# Meteorology: *Surface Air Flow, Pressure, and Fronts (W7)

## 34 terms · *Coriolis Force; Surface Air Flow; Convergence and Divergence; Fronts and Troughs

### vector

WIND VELOCITY is a ___________; it has a magnitude, or speed (25 mph is a breezy example) AS WELL AS a direction (from northwest, for example).

### acceleration

_____________ is a change in velocity over time.

### speed, direction

There are TWO ways for ACCELERATION to occur: by changing ___________ (you hit the gas to pass a truck or you brake to avoid a squirrel) OR by changing ___________ (you round a sharp curve in the road at constant speed, and your bottom slides outward as you negotiate the bend.)

### frame of reference, centrifugal force

By _________ ____ ____________, we mean the part of your immediate surroundings that you sense is unaccelerated (the interior of the car, in this case). Indeed, you perceive the car to be unaccelerated as it negotiates the curve at constant speed, even though it really accelerates toward the center of the curve. This perception leads you to falsely sense that some other force, which acts to accelerate you outward, is the "real" force at work. But it's not: This phantom force, called the ____________ __________, is only an "apparent" force (see Figure 6.21). The key point we want to make here is this: Our sense of what is accelerated and what is not accelerated sometimes depends on our frame of reference.

### Coriolis Force

The _________ _________ is due solely to the rotation of the earth.

### equator

Not all points on the planet move at the same speed. For points on the _____________, the posted eastward speed limit is about 1036 mph (466 m/s). The difference in these rotational speeds is a result of the SPHERICAL SHAPE OF THE EARTH.

### equator, polar

If the ___________ is the turbo supercharged racer of latitudes, then _________ regions, where latitudinal circles become progressively smaller with increasing altitude, turn out to be the golf carts.

### Coriolis Effect

The "apparent" deflection is a natural consequence of living on a spherical, rotating planet. This deflection is the ____________ __________, named after the French engineer and mathematician Gustav Coriolis.

### right, left

The CORIOLIS EFFECT always operates to the ________ OF THE WIND in the northern hemisphere, and to the ___________ of the wind in the southern hemisphere.

### Coriolis effect

The __________ __________ is NOT a true force in any traditional sense. It cannot cause motion. It is an apparent "effect" that simply results from an object moving over a spherical, rotating planet such as the earth.

### low

The CORIOLIS EFFECT is relatively small at _______ latitudes. Indeed, hurricanes (see Chapter 11), which need some contribution from the Coriolis effect in order to develop their characteristic spin, rarely form between 10N and 10S latitude because the Coriolis effect is simply too small.

### increases, speed

The Coriolis effect ____________ with increasing latitude. The apparent deflection caused by the Coriolis effect also depends on the __________ of the moving object.

### faster

The ___________ a moving object, the greater the APPARENT DEFLECTION caused by the CORIOLIS EFFECT

### counterclockwise, inward

The circulation of air around a SURFACE LOW-PRESSURE SYSTEM in the NORTHERN HEMISPHERE is in the ______________ sense with air moving slightly __________ toward the low.

### clockwise, outward

A similar diagram for high-pressure system in Figure 6.27d shows that the circulation of air around a HIGH PRESSURE in the NORTHERN HEMISPHERE is in the ______________ sense with air moving slightly ___________ from the high. Thus, air DIVERGES in the vicinity of SURFACE HIGH PRESSURE SYSTEM.

### 30, smaller

Over LAND, the wind crosses the ISOBARS at approximately _____ degrees or so, on average (see Figure 6.29). Over the ocean and other large bodies of water, the CROSSING ANGLE is generally ___________, owing to LESS friction over the smoother water (compared to rough land.).

### convergence, divergence

It is the _____________ in the vicinity of surface low pressure AND troughs, and ____________ in the vicinity of surface high pressure AND ridges, that gives each its characteristic weather.

### rise, sink

To avoid big traffic jams, air CONVERGING toward the center of a LOW must __________ to avoid a big congestion (see Figure 6.30). Rising air cools, and, if the air cools enough, clouds and precipitation can form. By the same token, air DIVERGING away from the center of a surface HIGH prompts air to gently _______ over the high in order to avoid any LARGE EVACUATION of air.

### weight

Using _______ as a proxy for PRESSURE, the column of air over the center of a surface low-pressure system (high-pressure system) must be the lightest (heaviest) column in the general vicinity.

### convergence

If more air leaves the air column over the center of low pressure than enters it via _____________ near the ground, the low can maintain OR lose weight and thrive.

### Diverging

______________ air away from the center of surface HIGH-PRESSURE SYSTEMS constitutes a weight-loss plan (not good for weighty highs).

### divergence

If MORE AIR enters the air column over the center of HIGH pressure than leaves it via ______________ near the ground, the high can gain or maintain its weight and thrive.

### pressure tendency

You can infer the weight loss or gain of air columns by watching your BAROMETER. As a low (high) pressure system approaches your town, local air columns start to lose (gain) weight and surface pressure starts to decrease (increase). In these situations, weather forecasters keep tabs on the ____________ ___________, which is simply the CHANGE IN PRESSURE OVER A GIVEN TIME PERIOD. In light of this definition, the standard units of pressure tendency are MILLIBARS PER HOUR (mb/hr). As a matter of practice, forecasters often choose THREE HOURS at the time period over which they measure pressure changes - that's generally long enough to capture the significant pressure rises and falls that accompany the movement of large-scale high and low pressure systems.

### 63, -8

For example, if the PRESSURE rose 6.3 MILLIBARS over the last three hours, "___" would appear in the station model. Or, assuming the pressure fell 0.8 MILLIBARS over the last three hours, "_____" would be the coded three-hour PRESSURE TENDENCY.

By way of example, moderate to strong cold air ADVECTION in the lower troposphere is typically associated with ____________ PRESSURE TENDENCIES. And ____________ is the HORIZONTAL transport of an atmospheric property (such as temperature or moisture) by the wind.

### negative

In similar fashion, moderate to strong warm air ADVECTION in the LOWER TROPOSPHERE is typically associated with ____________ PRESSURE TENDENCIES as the density of local air columns DECREASES with time.

### high

You may be surprised to learn that the instigators of confrontations BETWEEN AIR MASSES are areas of ______ pressure.

### surface

In a nutshell, AIR MASSES acquire their temperature AND moisture characteristics from the underlying ____________ (land OR water). For example, a chilly, dry air mass that enters the U.S. from Canada during winter became cold and dry while in contact with snow-covered ground and frozen bodies of water at relatively high latitudes, where moisture is sparse.

### high

In a nutshell, sprawling, relatively stationary ______-pressure systems promote persistently LIGHT WINDS that allow air to stay in contact with the earth long enough for the air to take on the underlying surface's characteristics.

### light

Winds are ________ around the cores of HIGH-PRESSURE SYSTEMS because the pressure gradients there are characteristically weak. In Figure 6.34, which shows a sprawling high-pressure system centered over the northern Gulf of Mexico on a July Day, you can see the prevalence of LIGHT winds around this relatively stationary high, allowing the air to TAKE ON the characteristics of the underlying warm ocean.

### lows

In contrast, the regions around the centers of windbag ________ are just too blustery TO ALLOW overlying air to stay in contact with the earth's surface for very long.

### fronts

As mentioned in Chapter 3, the narrow zones where differing air masses meet AND vie for control are called __________.

### low

Given that relatively tranquil HIGH-PRESSURE SYTEMS mark the cores of AIR MASSES, it makes sense that the outer edges of air masses must have relatively ______ pressure. Thus, any FRONT that forms when two air masses meet must be located in a zone of relatively ______ pressure - a trough.

### pressure

Because fronts lie in troughs and zones of typically LARGE HORIZONTAL TEMPERATURE CONTRASTS, they make favorable breeding grounds for centers of low ____________ to form and intensify.