Chapter 12 - Moisture, Clouds, and Precipitation

Questions and Answers

What is the primary factor that determines the dew point temperature?

The temperature of the air

The pressure of the air

The time of day

The amount of water vapor in the air (correct)

What is a characteristic of water vapor?

It has an odor

It is a green gas

It is odorless (correct)

It is a liquid at room temperature

What happens to the amount of water vapor needed for saturation when the temperature increases by 10°C?

It triples

It decreases by half

It remains the same

It doubles (correct)

What is unique about water?

It exists as a solid, liquid, and gas in the atmosphere Signup and view all the answers

What is the primary mechanism that accounts for the formation of dew and some types of fog?

Radiation cooling Signup and view all the answers

What happens to the bonds between water molecules when ice melts?

They break Signup and view all the answers

What is the purpose of a psychrometer?

To determine relative humidity Signup and view all the answers

What is the term for the energy involved in changing states of matter?

Latent heat Signup and view all the answers

What is indicated by a high dew point temperature?

Moist air Signup and view all the answers

What is required for the formation of dew, fog, or clouds?

Saturated air Signup and view all the answers

What is the approximate amount of energy required to convert 1 gram of water at 100°C into a gas?

600 calories Signup and view all the answers

What is the term for the process of converting a gas into a liquid?

Condensation Signup and view all the answers

What is the primary mechanism that accounts for the formation of clouds?

Convection Signup and view all the answers

What is displayed on weather maps?

Dew point temperature Signup and view all the answers

What is the term for the amount of water vapor in air?

Humidity Signup and view all the answers

What happens to the vapor pressure as water continues to evaporate?

It increases until the air is saturated Signup and view all the answers

What is the range of water vapor in the atmosphere?

1/10 of 1% to 4% Signup and view all the answers

What is an example of sublimation?

The shrinking of ice cubes left in the freezer for long periods of time Signup and view all the answers

What is the term used to describe the air when the number of vapor molecules returning to the water equals the number leaving the liquid?

Saturated Signup and view all the answers

What is the term for the conversion of a gas directly into a solid without passing through the liquid state?

Deposition Signup and view all the answers

What happens to the water-vapor capacity when heat is applied to saturated air?

It increases until a new balance is achieved Signup and view all the answers

What is the main drawback of using the mixing ratio method to express water vapor content?

It is a time-consuming process that requires direct sampling of the atmosphere Signup and view all the answers

What does relative humidity indicate?

How near the air is to saturation Signup and view all the answers

What are the two ways in which relative humidity can change?

Addition or removal of water vapor and change in temperature Signup and view all the answers

What is the main advantage of using relative humidity over mixing ratio?

It is less time-consuming Signup and view all the answers

Why is warm air able to hold more water vapor than cold air?

Because of its higher temperature Signup and view all the answers

What is the primary process responsible for most cloud formation?

Adiabatic temperature change Signup and view all the answers

What happens to the atmospheric pressure as altitude increases?

It decreases Signup and view all the answers

At what rate does unsaturated air cool for every 1,000 meters it rises?

10° C Signup and view all the answers

What is the term for the rate of cooling of air when it is saturated with water vapor?

Wet adiabatic rate Signup and view all the answers

What is the term for the localized pockets of air that rise due to buoyancy?

Thermals Signup and view all the answers

What is the primary reason why air resists vertical movement?

It stays at the Earth's surface Signup and view all the answers

What is orographic lifting an example of?

Forced ascent over a mountainous region Signup and view all the answers

What determines the stability of air?

The temperature of the surrounding air Signup and view all the answers

What happens to the air when it descends down the slope of the mountain after orographic lifting?

It warms adiabatically Signup and view all the answers

What happens to air that is cooler than its surrounding air?

It sinks Signup and view all the answers

What is the environmental lapse rate in the given illustration?

5°C per 1,000 meters Signup and view all the answers

What is the term for the process where warm, less dense air is forced over cooler, more dense air?

Frontal wedging Signup and view all the answers

What type of air develops when the environmental lapse rate is less than the wet adiabatic rate?

Absolute stability Signup and view all the answers

What is the result of convergence of air flow?

Upward movement Signup and view all the answers

What commonly occurs on clear nights when radiation cooling lowers the temperature dramatically near the Earth's surface?

Temperature inversion Signup and view all the answers

What is the term for the rate of cooling of unsaturated air for every 1,000 meters it sinks?

10° C Signup and view all the answers

What type of air develops when the environmental lapse rate is greater than the dry adiabatic rate?

Absolute instability Signup and view all the answers

What happens to the air at the surface if it is unsaturated and rises to 1,000 meters?

It becomes cooler and denser than its surrounding environment Signup and view all the answers

What prevents vertical mixing of the atmosphere and can trap pollutants near the Earth's surface?

Temperature inversion Signup and view all the answers

What type of clouds can form when the air is considerably warmer than air aloft?

Small, fluffy white clouds Signup and view all the answers

What is the primary reason why an average cloud droplet cannot reach the Earth's surface?

It evaporates before it reaches the ground Signup and view all the answers

What is the minimum diameter of a droplet for it to be considered rain?

0.5 millimeters Signup and view all the answers

In which region do clouds contain droplets that form through the Bergeron process?

Mid-latitudes Signup and view all the answers

What is the term for the fall of small particles of ice that are clear to translucent?

Sleet Signup and view all the answers

What is the definition of clouds?

Visible aggregates of minute droplets of water or tiny ice crystals Signup and view all the answers

What is the purpose of a rain gauge?

To collect and measure rainfall Signup and view all the answers

What is the ratio of snow to water equivalent?

10:1 Signup and view all the answers

What are the three basic forms of clouds?

Cirrus, Cumulus, and Stratus Signup and view all the answers

What is the characteristic of high clouds?

They have bases above 20,000 feet Signup and view all the answers

What is the term for the process of ice crystals growing through the consumption of excess water vapor?

Bergeron process Signup and view all the answers

What is the main difference between fog and clouds?

Fog is a cloud with its base at or very near the ground Signup and view all the answers

What is the primary difference between rain and drizzle?

Rain has a larger diameter than drizzle Signup and view all the answers

In which process do hygroscopic particles remove water vapor from the air?

Collision-coalescence process Signup and view all the answers

What is the term for clouds that have bases in the low range and extend into the middle and high ranges?

Clouds of vertical development Signup and view all the answers

What is the term for fog that occurs when warm, moist air moves over a cool surface?

Advection fog Signup and view all the answers

What is the term for the deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on objects?

Rime Signup and view all the answers

Why do some clouds produce precipitation, while others do not?

Because of the abundance of condensation nuclei Signup and view all the answers

What is the term for clouds that are associated with stable conditions, but can grow dramatically in height under the correct conditions?

Cumulus clouds Signup and view all the answers

What is the term for fog that occurs when cool air moves over warm water?

Steam fog Signup and view all the answers

Why do cloud droplets fall very slowly?

Because of their small size Signup and view all the answers

What is conditional instability in the atmosphere?

A type of instability that occurs when the environmental lapse rate is between the dry and wet adiabatic rates Signup and view all the answers

What happens when stable air is forced aloft?

The air forms widespread, vertically thin clouds with little precipitation Signup and view all the answers

What is necessary for condensation to occur in the atmosphere?

Both a surface for water vapor to condense on and saturated air Signup and view all the answers

What is unique about hygroscopic nuclei?

They are good condensation nuclei because they can absorb water Signup and view all the answers

What type of clouds are associated with lifting unstable air?

Cauliflower-shaped clouds that appear to be growing in height Signup and view all the answers

What happens when air is cooled to its dew point?

The air becomes saturated and condenses Signup and view all the answers

What is the role of condensation nuclei in the atmosphere?

They serve as a surface for water vapor to condense on Signup and view all the answers

What is characteristic of clouds formed from unstable air?

They are towering, cauliflower-shaped, and can produce thunderstorms Signup and view all the answers

Study Notes

Water Vapor and Atmospheric Processes

Water vapor is an odorless, colorless gas that can change states of matter at Earth's temperatures and pressures.
It is the only substance that exists as a solid, liquid, and gas in the atmosphere.
Composed of 2 hydrogen atoms bonded to an oxygen atom.

States of Matter of Water

Solid (ice):
- Water molecules are held together by molecular attractions, forming a tight, orderly arrangement.
- Molecules vibrate about fixed sites and are not free to move about.
- Melting occurs when molecular movements increase, breaking bonds between water molecules.
Liquid (water):
- Water molecules are tightly packed but move fast enough to slide past one another.
- As molecular movement increases, remaining molecular attractions are broken, allowing molecules to escape and become water vapor.
Gas (water vapor):
- Water molecules are widely spaced and exhibit energetic, random motion.
- Water vapor is compressible and distinguished from liquid water in this way.

Energy and State Changes

Changing states of matter requires heat (energy) to be absorbed or released.
Measured in calories.
Latent heat:
- Latent heat of melting: 80 calories to change 1 gram of ice at 0°C to liquid.
- Latent heat of fusion (freezing): 80 calories to change 1 gram of liquid water at 0°C to solid.
- Latent heat of vaporization: approximately 600 calories to change 1 gram of water at 100°C to gas.
- Latent heat of condensation: approximately 600 calories to change 1 gram of water vapor at 100°C to liquid.

Additional State Changes

Sublimation: conversion of a solid directly into a gas without passing through the liquid state.
Deposition: conversion of a gas directly into a solid without passing through the liquid state.

Water Vapor in the Atmosphere

Water vapor comprises a small amount of gases within the atmosphere (1/10 of 1% to 4%).
Most important gas in understanding atmospheric processes.
Humidity is the general term for the amount of water vapor in air.

Measures of Water Vapor Content

Mixing ratio: mass of water vapor in a unit of air compared to the remaining mass of dry air.
Relative humidity: ratio of air's actual water vapor content compared with the amount required for saturation at that temperature.
Dew-point temperature: temperature to which a parcel of air would need to be cooled to reach saturation.

Condensation and Cloud Formation

Condensation occurs when air is cooled to its dew point temperature.
Requires a surface on which water vapor can condense (e.g., particulate matter, condensation nuclei).
Clouds can consist of water droplets, ice crystals, or both.

Cloud Formation Mechanisms

Adiabatic temperature change: change in temperature that occurs without heat being added or removed.
Orographic lifting: air is forced to rise over a mountainous region, causing adiabatic cooling and cloud formation.
Frontal wedging: warmer, less dense air is forced over cooler, denser air, causing it to rise and cool.
Convergence: pileup of horizontal airflow results in upward movement.
Localized convective lifting: unequal surface heating causes localized pockets of air to rise due to buoyancy.

Air Stability

Stable air: when a parcel of air is cooler than the surrounding air, it will sink.
Unstable air: when a parcel of air is warmer than the surrounding air, it will rise.
Environmental lapse rate: rate of temperature change with altitude.
Adiabatic temperature change: changes in temperature caused by expansion or compression.

Types of Air Stability

Absolute stability: develops when the environmental lapse rate is less than the wet adiabatic rate.
Absolute instability: develops when the environmental lapse rate is greater than the dry adiabatic rate.
Conditional instability: develops when moist air has an environmental lapse rate between the dry and wet adiabatic rates.### Clouds
Clouds are visible aggregates of minute droplets of water or tiny ice crystals
Classified by form and height:
- Cirrus: high, white, and thin; can occur as patches, veil-like sheets, or wispy fibers
- Cumulus: globular individual cloud masses; normally have a flat base and the appearance of rising domes or towers
- Stratus: sheets or layers that cover much or all of the sky; typically have no distinct individual cloud units
Three levels of cloud height:
- High clouds: have bases above 20,000 feet
- Middle clouds: occupy heights from 6,500 to 20,000 feet
- Low clouds: form below 6,500 feet

Cloud Classification

High clouds:
- Cirrus
- Cirrostratus
- Cirrocumulus
Middle clouds:
- Altocumulus
- Altostratus
Low clouds:
- Stratus
- Stratocumulus
- Nimbostratus
Clouds of vertical development:
- Cumulus clouds can grow dramatically in height under correct conditions, forming cumulonimbus

Fog

Fog is a cloud with its base at or very near the ground
Typically reduces visibility
Officially reported when visibility is reduced to 1 km (0.6 miles) or less
Types of fog:
- Fogs Caused by Cooling:
  - Advection fog: occurs when warm, moist air moves over a cool surface
  - Radiation fog: forms on cool, clear nights when the Earth's surface cools rapidly by radiation
  - Upslope fog: occurs when relatively humid air moves up a sloping terrain
- Evaporation Fogs:
  - Steam fog: occurs when cool air moves over warm water, producing saturation
  - Frontal (or precipitation) fog: occurs when frontal wedging occurs, producing precipitation that evaporates to produce fog

Precipitation

Cloud droplets are very small (20 micrometers), requiring a large size to reach the Earth's surface
Two processes result in large volume droplets:
- Bergeron process:
  - Relies on supercooling and supersaturation
  - Dominant in mid-latitudes where cold clouds exist
- Collision-coalescence process:
  - Occurs in warm clouds where large droplets form
  - Requires giant condensation nuclei or hygroscopic particles
Types of precipitation:
- Rain: drops of water with a diameter of at least 0.5 millimeters
- Drizzle: fine uniform drops of water with a diameter of less than 0.5 millimeters
- Snow: precipitation in the form of ice crystals or aggregates of crystals
- Sleet: small particles of ice that are clear to translucent
- Glaze: raindrops that fall and freeze, resulting in a thick coating of ice
- Hail: hard, rounded, pellets or irregular lumps of ice
- Rime: deposit of ice crystals formed by the freezing of supercooled fog or cloud droplets on objects

Measuring Precipitation

Rain is measured using a rain gauge, which collects and conducts rain into a cylindrical measuring tube
Snow has two measurements: depth and water equivalent
- General ratio: 10 parts snow equals 1 part water
- Ratio varies based on physical properties of snow
Weather radar is used to estimate rainfall and rate of rainfall
- Radio waves are transmitted and reflected by larger raindrops, ice crystals, or hailstones
- Reflected signal (echo) is displayed

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Description

Learn about the physical properties of water vapor, its states of matter, and how temperature affects molecular motion. Understand the importance of water vapor in atmospheric processes.