8.4 The Atmosphere, Weather, the Water Cycle, and Astronomy

8.4 The Atmosphere, Weather, the Water Cycle, and Astronomy

Candidates who have sat the AdUCET consistently report science items on the layers of the atmosphere and on cloud types, so those two topics are treated here in full detail. This section then works outward through the water cycle, weather systems, Philippine climate and finally astronomy. Earth's interior, plate tectonics and rocks belong to section 8.3.

What the Atmosphere Is Made Of

Dry air near the surface is 78% nitrogen and 21% oxygen by volume. The remaining 1% is mostly argon (about 0.93%) plus carbon dioxide (about 0.04%) and trace gases. On top of that sits water vapour, which is highly variable — close to zero over a desert and up to about 4% over the warm seas around the Philippines. Water vapour, carbon dioxide and ozone are the small ingredients that do almost all of the atmospheric work.

The Five Layers of the Atmosphere

The layers are defined by how temperature changes with altitude, and the boundary between two layers is a pause — tropopause, stratopause, mesopause, thermopause.

A useful check: the trend alternates — down, up, down, up. Remember the order with "The Strong Man Threw Everything" for Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere.

The thermosphere is a favourite trap. Its molecules move extremely fast, so its temperature is enormous, yet the gas is so sparse that an astronaut would feel no heat from it at all.

Why the stratosphere warms with height

Ultraviolet radiation splits oxygen molecules, and the free atoms bond with other $\text{O}_2$ molecules to make ozone ($\text{O}_3$). Ozone then absorbs UV-B and UV-C, converting that radiation into heat. Because the absorption happens above the lower stratosphere, the top of the layer is warmer than its base — a temperature inversion. Chlorofluorocarbons (CFCs) destroy ozone catalytically, which is why the 1987 Montreal Protocol phased them out.

The greenhouse effect

Short-wave sunlight passes easily through the atmosphere and warms the ground. The ground re-emits that energy as long-wave infrared, and greenhouse gases — water vapour, carbon dioxide, methane, nitrous oxide — absorb it and radiate part of it back down. The natural effect is essential: without it Earth's average surface temperature would sit near -18 degrees C instead of about 15 degrees C. Burning fossil fuels and clearing forests strengthens the effect, driving climate change: higher mean temperatures, rising sea levels, and heavier rainfall extremes — a serious matter for an archipelago of low-lying coastal cities.

The Water Cycle

The hydrologic cycle is powered by solar energy and gravity, and every process has a name.

1. Evaporation — liquid water at the ocean or lake surface becomes vapour.
2. Transpiration — plants release vapour through their stomata; combined with evaporation this is evapotranspiration.
3. Sublimation — ice turns straight into vapour without melting first.
4. Condensation — vapour cools and turns back into droplets around microscopic condensation nuclei such as dust, sea salt or smoke, forming clouds and fog.
5. Precipitation — droplets coalesce until they are too heavy to stay aloft and fall as rain, drizzle, hail or snow.
6. Infiltration — water soaks into the soil, then percolates downward to recharge groundwater stored in aquifers below the water table.
7. Runoff — water that cannot soak in flows over the surface into creeks, rivers and back to the sea.

Humidity, Dew Point, and How Clouds Actually Form

Absolute humidity is the mass of vapour in a given volume of air. Relative humidity is the percentage of the maximum the air could hold at that temperature — and warm air holds far more than cold air, so relative humidity rises overnight even when no water is added. The dew point is the temperature at which the air becomes saturated and relative humidity reaches 100%.

Clouds form through one chain of events: air rises, the surrounding pressure drops, the parcel expands, expansion cools it (adiabatic cooling), and once it cools to the dew point the vapour condenses. The altitude where this happens is the cloud base, and it is why cumulus clouds in a fair-weather sky all have flat bottoms at the same level. Air is forced to rise in four ways: convection over a hot surface, orographic lifting up a mountain slope, frontal lifting where air masses meet, and convergence where winds flow together.

Worked example 1 - finding the cloud base

Air over Quezon City has a surface temperature of 32 degrees C and a dew point of 22 degrees C. Unsaturated rising air cools at about 10 degrees C per kilometre.

1. The parcel must cool by $32 - 22 = 10$ degrees C to reach saturation.
2. Altitude needed: $10 \div 10 = 1$ km.
3. The cloud base sits at about 1,000 m.

Worked example 2 - temperature at cruising altitude

The surface temperature at Manila is 30 degrees C and the environmental lapse rate is 6.5 degrees C per kilometre. What is the air temperature outside an aircraft at 10 km?

1. Total drop: $6.5 \times 10 = 65$ degrees C.
2. Temperature: $30 - 65 = -35$ degrees C.

Cloud Classification

Clouds are named from two Latin roots plus an altitude prefix: cirro- means wispy and high, alto- means middle, strato- means layered sheet, cumulo- means heaped, and nimbo- or -nimbus means it is raining.

The two rain-makers to memorise are the pair in bold: nimbostratus for long, steady, gentle rain, and cumulonimbus for short, violent, thundery rain.

Pressure, Wind, and Global Circulation

Air pressure at sea level averages 101.325 kPa, which is 1,013.25 millibars or 760 mm of mercury, and it falls with altitude. Wind is simply air flowing from high pressure toward low pressure; the steeper the pressure gradient, the stronger the wind. Winds are named for the direction they come from.

The Coriolis effect, caused by Earth's rotation, deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It is zero at the equator and strongest near the poles. Combined with uneven solar heating it produces three circulation cells per hemisphere:

• Hadley cell (0-30 degrees): air rises at the equator, dumps rain, sinks at 30 degrees, and returns as the trade winds. The sinking limb creates the world's great deserts.
• Ferrel cell (30-60 degrees): drives the prevailing westerlies.
• Polar cell (60-90 degrees): drives the polar easterlies.

Air Masses and Fronts

An air mass takes the temperature and moisture of the region it sat over: continental (dry) or maritime (moist), and polar (cold) or tropical (warm). A front is the boundary where two of them meet.

Philippine Weather and Climate

• Amihan, the northeast monsoon, blows roughly November to February, bringing cool, comparatively dry air and rain to the eastern seaboard.
• Habagat, the southwest monsoon, blows roughly June to September, bringing hot, very humid air and the heavy, days-long rain that floods western Luzon and the Visayas.
• The Intertropical Convergence Zone (ITCZ) is the belt where the trade winds of both hemispheres meet. The converging air rises, so the ITCZ carries thick cloud and heavy convective rain; it migrates north and south with the sun and regularly drenches Mindanao and the Visayas.

A tropical cyclone needs sea-surface temperatures of at least about 26.5 degrees C through a deep layer, a pre-existing disturbance, humid mid-level air, weak vertical wind shear, and a position at least about five degrees from the equator so the Coriolis effect can spin it. Its structure is a calm, clear eye of sinking air, an eyewall of the fiercest winds and rain, and outward spiral rainbands. PAGASA names any cyclone entering the Philippine Area of Responsibility and grades it by maximum sustained wind.

Across years, the El Nino Southern Oscillation dominates. El Nino warms the central and eastern Pacific, weakens the trade winds, and typically brings the Philippines below-normal rainfall, drought and water shortages. La Nina does the reverse, with cooler eastern Pacific water, stronger trades, above-normal rainfall and more flooding. Keep the definitions straight: weather is the state of the atmosphere at one place over hours or days, while climate is the average pattern over about 30 years.

Astronomy

The Sun holds more than 99% of the solar system's mass. The four inner terrestrial planets — Mercury, Venus, Earth, Mars — are small, rocky, dense and poor in moons. Beyond the asteroid belt the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune are huge, low-density and ringed, with dozens of moons each. Pluto was reclassified as a dwarf planet in 2006.

Seasons are caused by Earth's 23.5-degree axial tilt, not by distance from the Sun — Earth is in fact closest to the Sun in early January. When a hemisphere tilts toward the Sun it receives more direct rays and longer days, giving summer while the other hemisphere has winter; at the two equinoxes day and night are near equal everywhere. The Philippines lies close to the equator, so instead of four temperature seasons it experiences dry and wet seasons set by the monsoons.

Moon phases cycle over a synodic month of about 29.5 days: new, waxing crescent, first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent. They are produced by the changing Sun-Earth-Moon angle, not by Earth's shadow, and the Moon is tidally locked so the same face always points at us. A solar eclipse can only occur at new moon, when the Moon passes between Sun and Earth; a lunar eclipse only at full moon, when Earth passes between Sun and Moon. They are not monthly events because the Moon's orbit is tilted about five degrees from Earth's orbital plane.

Tides are the ocean's response to the Moon's gravity, helped by the Sun's. Two bulges give most coasts two high and two low tides a day. When Sun, Earth and Moon line up at new and full moon, the pulls add and produce the large-range spring tides; when the Sun and Moon pull at right angles at the quarter moons, they partly cancel and produce the small-range neap tides.

A star begins as a nebula of gas and dust that gravity squeezes into a protostar until hydrogen fusion ignites and it joins the main sequence, where the Sun has spent about 4.6 billion years. A Sun-like star then swells into a red giant, sheds its outer shell as a planetary nebula, and leaves a white dwarf that slowly cools. A star far more massive becomes a red supergiant, explodes as a supernova, and leaves behind a neutron star or, if massive enough, a black hole.

Worked example 3 - the scale of space

Light travels at about 300,000 km/s and the Sun is about 150 million km away. How long does its light take to reach us?

1. $t = 150{,}000{,}000 \div 300{,}000 = 500$ seconds.
2. $500 \div 60 \approx 8.3$ minutes, that is 8 minutes and 20 seconds.

Distances beyond the solar system use the light-year, the distance light covers in one year, about 9.5 trillion km. Our Milky Way is a barred spiral galaxy roughly 100,000 light-years across holding hundreds of billions of stars, and it is one of countless galaxies in a universe that began about 13.8 billion years ago and is still expanding, as the redshift of distant galaxies shows.

Common Traps

1. Placing the ozone layer in the troposphere. It is in the stratosphere, and it is the reason that layer warms with height.
2. Calling the thermosphere the hottest place you could stand. Its temperature is high but its density is almost nil.
3. Mixing up nimbostratus and cumulonimbus — steady all-day rain versus a short violent thunderstorm.
4. Swapping the monsoons: habagat is the wet southwest monsoon, amihan the cool northeast one.
5. Explaining seasons by distance from the Sun instead of by the 23.5-degree tilt.
6. Expecting an eclipse every month, and forgetting that solar eclipses need a new moon while lunar eclipses need a full moon.