Global Weather

Global Atmospheric Circulation

The movement of air around the planet; explains how thermal energy and storm systems move over Earth's surface.

Hadley Cell

The low-latitude atmospheric circulation cell where air moves toward the equator, heats and rises, then moves poleward in the upper atmosphere. Covers tropical and subtropical climates. Named for George Hadley (1735).

Ferrel Cell

The mid-latitude atmospheric circulation cell (~35°-60° N/S) where surface air flows poleward and eastward, while upper air moves equatorward and westward. Proposed by William Ferrell in 1856; first to account for mid-latitude westerly winds.

Polar Cell

The high-latitude circulation cell where air rises near 60° N/S, travels to the poles, sinks (forming polar highs), and returns toward lower latitudes as polar easterlies. The smallest and weakest circulation cell.

How many circulation cells exist in each hemisphere?

Three: the Hadley cell, the Ferrel cell, and the Polar cell.

George Hadley

English physicist and meteorologist who in 1735 proposed a single circulation cell per hemisphere to explain global wind patterns.

William Ferrell

19th-century scientist who in 1856 proposed the mid-latitude Ferrel cell, accounting for westerly winds between 35° and 60° N/S.

Polar Highs

Areas of high atmospheric pressure that form when sinking cold air accumulates over the poles.

Polar Easterlies

East-blowing surface winds that flow outward from the polar highs at high latitudes.

Pressure Band at 30° N/S

A band of HIGH pressure found near 30° North and South latitude; associated with fair, dry, and often hot weather; location of many of the world's major deserts.

Pressure Band at 50°-60° N/S

A band of LOW pressure found near 50°-60° N/S latitude; associated with rainy and stormy weather; west coasts at this latitude receive heavy precipitation.

Why do deserts form near 30° N/S?

Descending air at the 30° N/S high pressure belt is dry and warm, suppressing precipitation and creating desert conditions (e.g., Sahara, Australian Outback, American Southwest).

Coriolis Effect

The apparent deflection of moving air (and objects) due to Earth's rotation. Air moving toward the poles retains its eastward rotational momentum, causing it to deflect eastward relative to Earth's surface — making winds blow from west to east.

Why does Earth's rotational speed vary by latitude?

The circumference is largest at the equator, so the surface moves fastest there (~1,000+ mph). Speed decreases toward the poles, where it reaches zero. This difference causes the Coriolis Effect.

Jet Stream (definition)

Relatively narrow bands of strong wind in the upper levels of the atmosphere, typically near 30,000 ft (9,100 m) elevation, blowing from west to east.

Direction of jet stream winds

West to east (westerlies), because air moving poleward retains its eastward rotational momentum and moves faster than Earth's surface below.

Why are jet streams strongest in winter?

Temperature differences between cold polar air and warm equatorial air are greatest in winter; larger temperature gradients produce stronger winds.

Polar Jet Stream

Located near 50°-60° latitude in both hemispheres; forms at the boundary of the Ferrel and Polar cells; strongest and most well-known jet stream.

Subtropical Jet Stream

Located near 30° latitude in both hemispheres; forms at the boundary of the Hadley and Ferrel cells.

Jet stream altitude range

Typically 4 to 8 miles (roughly 20,000-40,000 ft) above Earth's surface.

Maximum jet stream speed

Can exceed 275 mph (239 knots / 442 km/h).

How does the jet stream shift seasonally?

It follows the Sun — moving poleward in spring and summer, and equatorward in fall and winter. In summer the polar jet often sits near the U.S.-Canada border; in winter it dips southward.

"Rivers of air"

A term used to describe jet streams because, like a river, wind is fastest at the core and weakens toward the edges.

Jet stream cross-section

The jet stream is not a single line; it is a wide, deep band — hundreds of miles wide and thousands of feet tall — with the strongest winds at the core.

How to view the current jet stream

Via NOAA's SPC Upper Air maps at the 250 mb level (found at spc.noaa.gov/obswx/maps/).

Jet stream eddies

Swirling air disturbances that form when the jet stream meanders, splits, or becomes disrupted; can influence surface weather patterns significantly.

Weather (definition)

The atmospheric conditions at a specific point in time; can be measured hourly, daily, weekly, monthly, or yearly.

Climate (definition)

The average or typical weather over a long period of time — usually 30 years or more. Changes slowly over years, centuries, or millennia.

Key difference between weather and climate

TIME: weather is short-term atmospheric conditions at a moment; climate is the long-term average of those conditions.

Climate Normals

The arithmetic average of weather values (temperature, precipitation, wind, etc.) calculated over a standard 30-year period. Currently based on 1991-2020 data; next set will cover 2001-2030.

NCEI

National Centers for Environmental Information — the NOAA division responsible for calculating and maintaining climate normals.

When will the next set of climate normals be updated?

In 2031, based on observed weather from 2001-2030.

"Unseasonably warm"

A phrase meaning current weather is warmer than the climate normal for that time of year and location.

Why can average temperature be "near normal" yet the day feel unusual?

A high early in the morning and a low late at night can average out to a normal value even if the temperature pattern throughout the day was atypical.

Large weather swings vs. climate change

Day-to-day or year-to-year weather extremes do NOT necessarily indicate rapid climate change. Weather, averaged over 30 years, defines the new normal.

Köppen-Geiger Climate Classification

A global climate classification system developed by Wladimir Köppen and Rudolf Geiger that divides world climates based on temperature (related to latitude) and precipitation patterns.

Wladimir Köppen

German climatologist and amateur botanist (1846-1940) who created the original climate classification system based on temperature profiles by latitude. Worked with Rudolf Geiger to update it.

Köppen Zone A — Tropical Climates

Moist tropical climates from about 0°-25° latitude. All months average above 64°F (18°C); annual precipitation exceeds 59 inches. No true cold season.

Köppen Zone B — Dry Climates

Climates where potential evaporation exceeds precipitation. Found from 20°-35° N/S and in large continental interiors surrounded by mountains.

Köppen Zone C — Moist Subtropical Mid-Latitude

Warm, humid summers with mild winters. Found from 30°-50° latitude on eastern and western continent borders. Mid-latitude cyclones dominate winters; thunderstorms dominate summers.

Köppen Zone D — Moist Continental Mid-Latitude

Warm to cool summers, very cold winters. Warmest month averages >50°F (10°C); coldest month averages

Köppen Zone E — Polar Climates

Year-round cold; warmest month averages below 50°F (10°C). Found in northern coastal areas of North America, Europe, Asia, and on Greenland and Antarctica.

Köppen Zone H — Highland (Complex Zone) Climates

Unique climates produced by rapid elevation changes in mountainous terrain; can encompass characteristics of any other climate zone over short distances.

Köppen second letter "f"

Wet year-round — no distinct dry season.

Köppen second letter "s"

Dry summer season.

Köppen second letter "w"

Dry winter season.

Köppen second letter "m"

Monsoon — pronounced wet season with a short dry season.

Köppen third letter "a"

Hot summer (average warmest month >72°F / 22°C).

Köppen third letter "b"

Warm summer (all months below 72°F / 22°C, at least 4 months above 50°F / 10°C).

Köppen third letter "c"

Cool summer (1-3 months above 50°F / 10°C).

Köppen third letter "d"

Very cold winters.

Af — Equatorial Rainforest

No dry season; driest month has ≥2.36" (60 mm) rainfall; all monthly averages >64°F (18°C). Rainfall evenly distributed year-round.

Am — Equatorial Monsoon

Pronounced wet season with a short dry season; one or more months

Aw — Equatorial Savanna (winter dry)

Winter dry season; more than two months with

BWh — Subtropical Desert

Low-latitude desert; evaporation far exceeds precipitation; average temperature >64°F (18°C); frost rare or absent.

BWk — Mid-Latitude Desert

Mid-latitude desert; evaporation far exceeds precipitation; average temperature

BSh — Subtropical Steppe

Low-latitude semi-arid; evaporation exceeds precipitation (but less than full desert); average temperature >64°F (18°C).

BSk — Mid-Latitude Steppe

Mid-latitude semi-arid; evaporation exceeds precipitation; average temperature

Cfa — Humid Subtropical

No dry season, hot summer (warmest month average >72°F / 22°C); coldest month

Cfb — Marine West Coast (warm summer)

No dry season, warm summer; all months

Cfc — Marine West Coast (cool summer)

No dry season, cool summer; all months

Csa — Mediterranean (hot summer)

Dry, hot summer; warmest month >72°F (22°C); at least 4 months >50°F (10°C); wettest winter month has ≥3× the rain of driest summer month; frost risk in winter.

Csb — Mediterranean (cool summer)

Dry, cool summer; no month averages >72°F (22°C); at least 4 months >50°F (10°C); same winter-wetter pattern as Csa. Example: coastal California.

Dfa — Humid Continental (hot summer)

Hot, humid summer; severe cold winter; no dry season. Example: Midwest U.S.

Dfb — Humid Continental (warm summer)

Warm summer; severe winter; no dry season. Example: northern Great Plains, Great Lakes region.

Dfc — Subarctic

Cool summer; severe winter; no dry season. Example: interior Alaska, northern Canada.

Dfd — Subarctic (very cold winter)

Very cold winter; no dry season; cool summer. Extreme continental interiors.

Dwc — Subarctic (dry winter)

Severe, dry winter; cool summer. Found in parts of Siberia.

ET — Tundra

Polar tundra; no true summer; warmest month averages 32°-50°F (0°-10°C); found on Arctic coasts.

EF — Ice Cap

Perennial ice; all months average below freezing; found in Antarctica and interior Greenland.

What causes global atmospheric circulation to be complex rather than simple?

Three factors: (1) Earth's tilt causing the Sun's direct radiation to shift location seasonally, (2) Earth's rotation producing the Coriolis Effect, and (3) uneven land/sea distribution creating semi-permanent highs and lows.

Why does the Northern Hemisphere have less uniform circulation than the Southern?

More landmass in the Northern Hemisphere creates more semi-permanent high and low pressure systems, disrupting the idealized three-cell pattern.

Latitude of polar jet stream

Between approximately 50°-60° N/S latitude.

Latitude of subtropical jet stream

Near approximately 30° N/S latitude.

What would global circulation look like on a non-rotating, water-covered planet?

A single circulation cell per hemisphere: hot air rising at the equator, moving to the poles at altitude, cooling and sinking, then returning along the surface to the equator.

How does the Coriolis Effect cause west-to-east jet stream flow?

Air rising from the equator carries fast eastward momentum (~1,000 mph). As it moves poleward, the Earth beneath it rotates slower. The air overtakes the surface, producing a west-to-east wind.

Surface rotational speed at equator vs. poles

~1,000+ mph (1,600 km/h) at the equator; 0 mph at the poles. The difference in speed is what drives the Coriolis Effect.

What pressure feature is associated with the 30° N/S latitude bands?

High pressure (descending dry air) — producing deserts and fair weather.

What pressure feature is associated with the equator?

Low pressure (rising moist air) — producing heavy rainfall and the ITCZ (Inter-Tropical Convergence Zone).

What pressure feature is associated with the 50°-60° N/S latitude bands?

Low pressure — associated with more precipitation and frequent storm systems, especially on west coasts of continents.

Examples of regions at 30° N/S with desert climates

American Southwest, northern Mexico, northern Africa (Sahara), and Australia.

Examples of regions at 50°-60° N/S with wet climates

Canada, Alaska, British Isles, and western Russia.