Weather and climate

Weather and climate

These are my personal notes from the Introduction to weather and climate short course by Mel Strong.

Cloud Types

• The traditional cloud classification system considers both altitude and shape. Shape can be cumuliform (“puffy,” with significant vertical and horizontal development), stratiform (“big solid gray sky,” a uniform layer), or cirriform (“wispy,” with very varied forms). Altitude can be high (cirro-), middle (alto-), or low (-).
• Cumulus (Cu): usually have flat bases.
• Altocumulus (Ac): appear smaller due to altitude and actually are smaller; often form patchy patterns (or sometimes in rows); cast shadows and look three-dimensional.
• Cirrocumulus (Cc): very small and thin; you can see through them; have few shadows and appear two-dimensional.
• Cumulonimbus (Cb): start as cumulus and then grow vertically through all altitude layers; the only cumuliform cloud that produces precipitation and the only cloud capable of forming an ice “anvil” at its top (its presence almost guarantees precipitation); rainfall has a sharply defined edge.
• Stratocumulus (Sc): large clusters of cumulus clouds touching (or nearly touching) one another; the most common cloud type worldwide (but mainly over the oceans).
• Stratus (St): gray sky with the sun obscured; may have a little texture.
• Altostratus (As): you can see the sun through them but it appears as a diffuse disk.
• Cirrostratus (Cs): made of ice and almost invisible; produce halos around the sun (often with rainbow colors).
• Nimbostratus (Ns): stratus clouds that produce precipitation; the rain boundaries are indistinct.
• Cirrus (Ci): composed entirely of ice; exhibit a wide variety of textures (feathery, hairlike, cobweb, dense, hooked, streaky) but always have sharply defined edges.

Some Interesting Subtypes in the Traditional System

• Lenticular: disc-shaped, typically form over mountain peaks, usually stationary.
• Mammatus: pouch-like formations hanging from the base of cumulonimbus clouds, caused by sinking cold air.
• Fog: stratus cloud at the Earth’s surface.
• Noctilucent: the highest-forming clouds (around 85 km altitude), found only near the poles, visible after sunset when illuminated by sunlight that doesn’t reach the surface.

Humidity

• Dry air (99.9% of the atmosphere) is composed of 78% N₂, 21% O₂, and 1% Ar. Water vapor isn’t included in those figures because it’s variable: it can range from 0% to 4% of the air’s composition.
• Humidity can be measured as a mixing ratio (grams of water vapor per kilogram of air).
• Air has an upper limit on how much water vapor it can hold (saturation). That limit on the mixing ratio depends exponentially on temperature, which means cold air is always dry and warm air has the capacity to be humid (though it might not be).
• When temperature drops, the air reaches a point where it can’t hold more water, and condensation (or freezing below 0 °C) begins. This temperature is called the dew point: low dew points indicate dry air, high dew points indicate humid air.
• Adding water vapor to air increases both the mixing ratio and the dew point (i.e., the air must be cooled less to reach saturation when it's more humid). The reverse also holds.
• Humidity can also be measured as relative humidity: the current mixing ratio divided by the maximum possible at a given temperature. It’s proportional to the amount of water and inversely proportional to temperature.

Evaporation

• Whenever a body of water is exposed to the atmosphere, molecules constantly exchange between liquid and gas. Temperature controls the rate at which molecules leave the liquid (higher temperature → more evaporation), while relative humidity controls the rate at which they return (higher % humidity → more condensation).
• The liquid-to-gas transition absorbs thermal energy, which is why sweating cools the body. If the air is very humid, sweat won’t evaporate.

Cloud Formation

• The principle of cloud formation is the same as dew formation, but instead of condensing on a surface, water vapor condenses on microscopic particles (about 0.0002 mm) in the atmosphere, such as rock dust, ash, salt, pollen, bacteria, etc.
• Cloud droplets are microscopic (0.02–0.05 mm), whereas raindrops are much larger (about 2 mm).
• Rising air expands and cools, while sinking air compresses and warms. This occurs at about 9.8 °C/km when humidity is below 100%, or 3.6–5.5 °C/km (depending on temperature) when humidity is at 100%, assuming no heat exchange with the environment. The cooling rate is lower in saturated air because condensation releases latent heat, partially offsetting adiabatic cooling.
• When a parcel of air near the surface is warmer than its surroundings, it rises until its temperature equals that of the environment and then sinks, forming a convection cell (thermal). If it reaches its dew point before temperatures equalize, a cloud forms.
• Convection is only one of the four main uplift mechanisms for cloud formation. The others are orographic uplift (see “Circulation”), convergence (see “Circulation”), and frontal uplift (see “Fronts”).
• A cloud will be higher if surface air is warmer (the rising parcel) and upper-level air is colder (the surrounding atmosphere).

Precipitation Formation

• At the top of a cumulonimbus cloud there’s ice. Slightly below, ice coexists with supercooled water droplets (microscopic droplets below 0 °C). Through the Bergeron process, supercooled droplets join ice crystals, growing flakes that eventually fall. They often melt in the cloud’s lower (liquid) region, producing rain. Note: in tropical regions with very high humidity, rainfall may form as water droplets directly.
• Besides the warm updraft in a cloud, precipitation also generates a cold downdraft. Downdrafts can reach high speeds (microbursts), striking the surface at up to 110 km/h and creating an outflow boundary that spreads sideways (sometimes across states). Virga occurs when rain evaporates before reaching the ground, leaving streaky “tentacles.”
• Hail formation isn’t fully understood but is thought to occur when strong updrafts keep hailstones suspended longer, allowing them to grow larger.
• For not-fully-understood reasons, negative charges accumulate in some cloud regions (usually lower) and positive charges in others (usually higher). When charge separation reaches a threshold, ionized gas is emitted in random paths (like a ladder). The negatively charged cloud base induces a positive charge on the ground, and when positive ions from Earth meet negative ions from the cloud, a lightning bolt forms. Lightning can also occur within a single cloud or between clouds.
• A lightning stroke heats the air to thousands of degrees, causing rapid expansion. When that heated air cools, the sudden collapse produces thunder.

Pressure and Wind

• We don’t feel atmospheric pressure because molecules push equally in all directions, canceling out net force. We do feel pressure imbalances.
• There’s a limit to how high you can draw liquid up a tube by suction: once the air is removed, atmospheric pressure on the liquid’s surface pushes it up the tube.
• When winds converge aloft at the tropopause, they force air downward (they can’t go above the tropopause), creating a surface high (H). When winds diverge aloft, they cause a surface low (L). These high- and low-pressure areas cover large regions, often spanning multiple countries.
• Near the equator there’s a persistent low (especially over oceans) because warm, humid surface air continually rises, generating abundant cumulonimbus development.
• Terrain elevation causes much larger pressure changes than weather does, so reported pressures are adjusted to sea level. This ensures a global average of about 1013 hPa everywhere.
• Air initially moves from high to low pressure due to the Pressure Gradient Force (PGF). Earth’s rotation introduces the Coriolis Force, deflecting winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• At the tropopause, PGF and Coriolis nearly balance, so winds flow parallel to isobars. Near the surface, friction slows winds, allowing them to cross isobars toward lower pressure. Wind speed increases with the pressure gradient’s strength.

Troposphere and Stratosphere

• Air temperature decreases with height up to the tropopause (around 11 km), the boundary between the troposphere and stratosphere. Above that, temperature increases with altitude because ozone absorbs ultraviolet radiation and releases heat.
• Chlorofluorocarbons once released into the atmosphere hinder ozone regeneration, enlarging the ozone hole. Following international agreements starting in 1987, damage bottomed out in 2006 and has been slowly recovering. CFCs were completely phased out by 2015.
• All clouds, 99% of atmospheric humidity, and 75% of the atmosphere’s mass are in the troposphere. Above it, the sky appears black.
• Heavier gas molecules tend to concentrate near the surface more than lighter ones (though gravity acts on all molecules). Nevertheless, molecular collisions keep gases well mixed.

Fronts

• Near the surface, air temperature generally decreases from the equator toward the poles, though continental temperatures vary more than oceanic ones (different heat capacities).
• Around 3 km altitude, temperatures are fairly uniform in a broad equatorial band, but contrast sharply toward the poles. The boundary between warm and cold air is the polar front, and its position shifts daily.
• When warm air advances into cold air along a polar front, a warm front forms: warm air slides up over cold with a gentle slope. As a warm front approaches you, you’ll see (in order): cirrus, cirrostratus, altostratus, stratus, nimbostratus.
• Conversely, when cold air pushes into warm air, a cold front forms: cold air undercuts warm with a steep slope. As a cold front approaches, you’re likely to see sudden cumulonimbus development.
• When neither air mass displaces the other, a stationary front forms, producing mixed effects of both warm and cold fronts.
• When a cold front overtakes a warm front moving in the same direction (because it’s faster), an occluded front forms.
• Due to Earth’s rotation, stationary fronts along the polar front begin to rotate, spawning both a warm and a cold front that circle around a low (L) until the cold front catches the warm one and occlusion occurs. This system is called an extratropical cyclone.
• Tropical cyclones (hurricanes) form by a different mechanism, linked to high evaporation over warm oceans. They don’t form right at the equator (Coriolis effect) but at low latitudes.

Circulation

• Earth’s rotation causes easterly trade winds to converge near the equator in the Intertropical Convergence Zone (ITCZ). As they converge, air rises due to high surface temperatures, forming a band of large cumulonimbus clouds and low pressure.
• The ITCZ aligns with the hottest surface band and isn’t exactly at the equator.
• Rising air in the ITCZ reaches the tropopause, then spreads north and south before sinking around 30° N and 30° S, creating two subtropical high-pressure belts. Because continents and oceans differ, these highs appear as more localized subtropical highs.
• There are also low-pressure belts near 60° N and 60° S, and high-pressure centers at the poles. In total, three circulation cells exist (from low to high latitude): Hadley (strongest effects), Ferrel, and Polar.
• A storm moving from ocean to land intensifies as it draws warmth; over mountains it intensifies further. On descending the lee side, the reverse happens but not symmetrically because precipitation depletes moisture (no replenishing water source). This “rain shadow” effect creates a dry side and a wet side of mountains.

Seasonal Changes

• Weather refers to current atmospheric conditions, predictable up to about five days (chaotic system). Climate refers to average conditions at a location, predictable for decades.
• Seasons are caused by Earth’s axial tilt, not changes in distance from the Sun.
• Seasonal changes shift the ITCZ and other low- and high-pressure belts throughout the year.

Monsoons

• Different heat capacities of land and ocean cause a seasonal reversal of wind direction, bringing distinct wet and dry seasons. This process is called a monsoon and is most pronounced in India and Southeast Asia, where the ITCZ passes overhead.
• In winter, warmer ocean air rises and is replaced by cold, dry mountain air. In summer, hot continental air rises, drawing in moist ocean air.

El Niño and La Niña (ENSO)

• Subtropical high-pressure winds carry cold polar waters toward the equator along eastern ocean boundaries, while warm equatorial waters move toward the poles along western boundaries. On eastern coasts, upwelling of cold deep water supports abundant fisheries.
• In the southern Pacific Ocean, subtropical highs sometimes weaken (or reverse to lows), allowing warm coastal waters to spread along South America—this is El Niño.
• The opposite can also occur: stronger subtropical highs bring colder coastal waters along western South America—this is La Niña.
• Both El Niño and La Niña are natural, unpredictable events lasting several months (\~4–8). They occur only in the Pacific Ocean (large enough to feel effects) but can impact global precipitation patterns.

Greenhouse Effect

• Most solar radiation reaching Earth’s surface is shortwave (up to near-infrared). The ground absorbs much of it and emits longwave (heat) radiation back to space.
• O₂ and N₂ are transparent to both shortwave and longwave. Greenhouse gases (H₂O, CO₂, CH₄) absorb longwave radiation and re-emit it in random directions. This greenhouse effect is essential for life on Earth.
• Water vapor doesn’t absorb shortwave, but clouds scatter and reflect it, reducing surface heating. Since water vapor is also a greenhouse gas, cloudy days are cooler but nights are warmer.
• Over the last 100 years, human emissions of greenhouse gases have risen unprecedentedly, gradually increasing global average temperatures.
• Although water vapor is the most abundant greenhouse gas, its atmospheric concentration can’t increase indefinitely because it precipitates quickly. Other greenhouse gases remain in the atmosphere for years.
• In the USA, 82.2% of greenhouse-effect contribution comes from CO₂ emissions. Over 90% of that CO₂ comes from fossil fuels, subdivided into: 38% energy generation, 34% transportation, 16% industry, 7% residential, and 5% commercial. Another 10% comes from CH₄ emissions (mainly agriculture, natural gas extraction, and landfills). The remaining 5.1% is from N₂O emissions, disproportionately from agricultural soil fertilization. (See the EPA’s graphs for global figures.)

Carbon Cycle

• Carbon reservoirs and net fluxes:
• Vegetation: 600 Tg (+2.6 Tg/yr)
• Atmosphere: 829 Tg (+4 Tg/yr)
• Ocean: 38,000 Tg (+2.3 Tg/yr)
• Soil: 2,000 Tg
• Fossil fuels: 4,100 Tg
• Carbonate rocks: 80,000,000 Tg
• Adding CO₂ to the oceans forms carbonic acid, which harms corals and their dependent biodiversity.
• The Keeling Curve shows atmospheric CO₂ over time has two components: a seasonal oscillation (due to differing vegetation cycles between hemispheres: trough in northern summer, peak in northern winter) and a quasi-linear rise from human emissions over the last 150 years.