Away from the tropics, weather travels east
For every pair of airports between 250 and 1,500 km apart we asked a simple question: does the first airport's unusual
pressure today predict the second airport's tomorrow better than the other way round? Summing the answers over
4,404 pairs gives each airport an arrow pointing the way its weather tends to move. Nobody told the
model about prevailing winds; the arrows come out of the reports alone. North of the tropics they point east, in the
south they point east to northeast, and near the equator they show no steady direction.
ABJ: weather tends to arrive moving west ABQ: weather tends to arrive moving southeast ABV: weather tends to arrive moving southwest ACC: weather tends to arrive moving west ACE: weather tends to arrive moving northeast ADD: weather tends to arrive moving northeast ADL: weather tends to arrive moving east AEP: weather tends to arrive moving east AGA: weather tends to arrive moving southwest AGP: weather tends to arrive moving east AKJ: weather tends to arrive moving northeast AKL: weather tends to arrive moving north ALB: weather tends to arrive moving east ALC: weather tends to arrive moving east ALG: weather tends to arrive moving east AMS: weather tends to arrive moving southeast AOJ: weather tends to arrive moving northeast ARN: weather tends to arrive moving east ATH: weather tends to arrive moving southeast ATL: weather tends to arrive moving east AUH: weather tends to arrive moving southeast AUS: weather tends to arrive moving southeast AVL: weather tends to arrive moving east AYT: weather tends to arrive moving east BCN: weather tends to arrive moving east BDL: weather tends to arrive moving east BEG: weather tends to arrive moving southeast BER: weather tends to arrive moving southeast BHM: weather tends to arrive moving east BIL: weather tends to arrive moving southeast BIO: weather tends to arrive moving east BKK: weather tends to arrive moving south BLR: weather tends to arrive moving northwest BNA: weather tends to arrive moving east BNE: weather tends to arrive moving northeast BOG: weather tends to arrive moving northeast BOI: weather tends to arrive moving southeast BOM: weather tends to arrive moving northwest BOS: weather tends to arrive moving east BRC: weather tends to arrive moving northeast BRU: weather tends to arrive moving southeast BSB: weather tends to arrive moving north BTS: weather tends to arrive moving southeast BTV: weather tends to arrive moving northeast BUD: weather tends to arrive moving southeast BUF: weather tends to arrive moving east BUR: weather tends to arrive moving northeast BWI: weather tends to arrive moving east CAI: weather tends to arrive moving northeast CAN: weather tends to arrive moving south CBR: weather tends to arrive moving northeast CCU: weather tends to arrive moving southwest CDG: weather tends to arrive moving east CGH: weather tends to arrive moving northeast CGK: weather tends to arrive moving north CHC: weather tends to arrive moving northeast CHS: weather tends to arrive moving east CJU: weather tends to arrive moving east CKG: weather tends to arrive moving south CLE: weather tends to arrive moving east CLT: weather tends to arrive moving east CMH: weather tends to arrive moving east CMN: weather tends to arrive moving northeast CNF: weather tends to arrive moving northeast CNS: weather tends to arrive moving northwest COR: weather tends to arrive moving east CPH: weather tends to arrive moving southeast CPT: weather tends to arrive moving northeast CRW: weather tends to arrive moving east CTA: weather tends to arrive moving southeast CTS: weather tends to arrive moving northeast CUN: weather tends to arrive moving east CVG: weather tends to arrive moving east DAL: weather tends to arrive moving east DAR: weather tends to arrive moving southeast DBV: weather tends to arrive moving southeast DCA: weather tends to arrive moving east DEL: weather tends to arrive moving south DEN: weather tends to arrive moving southeast DFW: weather tends to arrive moving east DJE: weather tends to arrive moving east DMK: weather tends to arrive moving south DOH: weather tends to arrive moving east DPS: weather tends to arrive moving east DSM: weather tends to arrive moving southeast DSS: weather tends to arrive moving north DTW: weather tends to arrive moving east DUB: weather tends to arrive moving east DUR: weather tends to arrive moving northeast DUS: weather tends to arrive moving southeast DXB: weather tends to arrive moving east EBB: weather tends to arrive moving east EDI: weather tends to arrive moving southeast EWR: weather tends to arrive moving east EZE: weather tends to arrive moving east FAR: weather tends to arrive moving southeast FCO: weather tends to arrive moving southeast FLL: weather tends to arrive moving southeast FRA: weather tends to arrive moving southeast FSD: weather tends to arrive moving southeast FSZ: weather tends to arrive moving east FUE: weather tends to arrive moving northeast FUK: weather tends to arrive moving east GDL: weather tends to arrive moving southwest GEG: weather tends to arrive moving southeast GIG: weather tends to arrive moving northeast GMP: weather tends to arrive moving east GOT: weather tends to arrive moving southeast GRR: weather tends to arrive moving east GRU: weather tends to arrive moving northeast GVA: weather tends to arrive moving southeast GYE: weather tends to arrive moving northeast HAM: weather tends to arrive moving southeast HAN: weather tends to arrive moving south HBA: weather tends to arrive moving southeast HEL: weather tends to arrive moving southeast HER: weather tends to arrive moving southeast HGH: weather tends to arrive moving east HIJ: weather tends to arrive moving northeast HKD: weather tends to arrive moving northeast HKG: weather tends to arrive moving south HND: weather tends to arrive moving east HNL: weather tends to arrive moving northwest HOU: weather tends to arrive moving southeast HPN: weather tends to arrive moving east HRG: weather tends to arrive moving northeast HYD: weather tends to arrive moving west IAD: weather tends to arrive moving east IAH: weather tends to arrive moving southeast IBZ: weather tends to arrive moving east ICN: weather tends to arrive moving east ICT: weather tends to arrive moving southeast IGR: weather tends to arrive moving northeast ILG: weather tends to arrive moving east IND: weather tends to arrive moving east ISG: weather tends to arrive moving northeast IST: weather tends to arrive moving southeast ITM: weather tends to arrive moving northeast IZO: weather tends to arrive moving east JAC: weather tends to arrive moving east JAN: weather tends to arrive moving east JAX: weather tends to arrive moving east JED: weather tends to arrive moving northeast JFK: weather tends to arrive moving east JNB: weather tends to arrive moving north KCZ: weather tends to arrive moving northeast KEF: weather tends to arrive moving southeast KIJ: weather tends to arrive moving northeast KIX: weather tends to arrive moving northeast KKJ: weather tends to arrive moving east KMG: weather tends to arrive moving south KMI: weather tends to arrive moving northeast KMJ: weather tends to arrive moving northeast KMQ: weather tends to arrive moving northeast KOA: weather tends to arrive moving northwest KOJ: weather tends to arrive moving northeast KUH: weather tends to arrive moving northeast KUL: weather tends to arrive moving south KWI: weather tends to arrive moving east LAS: weather tends to arrive moving east LAX: weather tends to arrive moving northeast LCA: weather tends to arrive moving northeast LGA: weather tends to arrive moving east LGB: weather tends to arrive moving northeast LGW: weather tends to arrive moving east LHR: weather tends to arrive moving east LIM: weather tends to arrive moving southwest LIN: weather tends to arrive moving southeast LIS: weather tends to arrive moving northeast LIT: weather tends to arrive moving east LJU: weather tends to arrive moving southeast LOS: weather tends to arrive moving west LPA: weather tends to arrive moving northeast LPB: weather tends to arrive moving northwest LTN: weather tends to arrive moving southeast LUX: weather tends to arrive moving southeast MAA: weather tends to arrive moving west MAD: weather tends to arrive moving east MAH: weather tends to arrive moving east MAN: weather tends to arrive moving southeast MCI: weather tends to arrive moving east MCO: weather tends to arrive moving southeast MCT: weather tends to arrive moving east MDE: weather tends to arrive moving northeast MDW: weather tends to arrive moving east MDZ: weather tends to arrive moving east MEL: weather tends to arrive moving east MEM: weather tends to arrive moving east MEX: weather tends to arrive moving south MHT: weather tends to arrive moving northeast MIA: weather tends to arrive moving southeast MKE: weather tends to arrive moving east MLA: weather tends to arrive moving southeast MMY: weather tends to arrive moving northeast MNL: weather tends to arrive moving north MSJ: weather tends to arrive moving northeast MSP: weather tends to arrive moving southeast MSY: weather tends to arrive moving east MTY: weather tends to arrive moving southeast MUC: weather tends to arrive moving southeast MVD: weather tends to arrive moving east MXP: weather tends to arrive moving southeast MYJ: weather tends to arrive moving northeast MYR: weather tends to arrive moving east NAP: weather tends to arrive moving southeast NBO: weather tends to arrive moving northeast NCE: weather tends to arrive moving southeast NGO: weather tends to arrive moving east NGS: weather tends to arrive moving east NRT: weather tends to arrive moving east OAK: weather tends to arrive moving northeast OIT: weather tends to arrive moving northeast OKA: weather tends to arrive moving northeast OKC: weather tends to arrive moving southeast OKJ: weather tends to arrive moving northeast OMA: weather tends to arrive moving southeast ONT: weather tends to arrive moving northeast OOL: weather tends to arrive moving northeast OPO: weather tends to arrive moving east ORD: weather tends to arrive moving east ORF: weather tends to arrive moving east ORY: weather tends to arrive moving east OSL: weather tends to arrive moving east OTP: weather tends to arrive moving southeast PDX: weather tends to arrive moving east PEK: weather tends to arrive moving southeast PHL: weather tends to arrive moving east PHX: weather tends to arrive moving east PIT: weather tends to arrive moving east PKX: weather tends to arrive moving southeast PMI: weather tends to arrive moving east PRG: weather tends to arrive moving southeast PRN: weather tends to arrive moving southeast PSP: weather tends to arrive moving east PTY: weather tends to arrive moving north PVD: weather tends to arrive moving east PVG: weather tends to arrive moving east PWM: weather tends to arrive moving northeast RAK: weather tends to arrive moving northeast RDU: weather tends to arrive moving east RIC: weather tends to arrive moving east RIX: weather tends to arrive moving southeast RMF: weather tends to arrive moving northeast RMO: weather tends to arrive moving southeast RNO: weather tends to arrive moving east ROC: weather tends to arrive moving east RSW: weather tends to arrive moving east RUH: weather tends to arrive moving east SAN: weather tends to arrive moving north SAT: weather tends to arrive moving southeast SAV: weather tends to arrive moving east SAW: weather tends to arrive moving southeast SCL: weather tends to arrive moving east SDF: weather tends to arrive moving east SDJ: weather tends to arrive moving northeast SEA: weather tends to arrive moving east SFO: weather tends to arrive moving northeast SHA: weather tends to arrive moving east SIN: weather tends to arrive moving south SJC: weather tends to arrive moving northeast SJJ: weather tends to arrive moving southeast SKP: weather tends to arrive moving southeast SLC: weather tends to arrive moving east SMF: weather tends to arrive moving northeast SNA: weather tends to arrive moving northeast SOF: weather tends to arrive moving southeast SRQ: weather tends to arrive moving east SSA: weather tends to arrive moving northeast SSH: weather tends to arrive moving northeast STL: weather tends to arrive moving east STN: weather tends to arrive moving southeast STR: weather tends to arrive moving southeast SVQ: weather tends to arrive moving northeast SYD: weather tends to arrive moving northeast SYR: weather tends to arrive moving east SZX: weather tends to arrive moving south TAK: weather tends to arrive moving northeast TFN: weather tends to arrive moving northeast TFS: weather tends to arrive moving northeast TFU: weather tends to arrive moving south TGD: weather tends to arrive moving southeast TIA: weather tends to arrive moving southeast TKS: weather tends to arrive moving northeast TLL: weather tends to arrive moving southeast TLV: weather tends to arrive moving northeast TNG: weather tends to arrive moving northeast TOY: weather tends to arrive moving northeast TPA: weather tends to arrive moving east TPE: weather tends to arrive moving northeast TTJ: weather tends to arrive moving northeast TUL: weather tends to arrive moving east TUN: weather tends to arrive moving southeast TUS: weather tends to arrive moving southeast UIO: weather tends to arrive moving north UKB: weather tends to arrive moving northeast VCE: weather tends to arrive moving southeast VCP: weather tends to arrive moving northeast VIE: weather tends to arrive moving southeast VLC: weather tends to arrive moving east VNO: weather tends to arrive moving southeast WAW: weather tends to arrive moving southeast WLG: weather tends to arrive moving northeast XIY: weather tends to arrive moving south YGJ: weather tends to arrive moving east YUL: weather tends to arrive moving northeast YVR: weather tends to arrive moving southeast YYC: weather tends to arrive moving southeast YYZ: weather tends to arrive moving east ZAG: weather tends to arrive moving southeast ZNZ: weather tends to arrive moving southeast ZRH: weather tends to arrive moving southeast Arrow length shows how consistent the direction is. Airports with no clear direction are left out. Show the numbers Region Weather moves toward
Bearing Airports Southern mid-latitudes northeast 59° 22 Tropics northwest 322° 46 Northern subtropics east 84° 141 Northern mid-latitudes east 107° 102
Pressure changes travel at about 79 km/h
Where one airport clearly leads another, the hour-by-hour pressure record shows how long a change takes to arrive.
Distance divided by that delay gives a speed. Across 2,130 leading pairs at least 600 km apart, the typical speed is
79 km/h, about 1,890 km a day. This is how fast the change in pressure spreads, which is quicker than
a storm centre itself moves. Either way, what happens 1,000 km upwind today shows up here tomorrow, which is why the
forecaster looks so far afield.
Tropics 80 km/h Subtropics 83 km/h Mid-latitudes 75 km/h Median speed of pressure changes between leading and following airports. Show the numbers Band Median Middle half
Pairs Tropics 80 km/h 56 to 107 km/h 12 Subtropics 83 km/h 62 to 121 km/h 1153 Mid-latitudes 75 km/h 60 to 95 km/h 965
Warm and cold spells fade in about 3.0 days
How alike is one day's temperature departure to the next? We measured it at every airport and turned it into the time
it takes a spell to lose most of its strength. Spells last longest in the mid-latitudes and fade fastest
nearer the equator. The shorter the spell, the less today tells you about tomorrow on its own.
Tropics 2.7 days Subtropics 3.0 days Mid-latitudes 3.6 days Median time for a temperature departure to fade to about a third of its size. Show the numbers Band Spell life
Day-to-day likeness Airports Tropics 2.7 days 0.69 53 Subtropics 3.0 days 0.71 160 Mid-latitudes 3.6 days 0.76 107
A falling barometer means rain is more likely
When pressure at an airport has dropped by 8 hPa or more over the day, it rains there the next day 62% of the
time, against 47% on a typical day for those places and dates. When it has risen as sharply, the chance falls to 37%, against a normal 46%.
The old barometer rule holds across our airports worldwide, and the data puts numbers on it.
0% 20% 40% 60% 80% −8 or more −4 0 +4 +8 or more Pressure change over the last 24 hours (hPa), measured at 18:00 Chance of rain tomorrow Rain tomorrow: 62% (pressure change -10 hPa) Rain tomorrow: 51% (pressure change -6 hPa) Rain tomorrow: 41% (pressure change -4 hPa) Rain tomorrow: 36% (pressure change -2 hPa) Rain tomorrow: 34% (pressure change -1 hPa) Rain tomorrow: 33% (pressure change +0 hPa) Rain tomorrow: 32% (pressure change +1 hPa) Rain tomorrow: 32% (pressure change +2 hPa) Rain tomorrow: 33% (pressure change +4 hPa) Rain tomorrow: 34% (pressure change +6 hPa) Rain tomorrow: 37% (pressure change +10 hPa) Rain tomorrow Normal chance All airports, evenings, 2021 to 2026. Show the numbers Pressure change Rain tomorrow
Normal chance Evenings -40 to -8 hPa 62% 47% 34,351 -8 to -5 hPa 51% 41% 44,841 -5 to -3 hPa 41% 37% 67,283 -3 to -1.5 hPa 36% 34% 64,711 -1.5 to -0.5 hPa 34% 34% 85,773 -0.5 to +0.5 hPa 33% 34% 98,144 +0.5 to +1.5 hPa 32% 34% 81,339 +1.5 to +3 hPa 32% 35% 84,957 +3 to +5 hPa 33% 38% 47,867 +5 to +8 hPa 34% 41% 37,207 +8 to +40 hPa 37% 46% 32,479
Fog needs a small dew point gap and a light wind
The dew point gap is how far the air would have to cool before it is saturated. With a gap under 0.5° and wind
under 3 kt in the evening, fog shows up in the next day's reports 32% of the time. Widen the gap or add wind
and the chance collapses.
under 0.5° under 0.5°, under 3 kt: 32% 32% under 0.5°, 3 to 7 kt: 25% 25% under 0.5°, 7 to 12 kt: 18% 18% under 0.5°, over 12 kt: 14% 14% 0.5 to 1° 0.5 to 1°, under 3 kt: 23% 23% 0.5 to 1°, 3 to 7 kt: 16% 16% 0.5 to 1°, 7 to 12 kt: 11% 11% 0.5 to 1°, over 12 kt: 8% 8% 1 to 2° 1 to 2°, under 3 kt: 17% 17% 1 to 2°, 3 to 7 kt: 12% 12% 1 to 2°, 7 to 12 kt: 8% 8% 1 to 2°, over 12 kt: 6% 6% 2 to 3° 2 to 3°, under 3 kt: 12% 12% 2 to 3°, 3 to 7 kt: 9% 9% 2 to 3°, 7 to 12 kt: 6% 6% 2 to 3°, over 12 kt: 5% 5% 3 to 5° 3 to 5°, under 3 kt: 8% 8% 3 to 5°, 3 to 7 kt: 6% 6% 3 to 5°, 7 to 12 kt: 4% 4% 3 to 5°, over 12 kt: 3% 3% 5 to 8° 5 to 8°, under 3 kt: 6% 6% 5 to 8°, 3 to 7 kt: 4% 4% 5 to 8°, 7 to 12 kt: 3% 3% 5 to 8°, over 12 kt: 2% 2% over 8° over 8°, under 3 kt: 3% 3% over 8°, 3 to 7 kt: 2% 2% over 8°, 7 to 12 kt: 1% 1% over 8°, over 12 kt: 1% 1% under 3 kt 3 to 7 kt 7 to 12 kt over 12 kt Wind speed at 18:00 Dew point gap Chance of fog at some point tomorrow, by conditions at 18:00 local time. Show the numbers Dew point gap Wind
Fog tomorrow Evenings under 0.5° under 3 kt 32% 3,898 under 0.5° 3 to 7 kt 25% 5,433 under 0.5° 7 to 12 kt 18% 3,261 under 0.5° over 12 kt 14% 1,437 0.5 to 1° under 3 kt 23% 6,679 0.5 to 1° 3 to 7 kt 16% 11,263 0.5 to 1° 7 to 12 kt 11% 7,685 0.5 to 1° over 12 kt 8% 3,717 1 to 2° under 3 kt 17% 7,982 1 to 2° 3 to 7 kt 12% 16,325 1 to 2° 7 to 12 kt 8% 12,699 1 to 2° over 12 kt 6% 5,946 2 to 3° under 3 kt 12% 7,928 2 to 3° 3 to 7 kt 9% 18,729 2 to 3° 7 to 12 kt 6% 15,674 2 to 3° over 12 kt 5% 7,335 3 to 5° under 3 kt 8% 14,413 3 to 5° 3 to 7 kt 6% 40,627 3 to 5° 7 to 12 kt 4% 39,115 3 to 5° over 12 kt 3% 18,137 5 to 8° under 3 kt 6% 14,716 5 to 8° 3 to 7 kt 4% 49,458 5 to 8° 7 to 12 kt 3% 56,100 5 to 8° over 12 kt 2% 28,177 over 8° under 3 kt 3% 25,525 over 8° 3 to 7 kt 2% 95,626 over 8° 7 to 12 kt 1% 108,173 over 8° over 12 kt 1% 53,509
Unusually humid air comes before thunder
When the dew point has run 6° or more above an airport's normal for a day, thunder is reported there the next day
2.1 times as often as usual for that place and date. After unusually dry air the chance drops to
0.3 times normal. The relationship climbs steadily in between.
0.0× 0.5× 1.0× 1.5× 2.0× 2.5× −6 or less −4 0 +4 +6 or more Dew point over the last 24 hours compared with normal (°C) Chance of thunder tomorrow, times normal Chance of thunder vs normal: 0.3× (dew point -8° from normal) Chance of thunder vs normal: 0.5× (dew point -4° from normal) Chance of thunder vs normal: 0.7× (dew point -2° from normal) Chance of thunder vs normal: 1.0× (dew point +0° from normal) Chance of thunder vs normal: 1.2× (dew point +2° from normal) Chance of thunder vs normal: 1.5× (dew point +4° from normal) Chance of thunder vs normal: 2.1× (dew point +8° from normal) Chance of thunder vs normal Normal All airports, evenings, 2021 to 2026. Comparing with each airport's own normal removes the fact that some places simply see more storms. Show the numbers Dew point vs normal Thunder tomorrow
Normal chance Times normal Evenings -30 to -6° 1% 5% 0.3× 40,775 -6 to -3° 3% 6% 0.5× 75,170 -3 to -1° 5% 7% 0.7× 109,594 -1 to +1° 11% 11% 1.0× 195,331 +1 to +3° 9% 8% 1.2× 137,860 +3 to +6° 8% 5% 1.5× 88,348 +6 to +30° 8% 4% 2.1× 31,741