點估幾時 3 / 8 / 9 / 10 號風球
0. 背景知識
首先就梗係要知道點先會有 3 號或以上風球啦。
3 號就係有強風，即係風速 41–62 km/h
8 號就係有烈風，即係風速 63–117 km/h
10 號就係有颶風，即係風速 118 km/h 以上
9 號就特別少少，喺以前嘅 practice 度會見到，掛得 9 號通常係個風直襲香港（進入 100 公里範圍），風力又得 8 號風球嘅烈風程度，但係風向就會突然轉變嘅時候。又或者係估計就嚟之後會掛 10 號嗰陣，就會掛 9 號。
- 天文台預測路徑
天文台熱帶氣旋路徑資訊
https://www.hko.gov.hk/wxinfo/currwx/tc_posc
第一樣就係大家都會睇嘅天文台路徑圖啦，天文台會根據唔同嘅模型預測路徑啦（廢話）。同埋可以睇埋圖入面個誤差圈嘅大細，嚟判斷天文台對估中曬條路徑係幾有信心。
2. 日本氣象廳預測 (JMA)
呢度主要就用嚟睇強風圈嘅大細嘅。佢入面有一項叫做「 15m/s以上の強風域」（54 km/h 以上嘅強風圈），大約就係會令天文台掛 3 號風球嘅風力水平（41–62 km/h）。
例如寫呢篇文嘅時候，2018 年 9 月 15 號 14 點 45 分，而家仲係 1 號風球，而超強颱風山竹就喺香港東南約 690 公里。喺 JMA 可以見到強風圈嘅西側係 560 公里，東側係 750 公里，咁因爲香港係喺個風嘅西側，所以個風去到大約 560 公里附近嘅話，天文台理論上就會有需要掛 3 號風球嘞。
3. 美國聯合颱風警報中心 (JTWC)
JTWC 入面嘅「TC Warning Text」 會有預測烈風圈、暴風圈同颶風圈嘅大細。爲咗方便解釋，所以就 copy 咗山竹嘅其中一個預報喺下面啦。
WTPN31 PGTW 150300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/TROPICAL CYCLONE WARNING//
RMKS/
1. TYPHOON 26W (MANGKHUT) WARNING NR 033
DOWNGRADED FROM SUPER TYPHOON 26W
01 ACTIVE TROPICAL CYCLONE IN NORTHWESTPAC
MAX SUSTAINED WINDS BASED ON ONE-MINUTE AVERAGE
WIND RADII VALID OVER OPEN WATER ONLY
---
WARNING POSITION:
150000Z --- NEAR 18.1N 120.6E
MOVEMENT PAST SIX HOURS - 275 DEGREES AT 16 KTS
POSITION ACCURATE TO WITHIN 030 NM
POSITION BASED ON CENTER LOCATED BY A COMBINATION OF
SATELLITE AND SYNOPTIC DATA
PRESENT WIND DISTRIBUTION:
MAX SUSTAINED WINDS - 115 KT, GUSTS 140 KT
WIND RADII VALID OVER OPEN WATER ONLY
RADIUS OF 064 KT WINDS - 080 NM NORTHEAST QUADRANT
075 NM SOUTHEAST QUADRANT
055 NM SOUTHWEST QUADRANT
075 NM NORTHWEST QUADRANT
RADIUS OF 050 KT WINDS - 160 NM NORTHEAST QUADRANT
140 NM SOUTHEAST QUADRANT
115 NM SOUTHWEST QUADRANT
145 NM NORTHWEST QUADRANT
RADIUS OF 034 KT WINDS - 300 NM NORTHEAST QUADRANT
290 NM SOUTHEAST QUADRANT
230 NM SOUTHWEST QUADRANT
245 NM NORTHWEST QUADRANT
REPEAT POSIT: 18.1N 120.6E
---
FORECASTS:
12 HRS, VALID AT:
151200Z --- 19.1N 117.8E
MAX SUSTAINED WINDS - 110 KT, GUSTS 135 KT
WIND RADII VALID OVER OPEN WATER ONLY
RADIUS OF 064 KT WINDS - 085 NM NORTHEAST QUADRANT
080 NM SOUTHEAST QUADRANT
060 NM SOUTHWEST QUADRANT
080 NM NORTHWEST QUADRANT
RADIUS OF 050 KT WINDS - 170 NM NORTHEAST QUADRANT
140 NM SOUTHEAST QUADRANT
120 NM SOUTHWEST QUADRANT
140 NM NORTHWEST QUADRANT
RADIUS OF 034 KT WINDS - 310 NM NORTHEAST QUADRANT
295 NM SOUTHEAST QUADRANT
220 NM SOUTHWEST QUADRANT
250 NM NORTHWEST QUADRANT
VECTOR TO 24 HR POSIT: 295 DEG/ 15 KTS
---
24 HRS, VALID AT:
160000Z --- 20.3N 114.8E
MAX SUSTAINED WINDS - 115 KT, GUSTS 140 KT
WIND RADII VALID OVER OPEN WATER ONLY
RADIUS OF 064 KT WINDS - 075 NM NORTHEAST QUADRANT
065 NM SOUTHEAST QUADRANT
050 NM SOUTHWEST QUADRANT
060 NM NORTHWEST QUADRANT
RADIUS OF 050 KT WINDS - 165 NM NORTHEAST QUADRANT
140 NM SOUTHEAST QUADRANT
120 NM SOUTHWEST QUADRANT
135 NM NORTHWEST QUADRANT
RADIUS OF 034 KT WINDS - 295 NM NORTHEAST QUADRANT
265 NM SOUTHEAST QUADRANT
210 NM SOUTHWEST QUADRANT
185 NM NORTHWEST QUADRANT
VECTOR TO 36 HR POSIT: 290 DEG/ 16 KTS
---
36 HRS, VALID AT:
161200Z --- 21.4N 111.5E
MAX SUSTAINED WINDS - 115 KT, GUSTS 140 KT
WIND RADII VALID OVER OPEN WATER ONLY
RADIUS OF 064 KT WINDS - 065 NM NORTHEAST QUADRANT
055 NM SOUTHEAST QUADRANT
045 NM SOUTHWEST QUADRANT
045 NM NORTHWEST QUADRANT
RADIUS OF 050 KT WINDS - 100 NM NORTHEAST QUADRANT
110 NM SOUTHEAST QUADRANT
080 NM SOUTHWEST QUADRANT
070 NM NORTHWEST QUADRANT
RADIUS OF 034 KT WINDS - 225 NM NORTHEAST QUADRANT
220 NM SOUTHEAST QUADRANT
175 NM SOUTHWEST QUADRANT
130 NM NORTHWEST QUADRANT
VECTOR TO 48 HR POSIT: 285 DEG/ 15 KTS
---
EXTENDED OUTLOOK:
48 HRS, VALID AT:
170000Z --- 22.2N 108.4E
MAX SUSTAINED WINDS - 060 KT, GUSTS 075 KT
WIND RADII VALID OVER OPEN WATER ONLY
DISSIPATING AS A SIGNIFICANT TROPICAL CYCLONE OVER LAND
RADIUS OF 050 KT WINDS - 040 NM NORTHEAST QUADRANT
050 NM SOUTHEAST QUADRANT
030 NM SOUTHWEST QUADRANT
030 NM NORTHWEST QUADRANT
RADIUS OF 034 KT WINDS - 120 NM NORTHEAST QUADRANT
130 NM SOUTHEAST QUADRANT
105 NM SOUTHWEST QUADRANT
080 NM NORTHWEST QUADRANT
VECTOR TO 72 HR POSIT: 285 DEG/ 13 KTS
---
72 HRS, VALID AT:
180000Z --- 23.3N 103.0E
MAX SUSTAINED WINDS - 020 KT, GUSTS 030 KT
WIND RADII VALID OVER OPEN WATER ONLY
DISSIPATED AS A SIGNIFICANT TROPICAL CYCLONE OVER LAND
---
REMARKS:
150300Z POSITION NEAR 18.4N 119.9E.
TYPHOON 26W (MANGKHUT), LOCATED APPROXIMATELY 211 NM NORTH OF
MANILA, PHILIPPINES, HAS TRACKED WESTWARD AT 16 KNOTS OVER THE
PAST SIX HOURS. NEXT WARNINGS AT 150900Z, 151500Z, 152100Z AND
160300Z.//
NNNN其實要睇嘅嘢就得好少，就係對返邊個時間啲風圈有幾大，再睇返天文台路徑睇下個風幾時會喺邊。例如睇下 12 個鐘後嘅預報啦，時間係 151200Z，即係 15 號嘅 12 點鐘 (UTC)，加返 8 個鐘就係香港時間，即係夜晚 8 點鐘。「RADIUS OF 034 KT WINDS」就係烈風圈嘞（KT 即係 knots，1 kt = 1.852 km/h；34 kts = 63 km/h）。烈風圈嘅大細就係：
RADIUS OF 034 KT WINDS - 310 NM NORTHEAST QUADRANT
295 NM SOUTHEAST QUADRANT
220 NM SOUTHWEST QUADRANT
250 NM NORTHWEST QUADRANT對返路徑嘅話，個風呢個時間應該會喺香港嘅東南邊，即係香港係喺個風嘅西北邊。預報度寫住西北邊嘅烈風圈係 250 NM 大 （1 NM = 1.852 km），即係 463 公里大。即係話，只要個風喺香港嘅東南邊大約 463 公里嘅話，就可能會吹烈風，就有需要掛 8 號風球。
「RADIUS OF 050 KT WINDS」就係暴風圈半徑，「RADIUS OF 064 KT WINDS」就係颶風圈半徑，可以用嚟睇 10 號風球嘅距離。
4. 實戰
首先，而家係下晝 4 點鐘，山竹喺香港東南約 660 公里，而西邊強風圈係 560 公里，而個風行走嘅速度係 30 km/h，咁就可以估到下晝 7 點就應該要已經掛咗 3 號（雖然天文台係呢個時候已經講咗今日下午三時至六時之間改發三號強風信號，so9sad，同埋結果 16:20 就掛咗）。
之後上面都有講到 JTWC 預計夜晚 8 點嘅時候，西北邊嘅烈風圈半徑係 463 公里大，東北邊嘅烈風圈半徑係 574 公里大！！！16 號朝早 8 點嘅時候，西北邊嘅烈風圈半徑 343 公里大，東北邊嘅烈風圈半徑係 546 公里大。再睇返天文台路徑同個風嘅速度嚟估，大約喺凌晨 1 點嘅時候，應該大約會喺香港 400 公里內。用最垃圾嘅直線估計嘅話，呢個時候香港應該會喺烈風圈入面，所以應該喺咁上下時間就會掛 8 號。
再進一步就係，16 號朝早 8 點嘅時候嘅颶風圈半徑大概係 130 公里。如果個風喺咁上下位置以內嘅話，就應該要掛 10 號風球。至於 9 號風球，因爲個風好勁，所以唔會係「直襲香港，風力又得烈風」嘅 case，而係要睇天文台啲人嘅心情。可能如果夠近（200 公里掛…），又預測個風會入到 130 公里左右嘅話，就會掛 9 號。
至於星期一（17 號）朝早會唔會仲係 8 號風球，烈風圈半徑預測係 240 公里，一睇路徑圖就知個風已經喺 240 公里以外好遠嘅地方，所以，都係早啲休息罷啦（除非黑雨啦）。
咁當然，咁樣睇其實都未必準，因爲首先要視乎返喺香港邊個方向會當風啲，邊個方向會冇咁當風啦。例如如果個風係喺香港嘅正東面嘅話，會冇咁當風，個風喺香港嘅西或者南面嘅話，就會好當風。
氣象冷知識：東登‧西登 （香港天文台）
https://www.youtube.com/watch?v=n3xyCLIBbDk
其次就係要睇下呢幾個預報準唔準嘞。不過，睇咗 JMA 同 JTWC 兩個預報，就大概可以估到個風去到邊個位置嘅時候，就會有幾大風，起碼多咗少少概念先。
