Chapter 4: atmospheric moisture and stability

why does indoor air become very dry in winter?

Cold outdoor air (which holds very little moisture even when saturated) is brought inside and heated. Heating increases the SMR, so the RH drops dramatically.

• cold dry air + heating = even lower RH indoors.

what is the lifting condensation level (LCL)?

the altitude where a rising air parcel cools to its dew point, reaches saturation (RH - 100%), and cloud formation begins due to condensation.

• LCL = cloud base altitude.

how much latent heat is released during condensation and why does this matter for storms?

600 calories per gram of water. When condensation occurs in the atmosphere, this heat is released to surrounding air, increasing its buoyancy, causing it to rise, and fueling storm growth.

• more condensation = more heat released = stronger updrafts.

what is capillary action what properties of water allow for it?

capillary action is the ability to flow through narrow spaces against gravity. it arises from water’s polarity, which creates adhesion (attraction to other surfaces) and cohesion (attract to itself).

What is Saturation?

A state of equilibrium where the rate of condensation equal the rate of evaporation the air is “full” of water vapor. Like a completely soaked sponge.

What is Sublimation vs. Deposition?

Sublimation = Solid → gas (eg Ice cubes shrinking in a freezer).

Deposition = Gas → solid direct skipping the liquid phase.

• dry ice ‘smoking’ sublimation. Frost on a window is deposition.

what is sensible heat?

energy gained or released within a single phase that DOES show up as a change in temperature measurable with a thermometer.

• ‘sensible’ = you can sense /feel it as a temp change.

how does a temperature inversion affect atmospheric stability and pollution?

a temperature inversion (warm air above cold air) is highly stable. Vertical mixing is inhabited. Pollutants near the surface cannot rise and disperese, becoming trapped in the lower atmosphere.

• inversions act like a lid on a pot → trapping everything below.

what is a sling psychrometer and how does it work?

a handheld instrument for measuring humidity using two thermometers one dry bulb and one wet bulb. The wet bulb is cooled by evaporation, so it always reads lower than the dry bulb. The greater the difference the lower the humidity.

• more evaporation = bigger wet/ dry bulb difference = direr air.

What are the 3 factors that ENHANCE atmospheric stability?

1) radiative cooling of earth’s surface after sunset

2) cooling of an air mass from below as it passes over a cold surface

3) subsidence (sinking) within an air column.

• Bottom cooling or sinking = more stability

what is relative humidity (RH)?

the ratio of the actual amount of water vapor in the air (mixing ratio) to the maximum it can hold at the temperature (saturation mixing ratio), expressed as a percentage.

• RH = (MR /SMR) x 100%

what is the dew point temperature?

the temperature to which air must be cooled (at constant pressure and water vapor content) to reach saturation (RH = 100%), causing condensation to begin.

• air temp = dew point → Condensation (fogs, dews, clouds)

what is the mixing ratio?

the actual amount of water vapor in a parcel of air, expressed as grams of water vapor per kilogram of dry air.

• it measures how much moisture IS in the air.

what condition defines Conditional instability?

WALR < ELR < DALR.

Stable when unsaturated (air must be forced up) but once it reaches the LCL and saturates it becomes unstable and rises freely on its own. Common setup for severe thunderstorms.

• ‘conditional’ = unstable only IF it reaches saturation first.

what is the Environmental Lapse Rate (ELR) and how does it relate to stability?

the ELR is actual measured temperature decrease with altitude in the atmosphere at a given time and place. Comparing it to the DALR and WALR tells us if air is stable or unstable.

• ELR is measured. DALR/WALR are rates of parcel change.

what is the level of free convection (LFC)?

the altitude in conditional instability where a forced rising saturated parcel becomes warmer than the surrounding environment and begins rising freely without any forcing.

• Above the LFC, the storm can sustain itself.

why is warm air forced above cold air in frontal lifting?

cold air is denser than warm air. When a cold air mass meets a warm air mass, the cold air acts as a wedge that forces the less dense warm air to rise over it.

• dense cold air = stays low

• less dense warm air = rides up

how do you find the dew point on a mixing ratio vs. temperature chart?

plot the current temperature and mixing ratio. keep the mixing ratio constant and follow it until it intersects the saturation mixing ratio curve. Read the temperature from the X- axis thats the dew point.

• dew point is where your mixing ratio line meets the SMR curve.

what is orographic lifting and what climate effect does it create?

air is forced to rise over a mountain range. the windward side gets heavy precipitation; the leeward side is very dry because sinking air warms and inhibits condensation creating a rain shadow.

• windward = wet

• leeward = dry rain shadow.

what is the dry adiabatic lapse rate (DALR)?

the rate at which unsaturated (RH < 100%) air cools as it rises, or warms as it sinks: + or - 10*C per 1 km of altitude change.

• Dry= unsaturated. 10*C/km

can warm air with low relative humidity contain more water vapor than cold air at 100% RH?

Yes because warm air has much higher SMR , 20% RH at 25*C can contain more actual water vapor than 100% RH at -10*C.

• so like Death valley at 20% RH vs. Chicago at 100% RH in January

what is saturation mixing ratio? (SMR)

the maximum amount of water vapor (in grams) that a kilogram of air can hold at a given temperature. Warmer air has a higher SMR>

• Warmer air = bigger ‘sponge’ capacity.

what happens to a saturated air parcel when it starts to sink?

it immediately begins to warm. Warming reduces RH below 100% so condensation stops and the parcel now wamrs at the faster DALR (10*C/km).

• sinking = warming= RH drops = always warms at DALR.

what happens to relative humidity if you add water vapor while keeping temperature constant?

RH increases, because more water vapor is being added toward the saturation limit while the SMR stays the same

• adding vapor → numerator rises → RH rises

what are the 3 examples of deserts created by the rain shadow effect?

The great basin desert (USA)

the global desert (mongolia)

the takla makan (china)

what is specific heat?

the energy (in calories) required to change the temperature of 1 gram of substance by 1*C. 1 calorie = 4.184 joules.

• water was very high specific heat so it resists temperature change.

why is water considered essential for habitability on other planets?

water is a good solvent, readily changes phases under earth like conditions, and is believed to be an essential component for life. Scientist search for liquid water as a key sign of habitability.

• ‘follow the water’ is a guiding principle in astrobiology.

what are the five processes that lift air in the atmosphere?

1) orographic lifting (mountains)

2) frontal lifting

3) Surface convergence

4) upper air divergence

5) localized convective heating (thermals)

• mountains fronts, convergence, divergence, convection → MFCDC

what is latent heat?

energy gained or released ONLY during a phase change (eg liquid → gas). Temperature does NOT change while latent heat is being exchanged.

• ‘latent’ = hidden no thermometer change during phase change.

What is the ‘Goldilocks Zone’ and why does it matter for water?

the habitable zone around a star where conditions are not too hot, not too cold allowing liquid water to exist on a planet’s surface. Earth is in this zone around the sun.

• just right temperature = liquid water possible.

what condition defines absolute instability?

ELR> DALR.

the environment cools faster than any rising parcel the parcel is always warmer than surroundings so it keeps rising freely. Produces towering cumulonimbus clouds and afternoon/evening thunderstorms.

• most common on hot summer afternoons when the surface heats rapidly.

What are three things that change relative humidity in nature?

1) Daily temperature changes (RH is highest near sunrise when temp is lowest)

2) Horizontal air movement into/out of a region changing temperature

3) vertical air movement (rising = cooling= higher RH)

• max RH usually occurs around sunrise

Why is the WALR less than the DALR?

Once air reaches saturation (at the LCL), condensation releases latent heat inside the parcel. this added heat partially offsets the cooling so the parcel cools more slowly than unsaturated air.

• latent heat release = internal heating = slower cooling rate

name four factors that ENHANCE atmospheric instability

1) Solar heating of the lowest atmosphere

2) heating of a cold air mass from below as it passes over a warm surface

3) orographic/frontal/convergence lifting

4) radiative cooling of cloud tops

• bottom heating or top cooling = more instability

why is water considered an extremely unique molecule on earth?

it is the most abundant liquid on earth and one of the few substances that exist in all three phases (solid, liquid, gas) under Earth’s surface conditions. Its solid form (ice) is also less dense than its liquid form rare among common molecules

• most solids sink in their liquid ICE FLOATS

what condition defines absolute stability?

ELR < WALR . the environment cools more slowly than even a saturated parcel the rising parcel is always cooler than its surroundings so it resists rising. results in thin, widespread clouds with light to moderate precipitation.

• smaller ELR = most stable. air must be forced to rise.

what is stable vs. unstable air?

stable air is cooler and denser than it surroundings it resist rising. Unstable air is warmer and less dense than surroundings it rises freely, producing clouds and precipitation.

• Stable = sinks back

• unstable= keeps rising

what are thermals and what causes them?

thermals are pockets of air that are warmer (and therefore less dense) than surrounding air, causing them to rise spontaneously. They are caused by unequal surface hating on warm days.

• glider pilots and hawks ride thermals to gain altitude.

what is the Dew Point Lapse Rate?

the rate at which dew point temperature decreases in a rising. UNSATURATED parcel: -2*C per km. Once the parcel saturates (at the LCL), the dew point stops decreasing at this rate.

• only applies when air is unsaturated (below the LCL).

what is the Wet Adiabatic Lapse Rate (WALR)?

the rate at which SATURATED (RH =100%) air cools as it rises: -5 to -9*C per km. It caries because the amount of moisture determines how much latent heat is released inside the parcel.

• wet =saturated. Slower cooling because latent heat is released.

what six factors allow Earth to maintain liquid water?

1) distance from the sun (Goldilocks zone)

2) greenhouse effect

3) planetary radius

4) Sufficient gravity to hold an atmosphere

5) Magnetic field maintaining a ‘closed’ system

• remove any one and water likely escapes to space or freezes.

what happens to relative humidity when air cools (with water vapor held constant)?

RH increases because the SMR decreases as temperature drops the cold air can hold less vapor, sot it gets closer to saturation. When RH hits 100% condensation begins.

• cooling = smaller sponge capacity = closer to full.