches, one begins to see high level (ex., cirrus) clouds, then the clouds get lower and thicker over 1-2 days. Details of cloud sequence are in Fig. 9-4. Eventually rain occurs, may not be very heavy but may last a few days. These fronts are slow (slow-er) moving features, their speed is on average 25 to 35 km/hr. After a warm front has passed, the temperature is higher than before the front arrived and the winds shift from the east direction to a southwest (see Fig. 9-11, for example).

Cold fronts - blue line with blue triangles in weather maps - These fronts have steeper slopes, resulting in clouds with more vertical development, often associated with cumulonimbus clouds, thunderstorms, wind, violent weather. Their characteristic slope is 1:100. They are faster-moving fronts compared to other types, their average speed is 35 to 50 km/hr. The associated precipitation doesn’t last as long as with warm fronts, but may be more violent. After a cold front has passed, the temperature is colder than before the front arrived and the winds shift southwest to northwest.

Sationary fronts

Much less predictable than either cold or warm fronts, they can remain in place, or move back and forth across a region, producing a few days that are generally overcast and rainy.
Within either air mass, the winds may be blowing in a direction perpendicular to the front, even though the front is not moving.

Occluded fronts

Occur when a cold front “catches up” with a warm front.
The cold-type of occlusion is when air behind the cold front (which is the front that is doing the 'catching up') is colder than the cool air of the section of the warm front that is being undertaken.
The warm-type is when cool air is 'catching up' to the cold air section of a cold fromt (ex., mP air invading cP or cA air in the West Coast)
Associated with very complex precipitation patterns, depending on the temperature, humidity, and stability of the air masses.
Found when a cyclone (disturbance) is well developed.

Drylines

Classification based on differences in hunidity across a boundary, rather than temperature.
More easily identified when comparing dew temperature of the dry air on one side of the boundary to the dew temperature of the moist air on the other side.
Severe weather can occur along these lines, like that weather that occurs along cold fronts

Note: the word disturbance is used for a small scale feature (in both space and time), smaller than general circulation features. Typically, the largest scales of disturbances in the atmosphere can be up to around 1600 km (1000 miles) in spatial extent, and last up to around a week.

II Midlatitude Cyclones ** Figs. 9-10, 9-12, 9-15, 9-16, 9-18 **

· Cyclonic (low) pressure systems that we studied previously
· Associated with convergence, rising air, condensation, and precipitation
· Usually occur near the polar front (a feature of the general circulation)
· Because of converging, counterclockwise motion, cold air from the north is pulled southward behind the low pressure center, and warm air from the south is pulled northward ahead of the low pressure center.

MOVEMENTS OF MID LATITUDE CYCLONE

· The entire system moves west to east. Why? They are more prevalent, and move faster, in winter than in summer. Why?
· Within the system, cyclonic flow
· The cold front moves faster than the warm front
· The warm front moves more slowly than the cold front

LIFE CYCLE OF A MIDLATITUDE CYCLONE

· First, something must initiate cyclogenesis. Can be either topography, uneven surface heating, or upper air flow that causes divergence aloft.
· Wave
· Cyclonic circulation
· Occlusion
· dissipation

RELATIONSHIP TO UPPER AIR FLOW

cyclones are associated with convergence at the surface and divergence aloft. So, cyclones are usually located underneath areas where there is divergence aloft.

Upper Air Conditions associated with divergence aloft
· strong pressure gradient (what does this say about the speed and location of the jet stream)
· meridional flow
· So, cyclones are usually located underneath the lee side of a trough in the jet stream
· As the jet stream moves, the surface cyclone usually follows, or is steered by, the jet stream

RELATIONSHIP TO OROGRAPHY (MOUNTAINS)

As a cyclonic system passes over a mountain range its rate of spinning is affected:
· As the system ascends, it is compressed vertically and spread out horizontally, therefore the rate of spinning is slower
· As the system descends on the leeward side, it is stretched vertically and compressed horizontally, therefore the rate of spinning is faster
· (think of ice skater)

In North America, three primary locations for this sort of cyclogenesis: Colorado, Alberta, east coast.

Midlatitude anti-cyclone: high pressure system (see Fig. 9-18) This is a pattern associated with

clear, dry weather due to subsiding air,
divergence at the surface and convergence aloft.

These weather systems can sometimes stay for a few or many days over the same spot whch can contribite to bad pollution episodes because this dryis very stable, hence there is little convection and mixing to 'dilute' the pollution and move it away. Anti-cyclones can also block cyclonic systems from moving through, diverting them either north or south, thereby affecting weather in other locations as well.

Case study of a midlatitude cyclone (pages 257 -265)

** Figs. 9-20, 9-21 and 9-22 ** and ** Table 9-2 **

Review this example in detail and make sure you can answer the questions on pages 264-265. This type of work was reviewed also in Laboratory sections.