Weather & Climatology: Understanding the Atmosphere
The Observatory Almanac — Living Environment Series
A comprehensive guide to reading the sky, interpreting atmospheric data, and understanding the invisible forces that shape daily life.
"Climate is what you expect; weather is what you get." — Mark Twain
The atmosphere is a machine — a global heat engine driven by solar radiation, modulated by the oceans, shaped by topography, and constantly seeking equilibrium it never quite achieves. Learning to read it is one of the oldest and most practical human skills. This guide translates the language of meteorology into tools you can use every day.
PART ONE: BAROMETRIC PRESSURE
What Is Atmospheric Pressure?
Atmospheric pressure is the weight of the air column above you pressing down. At sea level, standard pressure is 1013.25 hPa (hectopascals), equivalent to 29.92 inches of mercury (inHg) or 760 mmHg. This number represents the total mass of the atmosphere above a square meter of Earth's surface: approximately 10,330 kilograms.
Pressure varies with altitude (it drops roughly 1 hPa for every 8 meters of climb at sea level) and with weather systems. This variation is what makes pressure such a powerful forecasting tool.
Reading a Barometer
Units in use: - hPa (hectopascals) — international standard, used in scientific contexts - inHg (inches of mercury) — common in US consumer devices - mb (millibars) — older term, numerically identical to hPa
Conversion: 1 inHg = 33.86 hPa
Pressure and Weather: The Basic Framework
| Pressure (hPa) | Tendency | Interpretation |
|---|---|---|
| >1022 | Steady/rising | Fair weather, possible high pressure |
| 1013–1022 | Steady | Stable, typical conditions |
| <1013 | Falling | Weather may deteriorate |
| <1000 | Falling rapidly | Storm approaching |
| <980 | Any | Significant storm system |
| <960 | Any | Severe storm (strong hurricane, etc.) |
| <920 | Any | Extreme storm (Category 4–5 hurricane) |
Pressure Tendency — The Key Concept
The rate of change is more important than the absolute value.
3-hour tendency rules (the barometer forecaster's shorthand):
- Rising rapidly (+3 hPa in 3h): Clearing after storm; brief fair weather before next system
- Rising slowly (+1 hPa in 3h): Improving weather; extended fair spell likely
- Steady: Present conditions likely to persist 12–24 hours
- Falling slowly (-1 hPa in 3h): Gradual deterioration; rain likely within 24 hours
- Falling rapidly (-3 hPa in 3h): Storm within 12–24 hours; severity depends on other factors
- Falling very rapidly (-6+ hPa in 3h): Severe storm imminent; take precautions
High and Low Pressure Systems
High pressure (anticyclone): - Air descends in the center, diverges at the surface - In Northern Hemisphere: clockwise rotation - Brings clear skies (descending air suppresses cloud formation), dry conditions - Can persist for days to weeks ("blocking highs") - Extreme high pressure = very stable, possible fog in valleys at night
Low pressure (cyclone): - Air converges at surface, rises in the center - In Northern Hemisphere: counterclockwise rotation - Rising air cools → clouds form → precipitation likely - Fronts (cold, warm, occluded) associated with low pressure systems - Extreme low pressure = intense storms (hurricanes, nor'easters, bombs)
Pressure Gradient Force
The tighter the pressure gradient (isobars spaced closer together on a weather map), the stronger the wind. Isobars are lines connecting equal pressure values. Where they're packed close: high winds. Where they're spread apart: calm.
PART TWO: DEW POINT AND HUMAN COMFORT
What Is Dew Point?
Dew point is the temperature at which air becomes saturated — where water vapor condenses into liquid. Unlike relative humidity, which changes with temperature, dew point is an absolute measure of moisture in the air. A dew point of 60°F (16°C) means the same amount of water vapor whether it's 70°F or 95°F outside.
This makes dew point the gold standard for human comfort assessment.
The Dew Point Comfort Scale
| Dew Point °F (°C) | How It Feels | Classification |
|---|---|---|
| < 35°F (< 2°C) | Very dry, possibly uncomfortable | Arid |
| 35–45°F (2–7°C) | Comfortably dry, great for outdoor activity | Dry/Pleasant |
| 45–55°F (7–13°C) | Comfortable, typical of good summer days | Comfortable |
| 55–60°F (13–16°C) | Slightly humid; noticeable but not unpleasant | Moderate |
| 60–65°F (16–18°C) | Humid; sweat evaporation slowed | Noticeably Humid |
| 65–70°F (18–21°C) | Very humid; uncomfortable for most people | Oppressive |
| > 70°F (> 21°C) | Extremely oppressive; dangerous for exertion | Dangerous |
Why dew point beats relative humidity: A summer day with 50% relative humidity at 95°F (35°C) has a dew point of about 73°F — dangerously oppressive. A spring day with 50% relative humidity at 60°F (15°C) has a dew point of 41°F — perfectly comfortable. Same relative humidity, completely different experience.
When to worry: Dew points above 70°F (21°C) combined with temperatures above 90°F create conditions where the human body cannot effectively cool itself through sweating. This is when heat illness risk is severe.
PART THREE: WIND CHILL AND HEAT INDEX
Wind Chill
Wind chill is the perceived decrease in air temperature felt by exposed skin due to wind. Wind accelerates heat loss from the body by stripping away the thin layer of warm air that skin creates.
NWS Wind Chill Formula (°F):
Wind Chill = 35.74 + 0.6215T - 35.75(V^0.16) + 0.4275T(V^0.16)
Simplified table:
| Air Temp (°F) | Wind 10 mph | Wind 20 mph | Wind 30 mph | Wind 40 mph |
|---|---|---|---|---|
| 30°F | 21°F | 17°F | 15°F | 13°F |
| 20°F | 9°F | 4°F | 1°F | -1°F |
| 10°F | -4°F | -9°F | -12°F | -15°F |
| 0°F | -16°F | -22°F | -26°F | -29°F |
| -10°F | -28°F | -35°F | -39°F | -43°F |
Frostbite risk: At wind chill values below -20°F (-29°C), frostbite can occur in 30 minutes on exposed skin. Below -40°F, frostbite can occur in 10 minutes.
Metric formula (°C, km/h):
Wind Chill = 13.12 + 0.6215T - 11.37(V^0.16) + 0.3965T(V^0.16)
Heat Index
Heat index (apparent temperature) is the perceived temperature when heat and humidity combine. High humidity reduces the body's ability to cool through perspiration.
NWS Heat Index Formula (°F):
HI = -42.379 + 2.04901523T + 10.14333127RH
- 0.22475541T*RH - 0.00683783T²
- 0.05481717RH² + 0.00122874T²*RH
+ 0.00085282T*RH² - 0.00000199T²*RH²
Simplified Heat Index Scale:
| Heat Index | Condition | Risk |
|---|---|---|
| 80–90°F (27–32°C) | Caution | Fatigue possible with prolonged exposure |
| 90–103°F (32–39°C) | Extreme Caution | Heat cramps/exhaustion possible |
| 103–124°F (39–51°C) | Danger | Heat cramps/exhaustion likely; heat stroke possible |
| >125°F (>52°C) | Extreme Danger | Heat stroke imminent |
Quick approximation: For every 10% increase in relative humidity above 40%, add approximately 2°F to the apparent temperature when conditions are hot.
PART FOUR: READING WEATHER MAPS
Surface Analysis Maps
A surface analysis map (synoptic chart) shows current or forecast surface conditions:
Key symbols:
- H (Blue): High pressure center — expect fair, dry, settled weather
- L (Red): Low pressure center — expect clouds, precipitation, unsettled weather
- Solid blue lines: Cold fronts — triangles pointing in direction of movement
- Solid red lines: Warm fronts — semicircles pointing in direction of movement
- Alternating blue/red line: Occluded front — where cold front catches up to warm front
- Dashed lines: Troughs of low pressure — potential instability
- Dotted lines: Dry lines (SW USA) — sharp moisture boundary; often triggers thunderstorms
- Isobars: Thin black lines — connect equal pressure; tight spacing = strong wind
Wind Barbs
Wind direction and speed on weather maps use "barbs" on a staff: - Staff points toward the direction wind is blowing from - Barbs (short lines): Each full barb = 10 knots; half barb = 5 knots; pennant = 50 knots - Example: A staff pointing northwest with 2 full barbs = NW wind at 20 knots
Color-Coded Radar (NEXRAD)
| Color | dBZ (reflectivity) | Rain Rate |
|---|---|---|
| Light blue | 5–20 | Very light rain |
| Green | 20–35 | Light to moderate rain |
| Yellow | 35–45 | Moderate to heavy rain |
| Orange | 45–55 | Heavy rain |
| Red | 55–65 | Very heavy rain; possible hail |
| Purple/White | >65 | Extreme rain; large hail likely |
Velocity (Doppler) Radar: Shows wind rotation within a storm. Green = winds toward radar; red = winds away from radar. A couplet (tight red-green pair) indicates rotation — potential tornado.
PART FIVE: CLOUD IDENTIFICATION — THE TEN GENERA
The World Meteorological Organization recognizes ten cloud genera, organized by altitude and form.
HIGH CLOUDS (Above 20,000 ft / 6,000 m)
1. Cirrus (Ci)
"Mare's tails"
Wispy, fibrous white streaks high in the sky, composed entirely of ice crystals. They often indicate upper-level winds and can signal an approaching warm front 24–48 hours away. Isolated cirrus in an otherwise clear sky is often insignificant; thickening cirrus with cirrostratus spreading across the sky suggests incoming weather.
2. Cirrocumulus (Cc)
"Mackerel sky"
Small white puffs arranged in rippled rows or a dappled pattern. Much smaller than altocumulus (thumbnail or smaller held at arm's length). Composed of supercooled water droplets. Less common than other high clouds. A mackerel sky in the morning can indicate weather change within 24 hours.
3. Cirrostratus (Cs)
"Halo-maker"
A translucent veil of ice crystal cloud covering large portions of sky. Key identifier: it produces halos around the sun or moon (22° halo). It often follows cirrus and precedes altostratus — a classic progression of an approaching warm front.
MIDDLE CLOUDS (6,500–20,000 ft / 2,000–6,000 m)
4. Altostratus (As)
A gray or blue-gray sheet that covers the entire sky. The sun appears as through frosted glass — a "watery sun" with no distinct edges and no halo. Precipitation (often rain or snow reaching the ground) is common. Typically follows cirrostratus in the warm front sequence.
5. Altocumulus (Ac)
Gray or white patches, rolls, or waves arranged in layers. Larger than cirrocumulus (fist-sized at arm's length), smaller than stratocumulus. "Castellanus" form (towers on top) in the morning indicates afternoon thunderstorm risk. Iridescent or colored altocumulus (corona around sun/moon) indicates small droplet size and recent formation.
6. Nimbostratus (Ns)
The classic "rain cloud" — a thick, dark gray, featureless layer that blocks the sun completely and produces steady, prolonged rain or snow. Not dramatic; not dangerous on its own. Just persistently wet. Ceilings (base altitude) may be very low.
LOW CLOUDS (Surface to 6,500 ft / 2,000 m)
7. Stratus (St)
A flat, uniform low gray layer resembling fog that doesn't reach the ground. Produces drizzle rather than rain. Common in coastal areas (particularly California coast) and in stable, moist air masses. Often forms overnight, burns off by mid-morning when present.
8. Stratocumulus (Sc)
The most common cloud type on Earth. Lumpy gray or white patches arranged in rolls or masses, usually covering most of the sky. Often arranged in "streets" parallel to wind direction. Generally produces light precipitation at most. Persistent stratocumulus (marine stratus) can suppress coastal temperatures significantly.
9. Cumulus (Cu)
The classic "fair weather" cloud — distinct, puffy white mounds with flat bases and dome or cauliflower tops. When small and widely scattered (cumulus humilis), they indicate instability but little threat. When growing vertically (cumulus mediocris, congestus), conditions may produce afternoon showers or storms.
CLOUDS OF VERTICAL DEVELOPMENT
10. Cumulonimbus (Cb)
The storm cloud. Towers can reach 60,000 feet (18,000 m) into the stratosphere. The anvil-shaped top (cirrus anvil), characteristic of mature thunderstorms, marks where rising air hits the tropopause and spreads outward. Contains: lightning, thunder, violent updrafts/downdrafts, heavy rain, hail, possible tornadoes (supercell variety). The cumulonimbus commands respect. Give it a wide berth if outdoors.
PART SIX: SEVERE WEATHER PREPAREDNESS BY TYPE
Thunderstorms
Preparation: - 30-30 rule: If thunder follows lightning by 30 seconds or less, seek shelter immediately; wait 30 minutes after last thunder to resume outdoor activity - Indoors: Avoid windows, plumbing, corded phones, electrical equipment - Outdoors: Seek low ground; avoid tall isolated trees; crouch low if caught in open; never lie flat - Vehicle: A hard-top vehicle provides good protection; convertibles/open vehicles do not
Hail: Most injuries occur when people are caught outside or near windows. Protect your head; shelter in an interior room.
Tornadoes
Watch vs. Warning: - Watch: Conditions favorable for tornado development; stay alert - Warning: Tornado detected by radar or confirmed sighting; take shelter immediately
Shelter: Lowest floor, interior room, away from windows. Under a staircase. Bathtub with mattress over you if no basement. Do not stay in mobile homes — evacuate to a sturdy building or a shelter.
If outdoors: Do not try to outrun a tornado in a car (unless it's clearly moving away). Abandon vehicle; lie flat in a ditch or low area, covering your head.
Flooding
Two rules: 1. Turn Around, Don't Drown: Six inches of moving water can knock a person down; two feet can float a car 2. Never drive around barricades on flooded roads
Flash flood watches vs. warnings mirror thunderstorm protocol. Flash flooding is the #1 weather-related killer in the US.
Winter Storms
Hypothermia: Core temperature below 95°F (35°C). Symptoms: intense shivering, confusion, loss of coordination. Treatment: move indoors, remove wet clothing, warm core (not extremities) first.
Blizzard conditions: Winds 35+ mph, visibility less than 1/4 mile, 3+ hours duration. Disorientation is the leading cause of death; people can become lost mere feet from shelter.
Ice storms: More dangerous than snow for infrastructure and falls. A 1/4-inch coating of ice can cause widespread power outages; tree branches and power lines fail.
Tropical Cyclones (Hurricanes/Typhoons)
Saffir-Simpson Scale: | Category | Wind Speed | Storm Surge | Typical Damage | |----------|-----------|-------------|----------------| | 1 | 74–95 mph | 4–5 ft | Minimal — some shingles, branches | | 2 | 96–110 mph | 6–8 ft | Moderate — roofs, trees, power | | 3 | 111–129 mph | 9–12 ft | Extensive — major structural damage | | 4 | 130–156 mph | 13–18 ft | Extreme — most walls fail | | 5 | >157 mph | >18 ft | Catastrophic — complete destruction |
Storm surge, not wind, kills most people in hurricanes. A 20-foot storm surge along a low-lying coastline is a wall of ocean pushed inland by the storm.
Evacuation: When authorities order evacuation, leave. Every storm has exceptions that kill people who stayed to prove it wouldn't be that bad.
Heat Emergencies
Heat exhaustion: Heavy sweating, weakness, cold pale clammy skin, fast/weak pulse, nausea. Move to cool location; sip cool water; cool cloths to skin.
Heat stroke: Hot red dry or damp skin, rapid strong pulse, possible confusion. This is a medical emergency. Call 911. Cool the person rapidly by any means available (ice bath, cool water, fanning). Do not give fluids to someone who is unconscious or confused.
Further reading: National Weather Service (weather.gov), The AMS Weather Book (Jack Williams), Storm King (David Brooks), Meteorology Today (Ahrens & Henson).