Chapter 5 Questions for APS 101

- What is Doppler radar? - How does Doppler radar measure the intensity of precipitation?

- Doppler radar (also used to be known as next generation radar, or NEXRAD) uses the principle of Doppler shift in frequency of microwave radiation to measure the velocity of water droplets or ice crystals as they move toward or away from the radar transceiver. - It measures the intensity of precipitation by sending out a pulse of electromagnetic energy, then measuring the strength (and time delay) of that portion of the pulse that is scattered back to the radar transceiver by precipitation in the atmosphere (rain drops and snow or ice pellets).

- Why do heavy showers usually fall from cumuliform clouds? - Why does steady precipitation normally fall from stratiform clouds?

- If the shower is excessively heavy, it may be informally called a cloudburst. Beneath a cumulonimbus cloud, which normally contains large convection currents of rising and descending air, it is entirely possible that one side of a street may be dry (updraft side), while a heavy shower is occurring across the street (downdraft side) -Continuous rain, on the other hand, usually falls from a layered cloud that covers a large area and has smaller vertical air currents. These are the conditions normally associated with nimbostratus clouds.

How can the atmosphere be made more stable? More unstable?

- The atmosphere is stable when the environmental lapse rate is small. Consequently, the atmosphere tends to become more stable as the air aloft warms or the surface air cools. - A stable atmosphere can be made unstable when the environmental lapse rate steepens, which occurs whenair temperature drops rapidly with increasing height.

How would you be able to distinguish between virga and fallstreaks?

-Occasionally, the rain falling from a cloud never reaches the surface because the low humidity causes rapid evaporation. As the drops become smaller, their rate of fall decreases, and they appear to hang in the air as a rain streamer. These evaporating streaks of precipitation are called virga* -When ice crystals and snowflakes fall from high cirrus clouds they are called fallstreaks. -Fallstreaks behave in much the same way as virga—as the ice particles fall into drier air, they usually disappear as they change from ice into vapor (called sublimation). Because the wind at higher levels moves the cloud and ice particles horizontally more quickly than do the slower winds at lower levels, fallstreaks often appear as dangling white streamers. Moreover, fallstreaks descending into lower, supercooled clouds may actually seed them.

What is the primary dierence between a cloud droplet and a raindrop?

A cloud droplet is much smaller (by a factor of about 100) than a raindrop.

What is an adiabatic process?

An adiabatic process is one in which a parcel of air expands and cools, or compresses and warms, without exchanging heat with the air surrounding it.

There are usually large spaces of blue sky between cumulus clouds. Explain why this is so.

Because the thermals that form them on the ground are usually shaded/etc by the cloud that was just formed, and take time for the thermals to build up again.

What type of clouds would you most likely expect to see in a stable atmosphere? In a conditionally unsta-ble atmosphere?

Clouds in a stable atmosphere tend to spread out horizontally, resulting in cirrostratus, altostratus, nimbostratus or stratus forming in the stable air. Clouds are more likely to grow vertically in an unstable/conditionally unstable atmosphere, as the warm air parcel from the ground tends to keep rising upwards.

Why do typical cloud droplets seldom reach the ground as rain?

Clouds, are composed of many small droplets—too small to fall as rain. These minute droplets require only slight upward air currents to keep them suspended. Those droplets that do fall, descend slowly and evaporate in the drier air beneath the cloud.

Describe how the process of collision and coalescence produces rain.

Collision and coalescence is a way of "growing" small drops into large drops by means of the large drops physically bumping into and combining with smaller drops as they move around in a cloud. Eventually, drops will grow large enough and heavy enough to overcome any rising motions in the cloud, and will fall out of the cloud as rain.

Why are cumulus clouds more frequently observed during the afternoon?

Cumulus clouds are more frequently observed in the afternoon, because convection due to surface heating provides more lifting impetus to rising parcels in the afternoon when the ground is warmest, and because surface heating results in a more unstable actual lapse rate in the lower atmosphere in the afternoon.

How do the atmospheric conditions that produce sleet dier from those that produce hail?

Hail typically occurs in the summer, in large thunder clouds that cycle supercooled water droplets for 5-10 minutes. Sleet, typically occurs during winter time, and involves a falling snowflake, that melts and refreezes before hitting earths surface.

If the atmosphere is conditionally unstable, what does this mean? What condition is necessary to bring on instability?

If the atmosphere is conditionally unstable, that means it will be stable with respect to a rising DRY parcel, but UNSTABLE with respect to a rising saturated (or "moist") parcel. Therefore, the "condition" necessary to bring on instability in a conditionally unstable atmosphere is the presence of a rising saturated parcel of air.

How are satellites able to measure precipitation intensity inside clouds?

Launched in April 2006, the satellite CloudSat circles the earth in an orbit about 700 km (430 mi) above the surface. Onboard CloudSat, a very sensitive radar is able to peer into a cloud and provide a vertical view of its tiny cloud droplets and ice particles, as well as precipitation. (An example of CloudSat is shown is Chapter 4 in Fig. 4.51.) Such vertical profiling of liquid water and ice will hopefully provide scientists with a better understanding of precipitation processes that go on inside the cloud and the role that clouds play in the earth's global climate system.

How does rain differ from drizzle?

Rain - diameter equal to, or greater than, 0.5 mm (0.02 in) Drizzle - diameter smaller than 0.5, most drizzle falls from stratus clouds

What is the difference between freezing rain and sleet?

Sleet is a form of frozen pellet precipitation (pellet diameter 5 mm or less). It forms by refreezing of raindrops as they pass through a low level subfreezing layer of air. Freezing rain is precipitation that falls to the ground as rain, but freezes upon contact with the ground or other surface that has a temperature below freezing. This forms a glaze of ice on any surface it reaches

How can the depth of snow be measured?

Snow accumulation rates may be measured at a single site by using a snowboard, a small wooden platform resting near the ground that is cleared off after each measurement. Snow depth may also be measured by removing the collector and inner cylinder of a standard rain gauge and allowing snow to accumulate in the outer tube. Automated gauges are now used in many areas to collect snowfall and measure the amount of liquid water it contains. Typically, these gauges are surrounded by one or more octagonal fences that help to block wind and produce more accurate readings.

How does the ice-crystal (Bergeron) process produce precipitation? What is the main premise behind this process?

The Bergeron Process proposes that ice crystals and liquid cloud droplets must co-exist in clouds at temperatures below freezing. This process of rain formation is extremely important in middle and high latitudes, where clouds are able to extend upwards into regions where air temperatures are below freezing. These are known as cold clouds. Above the freezing level, liquid water droplets are known as supercooled liquid droplets. The reason behind this is the lack of ice nuclei, which typically consist of clay minerals and bacteria that share similar geometry to an ice crystal. In the subfreezing air of a cloud, many supercooled liquid droplets will surround each ice crystal. Both the liquid droplets and the ice crystals are in equilibrium, meaning that the number of molecules leaving the surface of both the droplet and the ice crystal must equal the number of molecules returning. This difference in vapor pressure causes water vapor molecules to move (diffuse) from the droplet toward the ice crystal. The removal of vapor molecules reduces the vapor pressure above the droplet. Since the droplet is now out of equilibrium with its surroundings, it evaporates to replenish the diminished supply of water vapor above it. This process provides a continuous source of moisture for the ice crystal, which absorbs the water vapor and grows rapidly. The ice crystal then grows even larger by colliding with supercooled liquid droplets, at which point the liquid droplets stick to the ice crystal. This is called accretion. The icy matter that forms is called graupel, which falls, splinters and forms ice crystals that act as seeds. These crystals then form snowflakes, which are an aggregate of crystals, which may melt before it reaches the ground.

Why are the moist and dry adiabatic rates of cooling different?

The dry and moist adiabatic rates are different because in a saturated parcel, latent heat is released during condensation, and adds to the heat within a rising parcel. Therefore, the parcel will not cool as rapidly.

Explain the main principle behind cloud seeding.

The primary goal in many experiments concerning cloud seeding is to inject (or seed) a cloud with small particles that will act as nuclei, so that the cloud particles will grow large enough to fall to the surface as precipitation.

Why do most thunderstorms have flat tops?

The reason for this shape is due to the fact that the cloud has reached the stable part of the atmosphere, and the rising air is unable to puncture very far into this stable layer. Consequently, the top of the cloud spreads laterally as high winds at this altitude (usually above 10 km or 33,000 ft) blow the cloud's ice crystals horizontally.

Describe how a standard rain gauge measures precipitation.

They consistof a funnel-shaped collector attached to a long measuring tube. The cross-sectional area of the collector is ten times that of the tube. Hence, rain falling into the collector is amplified tenfold in the tube, permitting measurements of great precision—to as low as one-hundredth (0.01) of an inch.

Explain why an inversion represents an extremely stable atmosphere.

With an inversion, warm air overlies cold air; rising air is becoming colder while the air around it is getting warmer. This causes the rising air to sink, creating stability.

How would one normally obtain the environmental lapse rate?

You could measure the environmental lapse rate by sending up an instrumented weather balloon, or by aircraft, or remotely sensed (ground-based or satellite) measurements.

List four primary ways in which clouds form.

convection, orographic lifting, surface convergence, and lifting along fronts.

Why is it never too cold to snow?

the air always contains water vapor that could produce snow

Explain why rain shadows form on the downwind (leeward) side of mountains.

the dew-point temperature of the air on the leeward side is lower than it was before the air was lifted over the mountain. The lower dew point and, hence, drier air on the leeward side is the result of water vapor condensing and then remaining as liquid cloud droplets and precipitation on the windward side.

Explain how clouds can be seeded naturally.

when cirriform clouds lie directly above a lower cloud deck, ice crystals may descend from the higher cloud and seed the cloud below. As the ice crystals mix into the lower cloud, supercooled droplets are converted to ice crystals, and the precipitation process is enhanced. Sometimes the ice crystals in the lower cloud may settle out, leaving a clear area or "hole" in the cloud. When the cirrus clouds form waves downwind from a mountain chain, bands of precipitation often form—producing heavy precipitation in some areas and practically no precipitation in others

On which side of a mountain (windward or leeward) would lenticular clouds most likely form?

windward