CMDL Annual Report 24: 1.5.2 Mauna Loa Climate

1.5.2 Mauna Loa Climate

The climatology of MLO is best understood when it is considered in two distinctive wind direction regimes, the night (downslope) period (18000559) Hawaiian Standard Time (HST) and the day (upslope) period (06001759 HST). The 19-year night and day wind roses illustrate the two distinct wind patterns (Figure 1.3).

Wind roses of the surface winds for MLO for 1977-1995 night (left) and day (right)

Fig. 1.3. Wind roses of the surface winds for MLO for 1977 through 1995 night (left) and day (right). The distribution of resultant wind direction and speed are given in units of percent occurrence for the 19-year period. Wind speed is displayed as a function of direction in three speed classes.

For the night regime, the 19-year wind rose (Figure 1.3) shows that 91.3% of all winds observed had a southerly component. The percentage of occurrence of southerly winds in 1996 was 91.8% (Figure 1.4) and 90.9% in 1997 (Figure 1.5). Pressure gradient controlled winds (WS ³ 10 ms^-1) from predominately westerly and southeasterly directions occurred 10.0% in 1996 and 8.8% in 1997 while the 19-year average shows a 7.0% occurrence. The annual average wind speed for both 1996 and 1997 was slightly above the long term mean (Tables 1.9 and 1.10). The upslope or northerly component winds (north-northwest through east-northeast) that occurred 2.4% of the time in 1996 and 1.8% in 1997, are the result of daytime, upslope flow extending into the early evening hours.

Wind roses of the surface winds for MLO for 1996 night (left) and day (right)

Fig. 1.4. Wind roses of the surface winds for MLO for 1996 night (left) and day (right). The distribution of resultant wind direction and speed are given in units of percent occurrence. Wind speed is displayed as a function of direction in three wind speed classes.

Wind roses of the surface winds for MLO for 1997 night (left) and day (right)

Fig. 1.5. Wind roses of the surface winds for MLO for 1997 night (left) and day (right). The distribution of resultant wind direction and speed are given in units of percent occurrence. Wind speed is displayed as a function of direction in three wind speed classes.

For the day regime, the 1996 and 1997 wind roses (Figures 1.4 and 1.5) indicate that winds from the west-northwest through east-northeast occurred 55.9% of the time in 1996 and 53.8% of the time in 1997, compared with 58.6% for the 19-year climatology (Figure 1.3). Pressure gradient controlled winds (WS ³ 10 ms^-1) occurred 9.2% of the time in 1996 and 7.8% in 1997 while the 19-year average shows an occurrence of 5.8%. In 1996 and 1997, the pressure gradient winds, which are usually associated with storms, followed the expected pattern of fewer occurrences during the day regime. The day wind rose is more uniformly distributed in the light wind classes than the night wind rose. This is due to the occurrence of variable wind directions during the transition periods at dawn and dusk, most of which are included in this regime.

The average ambient temperature for 1996 and 1997 (Tables 1.9 and 1.10), combining both day and night records, was 7.0°C, which is close to the long-term average of 7.1°C. February 1997 tied the record low temperature for the month (7°C). The average barometric pressure for 1996 (679.5 hPa) and 1997 (679.6 hPa) were both lower than the long-term average of 680.5 hPa. May and June 1996 each tied its high-pressure record. March 1996 and February 1997 set new record low pressures while June and July 1997 tied their record low pressures. The precipitation amount for 1996 (671 mm) was significantly higher than the long-term average of 460 mm while the amount for 1997 (391 mm) was slightly lower than normal.

TABLE 1.9. MLO 1996 Monthly Climate Summary

		Jan.	Feb.		March	April	May	June	July	Aug.	Sept.	Oct.	Nov.	Dec.	Year
									Night
Prevailing wind	SSE			SW	SW	SSE	SSE	SSE	SE	SSE	SSE	SE	SSE	SW	SSE
direction
Average wind	4.9		6.1		6.3	4.6	6.9	4.0	4.2	4.7	2.9	4.5	3.6	7.7	5.1
speed (m s^-1)
Maximum wind	18		17		22	13	16	12	13	13	10	12	14	19	22
speed* (m s^-1)
Direction of max.	226		235		232	227	250	164	162	164	159	160	156	224	232
wind* (deg.)
Average station	678.1		677.5		677.3	680.4	681.1	681.1	680.8	680.9	680.1	680.5	679.1	677.1	679.5
pressure (hPa)
Maximum pressure*	682		682		684	684	686	686	685	684	682	684	683	681	686
(hPa)
Minimum pressure*	672		673		669	675	674	678	678	678	678	676	675	670	669
(hPa)
Average air	3.3		3.1		2.4	5.5	7.0	5.8	6.1	6.7	6.8	6.0	4.8	3.6	5.1
temperature (°C)
Maximum temperature*	8		10		8	12	13	13	14	14	14	11	9	8	14
(°C)
Minimum temperature*	0		-2		-2	1	4	1	2	2	2	2	1	-1	-2
(°C)
Average dewpoint	-7.8		-9.4		-14.7	-14.2	-12.0	-9.3	-10.3	-9.2	-7.2	-11.2	-4.2	-12.6	-10.2
temperature (°C)
Maximum dewpoint	3		5		4	5	7	7	7	8	8	8	7	6	8
temperature (°C)
Minimum dewpoint	-28		-32		-33	-30	-31	-29	-30	-27	-27	-27	-30	-33	-33
temperature (°C)
Precipitation (mm)	35		68		57	0	0	0	2	1	1	0	21	64	249
									Day
Prevailing wind	SSE			WSW	WNW	N	NW	NNW	N	NE	NNW	ENE	NNW	WSW	NNW
direction
Average wind	4.7		5.7		6.7	3.9	6.3	3.3	3.3	4.4	2.7	3.8	3.5	7.4	4.7
speed (m s^-1)
Maximum wind	20		16		21	10	17	11	12	12	8	11	15	20	21
speed* (m s^-1)
Direction of max.	229		232		223	122	261	164	157	139	165	161	157	240	223
wind* (deg.)
Average station	678.1		677.5		677.4	680.5	681.3	681.3	681.0	681.1	680.1	680.5	679.0	677.0	679.5
pressure (hPa)
Maximum pressure*	683		682		684	684	685	685	684	684	682	683	683	682	685
(hPa)
Minimum pressure*	671		673		668	676	675	679	678	679	678	676	675	670	668
(hPa)
Average air	6.1		6.2		6.7	10.0	12.0	10.1	10.4	11.1	10.6	9.9	7.6	7.0	8.9
temperature (°C)
Maximum temperature*	14		12		13	16	17	17	17	18	16	15	14	14	18
(°C)
Minimum temperature*	0		-1		-1	2	5	3	3	3	4	3	1	-1	-1
(°C)
Average dewpoint	-4.5		-6.3		-9.5	-7.2	-6.9	-3.9	-3.4	-3.4	-2.0	-6.3	-2.0	-9.0	-5.4
temperature (°C)
Maximum dewpoint	6		7		5	6	8	8	9	9	9	8	8	8	9
temperature (°C)
Minimum dewpoint	-28		-32		-33	-27	-25	-28	-28	-26	-26	-24	-33	-31	-33
temperature (°C)
Precipitation (mm)	44		47		116	2	2	20	54	0	6	21	49	59	422

Instrument heights: wind, 10.2 m; pressure, 3398.4 m (MSL); air temperature, 2.0 m; dewpoint temperature, 2.0 m. Wind and temperature instruments are on a tower 15 m southwest of the main building.

*Maximum and minimum values are hourly averages.

TABLE 1.10. MLO 1997 Monthly Climate Summary

		Jan.	Feb.		March	April	May	June	July	Aug.	Sept.	Oct.	Nov.	Dec.	Year
									Night
Prevailing wind	SW			SSE	SSE	SE	SW	SE	SSE	SSE	SSE	SSE	SSW	SSE	SSE
direction
Average wind	7.0		5.5		4.7	3.3	5.7	6.6	5.2	3.3	3.2	4.5	3.5	4.7	4.9
speed (m s^-1)
Maximum wind	21		16		13	10	15	13	16	7	8	12	16	12	21
speed* (m s^-1)
Direction of max.	262		171		236	129	231	128	159	153	152	162	233	164	262
wind* (deg.)
Average station	677.8		678.3		678.2	679.6	678.4	680.7	680.2	681.4	681.3	680.7	680.4	680.1	679.6
pressure (hPa)
Maximum pressure*	683		683		683	683	683	683	683	685	684	683	684	683	685
(hPa)
Minimum pressure*	673		667		674	675	675	677	677	679	677	679	675	677	667
(hPa)
Average air	4.1		3.2		2.5	4.3	5.4	6.3	7.0	6.6	6.1	6.0	4.7	4.9	5.0
temperature (°C)
Maximum temperature*	9		11		8	11	13	13	15	13	11	12	10	10	15
(°C)
Minimum temperature*	-2		-3		-1	-2	0	1	2	3	2	1	1	1	-3
(°C)
Average dewpoint	-14.3		-16.4		-4.7	-11.6	-9.8	-8.6	-10.0	-7.9	-9.4	-14.7	-7.9	-27.3	-12.2
temperature (°C)
Maximum dewpoint	7		1		5	7	7	7	8	7	7	7	8	3	8
temperature (°C)
Minimum dewpoint	-33		-33		-23	-27	-30	-26	-29	-20	-28	-32	-25	-39	-39
temperature (°C)
Precipitation (mm)	83		5		12	6	0	9	36	0	1	0	48	0	201
									Day
Prevailing wind	WSW			ENE	NNW	NNW	WSW	SE	NNW	NNW	NNW	SE	NNW	SE	NNW
direction
Average wind	6.7		4.6		4.7	3.1	6.5	5.6	4.4	3.2	3.0	4.0	3.4	3.7	4.5
speed (m s^-1)
Maximum wind	17		15		14	10	16	13	16	8	8	9	16	12	17
speed* (m s^-1)
Direction of max.	259		171		246	280	241	143	153	158	151	145	230	290	259
wind* (deg.)
Average station	677.9		678.2		678.2	679.6	678.5	680.8	680.5	681.6	681.3	680.7	680.3	680.1	679.7
pressure (hPa)
Maximum pressure*	683		682		683	683	682	684	683	684	684	683	684	684	684
(hPa)
Minimum pressure*	672		668		673	676	675	677	677	679	678	678	674	677	668
(hPa)
Average air	7.7		7.0		5.8	8.7	9.9	10.9	11.2	11.7	10.2	10.4	8.4	8.8	9.1
temperature (°C)
Maximum temperature*	14		15		13	16	17	18	18	18	16	16	15	15	18
(°C)
Minimum temperature*	-2		-1		-1	0	1	2	4	4	3	3	1	1	-2
(°C)
Average dewpoint	-10.3		-12.0		-1.8	-5.7	-3.8	-3.9	-3.3	-3.3	-2.9	-8.9	-2.5	-18.6	-6.7
temperature (°C)
Maximum dewpoint	6		5		6	8	8	9	9	9	11	7	9	5	11
temperature (°C)
Minimum dewpoint	-31		-33		-23	-26	-29	-25	-27	-22	-26	-29	-24	-37	-37
temperature (°C)
Precipitation (mm)	9		2		63	22	1	11	31	7	6	32	5	0	191

Instrument heights: wind, 10.2 m; pressure, 3398.4 m (MSL); air temperature, 2.0 m; dewpoint temperature, 2.0 m. Wind and temperature instruments are on a tower 15 m southwest of the main building.

*Maximum and minimum values are hourly averages.

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