Johnston's Harbour
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Johnston's Harbour clear sky chart from 2005-11-06 to 2024-04-03.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	15% 11% 5% 8% 6% 3 5% 4 5% 6% 31%	78555
	Night	16% 13% 5% 10% 8% 4 5% 4 4 5% 26%	81592
Transparency	Day	37% 37% 8% 12% 6% 1	78260
	Night	43% 35% 7% 9% 4 1	81422
Seeing	Day	19% 8% 24% 29% 16% 3	14355
	Night	21% 11% 26% 26% 13% 2	23821

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	17% 23% 13% 14% 10% 3 4 4 2 2 8%	2219
	11th to 20th	Day	12% 21% 11% 23% 14% 3 5% 2 2 2 4	2530
	21st to end	Day	20% 21% 10% 12% 8% 4 6% 4 4 3 7%	3067
February	1st to 10th	Day	13% 20% 7% 14% 11% 4 5% 4 3 4 15%	1710
	11th to 20th	Day	12% 16% 7% 11% 10% 5% 6% 5% 5% 6% 19%	1663
	21st to end	Day	12% 18% 7% 11% 6% 5% 6% 4 4 5% 21%	1541
March	1st to 10th	Day	14% 10% 3 8% 7% 4 5% 4 4 6% 34%	2001
	11th to 20th	Day	12% 12% 5% 9% 6% 3 5% 5% 5% 6% 32%	2090
	21st to end	Day	16% 10% 4 7% 5% 3 6% 4 4 6% 35%	2299
April	1st to 10th	Day	18% 12% 4 6% 6% 3 4 5% 5% 7% 30%	2114
	11th to 20th	Day	18% 9% 3 6% 4 3 4 3 5% 6% 40%	2312
	21st to end	Day	18% 9% 4 5% 4 2 4 5% 5% 7% 37%	2340
May	1st to 10th	Day	17% 7% 3 5% 5% 2 5% 5% 5% 7% 39%	2340
	11th to 20th	Day	15% 7% 3 4 5% 3 4 4 5% 7% 43%	2391
	21st to end	Day	11% 5% 2 4 4 3 4 5% 6% 8% 47%	2743
June	1st to 10th	Day	12% 6% 3 5% 6% 3 5% 5% 6% 7% 42%	2700
	11th to 20th	Day	10% 6% 2 5% 5% 3 5% 4 6% 10% 44%	2700
	21st to end	Day	12% 7% 3 6% 5% 3 4 5% 6% 8% 43%	2700
July	1st to 10th	Day	7% 5% 2 4 4 3 4 4 6% 9% 51%	2700
	11th to 20th	Day	9% 6% 2 5% 5% 3 5% 5% 6% 9% 46%	2653
	21st to end	Day	9% 5% 2 4 5% 3 5% 5% 6% 10% 48%	2599
August	1st to 10th	Day	12% 5% 2 4 5% 2 4 4 6% 8% 49%	2327
	11th to 20th	Day	10% 4 2 5% 5% 3 4 4 6% 9% 47%	2303
	21st to end	Day	12% 7% 3 5% 5% 3 5% 5% 6% 9% 41%	2574
September	1st to 10th	Day	12% 6% 2 6% 5% 3 5% 4 6% 7% 43%	2127
	11th to 20th	Day	13% 5% 3 5% 4 3 5% 5% 6% 8% 44%	1980
	21st to end	Day	17% 7% 3 6% 5% 4 5% 5% 5% 7% 36%	1980
October	1st to 10th	Day	17% 10% 4 6% 6% 3 5% 5% 5% 6% 32%	1838
	11th to 20th	Day	26% 12% 4 8% 6% 4 5% 4 4 6% 21%	1800
	21st to end	Day	26% 15% 5% 8% 6% 4 4 4 4 5% 18%	1877
November	1st to 10th	Day	18% 15% 5% 9% 6% 4 6% 4 5% 6% 21%	1639
	11th to 20th	Day	18% 16% 5% 10% 9% 5% 5% 5% 5% 7% 16%	1520
	21st to end	Day	22% 20% 8% 10% 8% 4 6% 4 3 4 10%	1520
December	1st to 10th	Day	16% 28% 8% 14% 9% 4 4 3 4 3 7%	1520
	11th to 20th	Day	19% 22% 8% 14% 9% 3 4 4 4 3 9%	1707
	21st to end	Day	26% 24% 8% 16% 8% 3 3 3 2 3 5%	2431
January	1st to 10th	Night	16% 21% 10% 17% 12% 4 5% 3 3 3 7%	4734
	11th to 20th	Night	14% 17% 9% 19% 16% 5% 5% 4 3 3 6%	4440
	21st to end	Night	16% 18% 8% 15% 10% 5% 6% 4 3 4 11%	4588
February	1st to 10th	Night	11% 19% 7% 15% 10% 4 4 4 5% 5% 15%	2470
	11th to 20th	Night	13% 15% 8% 12% 8% 4 6% 5% 4 5% 20%	2403
	21st to end	Night	15% 14% 6% 11% 8% 3 6% 5% 5% 4 24%	1908
March	1st to 10th	Night	13% 10% 4 9% 7% 5% 4 4 5% 6% 34%	2166
	11th to 20th	Night	13% 12% 5% 7% 6% 3 5% 4 4 6% 34%	2089
	21st to end	Night	15% 8% 4 8% 7% 4 4 5% 6% 6% 35%	2299
April	1st to 10th	Night	21% 10% 4 6% 5% 3 4 4 5% 6% 33%	1906
	11th to 20th	Night	18% 7% 4 5% 5% 2 4 5% 5% 7% 38%	1597
	21st to end	Night	19% 10% 3 7% 6% 3 4 4 5% 5% 33%	1620
May	1st to 10th	Night	15% 7% 4 5% 5% 4 5% 6% 5% 8% 36%	1620
	11th to 20th	Night	13% 7% 3 5% 5% 4 5% 5% 5% 6% 41%	1501
	21st to end	Night	11% 6% 3 5% 4 3 5% 5% 5% 8% 46%	1528
June	1st to 10th	Night	12% 8% 3 5% 6% 3 5% 5% 5% 8% 40%	1260
	11th to 20th	Night	9% 6% 3 5% 5% 4 5% 6% 6% 9% 44%	1260
	21st to end	Night	10% 6% 3 4 6% 3 5% 6% 5% 8% 42%	1260
July	1st to 10th	Night	7% 4 2 4 4 2 5% 4 6% 9% 54%	1260
	11th to 20th	Night	8% 5% 2 6% 5% 4 4 5% 8% 9% 46%	1304
	21st to end	Night	7% 5% 2 4 4 3 5% 5% 7% 8% 50%	1719
August	1st to 10th	Night	8% 5% 3 5% 3 3 4 5% 6% 8% 50%	1611
	11th to 20th	Night	8% 5% 2 5% 4 3 6% 4 7% 9% 47%	1598
	21st to end	Night	11% 7% 2 6% 5% 3 4 6% 5% 7% 44%	1782
September	1st to 10th	Night	11% 5% 2 5% 5% 3 4 5% 6% 7% 47%	1812
	11th to 20th	Night	12% 5% 2 4 5% 4 5% 4 6% 6% 46%	1980
	21st to end	Night	16% 8% 4 6% 5% 3 4 4 4 7% 37%	1980
October	1st to 10th	Night	18% 9% 4 8% 6% 4 5% 4 3 6% 32%	2121
	11th to 20th	Night	24% 12% 3 8% 8% 4 4 4 5% 6% 23%	2160
	21st to end	Night	25% 14% 5% 9% 7% 3 5% 4 4 5% 19%	2474
November	1st to 10th	Night	20% 13% 4 9% 6% 4 4 5% 5% 5% 24%	2439
	11th to 20th	Night	17% 17% 6% 12% 8% 4 5% 5% 4 6% 17%	2660
	21st to end	Night	23% 19% 6% 11% 9% 4 5% 4 4 5% 12%	2660
December	1st to 10th	Night	20% 23% 10% 13% 9% 3 5% 4 3 3 8%	2660
	11th to 20th	Night	18% 21% 8% 15% 8% 4 5% 4 4 3 11%	3487
	21st to end	Night	24% 21% 8% 15% 8% 4 4 3 3 3 7%	5236

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	66% 27% 5% 2	2220
	11th to 20th	Day	72% 25% 2 1	2530
	21st to end	Day	65% 26% 5% 3	3067
February	1st to 10th	Day	51% 36% 6% 7% 1	1710
	11th to 20th	Day	48% 36% 8% 6% 1	1672
	21st to end	Day	44% 36% 9% 9% 2	1541
March	1st to 10th	Day	35% 36% 11% 12% 5% 1	2004
	11th to 20th	Day	36% 36% 10% 12% 5% 1	2090
	21st to end	Day	33% 36% 10% 12% 7% 1	2299
April	1st to 10th	Day	33% 37% 10% 13% 7% 1	2114
	11th to 20th	Day	29% 37% 6% 16% 11% 1	2312
	21st to end	Day	31% 38% 8% 12% 8% 2	2340
May	1st to 10th	Day	28% 37% 7% 16% 10% 2	2340
	11th to 20th	Day	28% 38% 6% 15% 13% 1	2395
	21st to end	Day	23% 38% 10% 16% 11% 2	2743
June	1st to 10th	Day	27% 39% 9% 16% 9%	2700
	11th to 20th	Day	22% 37% 9% 20% 12%	2700
	21st to end	Day	26% 39% 10% 16% 7% 1	2700
July	1st to 10th	Day	20% 40% 12% 18% 10% 1	2626
	11th to 20th	Day	21% 41% 11% 18% 8% 1	2572
	21st to end	Day	21% 42% 11% 17% 9%	2455
August	1st to 10th	Day	24% 38% 11% 18% 7% 1	2314
	11th to 20th	Day	20% 37% 14% 19% 10%	2303
	21st to end	Day	26% 38% 10% 17% 9% 1	2574
September	1st to 10th	Day	25% 38% 9% 19% 8% 1	2127
	11th to 20th	Day	27% 39% 12% 17% 5%	1980
	21st to end	Day	32% 39% 10% 14% 5%	1980
October	1st to 10th	Day	33% 38% 8% 13% 6%	1838
	11th to 20th	Day	45% 37% 8% 7% 4	1800
	21st to end	Day	44% 40% 7% 7% 2	1877
November	1st to 10th	Day	37% 42% 9% 10% 3	1639
	11th to 20th	Day	42% 42% 7% 5% 4	1520
	21st to end	Day	49% 41% 6% 3	1520
December	1st to 10th	Day	55% 39% 4 2	1520
	11th to 20th	Day	58% 33% 6% 3	1707
	21st to end	Day	67% 27% 4 1	2431
January	1st to 10th	Night	66% 26% 5% 3	4734
	11th to 20th	Night	69% 26% 3 2	4440
	21st to end	Night	63% 26% 7% 4	4588
February	1st to 10th	Night	49% 36% 6% 7% 1	2470
	11th to 20th	Night	50% 38% 5% 6% 1	2405
	21st to end	Night	43% 35% 11% 9% 2	1908
March	1st to 10th	Night	36% 36% 10% 12% 5%	2176
	11th to 20th	Night	36% 35% 8% 12% 7% 1	2090
	21st to end	Night	33% 36% 9% 14% 7% 1	2299
April	1st to 10th	Night	31% 36% 8% 16% 8% 1	1906
	11th to 20th	Night	29% 36% 6% 15% 12% 1	1597
	21st to end	Night	34% 36% 5% 12% 11% 2	1620
May	1st to 10th	Night	30% 38% 7% 12% 10% 2	1620
	11th to 20th	Night	27% 38% 5% 17% 12% 2	1502
	21st to end	Night	22% 38% 11% 15% 11% 3	1529
June	1st to 10th	Night	32% 38% 7% 13% 10% 1	1260
	11th to 20th	Night	24% 38% 8% 17% 13%	1260
	21st to end	Night	28% 40% 10% 15% 7% 1	1260
July	1st to 10th	Night	19% 38% 9% 20% 13% 2	1225
	11th to 20th	Night	23% 40% 8% 18% 10% 2	1262
	21st to end	Night	20% 42% 10% 19% 9%	1623
August	1st to 10th	Night	24% 39% 10% 18% 8% 1	1600
	11th to 20th	Night	19% 38% 14% 16% 13% 1	1598
	21st to end	Night	25% 38% 10% 17% 8% 2	1782
September	1st to 10th	Night	23% 38% 10% 17% 11% 1	1812
	11th to 20th	Night	26% 38% 12% 19% 5%	1980
	21st to end	Night	32% 37% 10% 15% 5%	1980
October	1st to 10th	Night	34% 37% 7% 14% 7%	2121
	11th to 20th	Night	44% 38% 7% 8% 3	2160
	21st to end	Night	48% 38% 5% 7% 2	2474
November	1st to 10th	Night	34% 42% 8% 12% 3	2439
	11th to 20th	Night	44% 41% 6% 5% 2 1	2660
	21st to end	Night	48% 41% 7% 4	2660
December	1st to 10th	Night	55% 40% 4 1	2660
	11th to 20th	Night	55% 33% 8% 3	3486
	21st to end	Night	67% 26% 6% 1	5236

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	25% 15% 30% 22% 7% 2	186
	11th to 20th	Day	22% 11% 37% 21% 9%	270
	21st to end	Day	28% 7% 28% 28% 9%	297
February	1st to 10th	Day	23% 21% 31% 19% 7%	270
	11th to 20th	Day	18% 16% 34% 27% 5%	270
	21st to end	Day	23% 23% 34% 18% 2	225
March	1st to 10th	Day	19% 10% 36% 27% 7% 1	374
	11th to 20th	Day	19% 12% 31% 28% 8% 2	460
	21st to end	Day	23% 9% 30% 29% 8% 1	506
April	1st to 10th	Day	20% 9% 33% 28% 9% 2	428
	11th to 20th	Day	25% 9% 26% 22% 13% 5%	412
	21st to end	Day	24% 8% 25% 30% 11% 2	420
May	1st to 10th	Day	22% 4 26% 34% 13% 2	423
	11th to 20th	Day	18% 4 21% 32% 21% 5%	447
	21st to end	Day	12% 3 20% 37% 21% 7%	503
June	1st to 10th	Day	12% 3 24% 40% 19% 3	630
	11th to 20th	Day	13% 4 22% 37% 21% 4	630
	21st to end	Day	15% 4 19% 38% 20% 3	630
July	1st to 10th	Day	8% 3 15% 40% 30% 4	621
	11th to 20th	Day	12% 3 18% 36% 24% 7%	618
	21st to end	Day	12% 2 19% 35% 27% 4	540
August	1st to 10th	Day	14% 5% 15% 32% 28% 6%	446
	11th to 20th	Day	12% 3 10% 34% 31% 9%	439
	21st to end	Day	17% 6% 19% 28% 23% 7%	495
September	1st to 10th	Day	16% 5% 20% 27% 24% 8%	450
	11th to 20th	Day	12% 6% 18% 28% 28% 7%	450
	21st to end	Day	19% 8% 21% 25% 22% 6%	450
October	1st to 10th	Day	24% 11% 22% 22% 17% 4	450
	11th to 20th	Day	28% 14% 28% 22% 6% 1	450
	21st to end	Day	33% 15% 26% 21% 4 1	392
November	1st to 10th	Day	25% 13% 36% 18% 8%	255
	11th to 20th	Day	24% 19% 34% 19% 3 1	180
	21st to end	Day	28% 19% 35% 16% 2	180
December	1st to 10th	Day	28% 21% 31% 14% 6%	180
	11th to 20th	Day	26% 26% 27% 18% 3	180
	21st to end	Day	31% 22% 36% 8% 2 1	198
January	1st to 10th	Night	25% 19% 29% 21% 6% 1	950
	11th to 20th	Night	27% 16% 29% 21% 6% 1	950
	21st to end	Night	21% 10% 32% 26% 9% 2	866
February	1st to 10th	Night	19% 17% 36% 22% 5% 1	760
	11th to 20th	Night	21% 11% 34% 26% 7% 1	743
	21st to end	Night	21% 12% 36% 24% 5%	628
March	1st to 10th	Night	17% 9% 35% 30% 8% 1	760
	11th to 20th	Night	18% 13% 33% 27% 9% 1	760
	21st to end	Night	19% 6% 29% 32% 13% 1	836
April	1st to 10th	Night	25% 9% 28% 30% 8% 1	707
	11th to 20th	Night	21% 9% 25% 29% 13% 3	533
	21st to end	Night	23% 8% 23% 29% 15% 2	540
May	1st to 10th	Night	17% 2 23% 36% 19% 2	540
	11th to 20th	Night	18% 3 16% 38% 24% 2	440
	21st to end	Night	13% 3 19% 39% 22% 4	389
June	1st to 10th	Night	17% 4 19% 38% 19% 3	360
	11th to 20th	Night	13% 3 18% 40% 23% 3	360
	21st to end	Night	11% 3 20% 34% 29% 3	360
July	1st to 10th	Night	8% 3 12% 47% 25% 4	350
	11th to 20th	Night	10% 2 18% 33% 32% 5%	392
	21st to end	Night	9% 3 14% 41% 29% 3	591
August	1st to 10th	Night	10% 2 16% 36% 29% 6%	537
	11th to 20th	Night	11% 3 15% 30% 32% 9%	534
	21st to end	Night	14% 5% 20% 30% 27% 6%	594
September	1st to 10th	Night	13% 5% 17% 32% 24% 9%	693
	11th to 20th	Night	13% 6% 21% 29% 24% 7%	720
	21st to end	Night	20% 8% 19% 22% 23% 8%	720
October	1st to 10th	Night	23% 8% 24% 24% 18% 3	720
	11th to 20th	Night	29% 14% 23% 24% 8% 2	720
	21st to end	Night	34% 12% 27% 22% 5% 1	792
November	1st to 10th	Night	27% 15% 26% 22% 9% 1	740
	11th to 20th	Night	25% 22% 27% 19% 7%	760
	21st to end	Night	34% 17% 28% 17% 4	760
December	1st to 10th	Night	33% 19% 29% 12% 7%	760
	11th to 20th	Night	27% 23% 33% 13% 5%	911
	21st to end	Night	33% 16% 32% 15% 4	1045

Page generated at 2024-03-31 06:08:55. Version ac07

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