Weather Indices - Weather Underground of Hong Kong

Heat Index

In an effort to alert the public to the hazards of prolonged heat/humidity episodes, the National Weather Service of the United States derived the Heat Index. The heat index is an accurate measure of how hot it really feels when the effects of humidity are added to high temperature.

The human body contains several mechanisms to maintain its internal operating temperature at 37C. When threatened with above normal temperatures, the body will try to dissipate excess heat by varying the rate and depth of blood circulation, by losing water through the skin and sweat glands, and, as a last resort, by panting. When weather conditions force the air temperature above 32C and the relative humidity is high, the body is doing everything it can to maintain its normal temperature. Unfortunately, conditions can exceed the body's ability to cope with the combined affects of heat and humidity. At such times the body may sucumb to any of a number of heat disorders including sun stroke, heat cramp, heat exhaustion and heat stroke.

To use the heat index chart below, find the temperature on the left of the chart. Read across until you reach the desired relative humidity. The number which appears at the intersection of the temperature and relative humidity is the Heat Index. Note that the Heat Index under direct sunlight will be 8C higher than the number shown in the chart.

Relative Humidity (%) Temperature (C)	50	55	60	65	70	75	80	85	90	95	100
28	28.2	28.6	29.1	29.7	30.2	30.9	31.6	32.3	33.1	33.9	34.7
29	29.5	30.1	30.8	31.6	32.5	33.4	34.4	35.5	36.7	37.9	39.3
30	31.0	31.9	32.8	33.9	35.0	36.3	37.7	39.1	40.7	42.4	44.2
31	31.9	32.9	33.9	35.1	36.4	37.9	39.4	41.1	42.9	44.8	46.8
32	33.8	35.0	36.3	37.8	39.4	41.2	43.2	45.3	47.5	49.9	52.4
33	35.8	37.3	39.0	40.8	42.8	44.9	47.3	49.8	52.5	55.4	58.4
34	38.2	39.9	41.9	44.0	46.4	49.0	51.7	54.7	57.9	61.3	64.8
35	40.7	42.7	45.1	47.6	50.3	53.3	56.5	60.0	63.7	67.6	71.7
36	42.0	44.3	46.7	49.5	52.4	55.6	59.1	62.8	66.7	70.9	75.3
37	44.9	47.5	50.3	53.4	56.8	60.5	64.4	68.6	73.1	77.8	82.8
38	48.0	50.9	54.2	57.7	61.5	65.7	70.1	74.8	79.8	85.1	90.7
39	51.3	54.6	58.3	62.3	66.6	71.2	76.1	81.4	87.0	92.9	99.1
40	54.8	58.5	62.6	67.1	71.9	77.0	82.5	88.3	94.5	101.0	107.9

Note:
>30C Extreme Caution: Heat cramp and exhaustion possible
>40C Danger: Heat exhaustion likely
>55C Extreme Danger: Heat stroke imminent

Wind Chill

When weather is cold and windy, the heat generated by the human body will be blown away by the wind quickly and the human being will have a feeling that the temperature is colder than the actual temperature. This apparent temperature is called the Wind Chill and is a function of temperature and wind speed. When the wind chill becomes low, it is advised not to expose outdoors for a prolonged period of time to prevent excessive heat loss from the human body which is extremely dangerous.

Wind Speed (km/h) Temperature (C)	5	10	15	20	25	30	35	40	45	50
12	11.7	10.6	10.0	9.5	9.1	8.8	8.5	8.2	8.0	7.8
11	10.4	9.3	8.6	8.1	7.7	7.3	7.0	6.7	6.5	6.3
10	9.8	8.6	7.9	7.4	7.0	6.6	6.3	6.0	5.7	5.5
9	8.5	7.3	6.6	6.0	5.5	5.1	4.8	4.5	4.2	4.0
8	7.2	6.0	5.2	4.6	4.1	3.7	3.3	3.0	2.7	2.5
7	6.0	4.7	3.8	3.2	2.7	2.2	1.9	1.5	1.2	1.0
6	4.7	3.3	2.4	1.8	1.2	0.8	0.4	0.0	-0.3	-0.6
5	4.1	2.7	1.8	1.1	0.5	0.1	-0.3	-0.7	-1.0	-1.3
4	2.8	1.3	0.4	-0.3	-0.9	-1.4	-1.8	-2.2	-2.5	-2.8
3	1.6	0.0	-1.0	-1.7	-2.3	-2.8	-3.3	-3.7	-4.0	-4.3
2	0.3	-1.3	-2.3	-3.1	-3.7	-4.3	-4.7	-5.1	-5.5	-5.9
1	-0.9	-2.6	-3.7	-4.5	-5.2	-5.7	-6.2	-6.6	-7.0	-7.4
0	-1.6	-3.3	-4.4	-5.2	-5.9	-6.5	-6.9	-7.4	-7.8	-8.1

Δp(972) and Δp(S)

Δp(972) and Δp(S) are pressure indices derived by Hong Kong Observatory to forecast the arrival time and intensity of winter monsoon surges in Hong Kong between October and March/April.

Δp(972) is the pressure difference between Chenzhou (WMO station 57972, 25.8N, 113.0E) and Hong Kong. Positive values indicate pressure over Chenzhou is higher than Hong Kong. According to Lai(1989)¹, a Δp(972) value of 7 hPa or above indicates a northerly surge is on the way to Hong Kong:

Δp(972) (hPa)	Arrival time (hours)
6	18
7	15
8	12
9	9

Look for duplicate signals (Δp(972) stays at 7 hPa) which may suggest a delay of the surge. If Δp(972) increases by 2 hPa in 6 hours or Δp(972) is expected to reach 10 hPa, the surge may bring strong northerlies to Hong Kong.

Δp(S) is the pressure difference between Shanghai (WMO station 58362, 31.4N, 121.5E) and Hong Kong. Positive values indicate pressure over Shanghai is higher than Hong Kong. According to Chang(1989)², a Δp(S) value of 8 hPa or above may suggest the arrival of easterly surge to Hong Kong with strong easterlies over offshore areas overnight.

In late winter, the arrival of northerly surge may be affected by other factors, e.g. cold air blocked by Nanling. Care should be taken when interpreting the indices, especially when Δp(972) and Δp(S) are of comparable magnitudes.

References:
¹Lai, S.T. (1989), Short-range forecasting of northerly surges, Hong Kong Observatory Technical Note 83.
²Chang, K.M. (1989), Prediction of the strength of overnight easterly winds in Hong Kong in winter, Hong Kong Observatory Technical Note No. 79.