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 2024-11-04.

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	14% 11% 4 9% 5% 4 4 3 5% 6% 34%	101522
	Night	15% 13% 5% 11% 6% 5% 4 3 5% 6% 28%	97562
Transparency	Day	27% 47% 7% 12% 6% 1	105066
	Night	30% 48% 6% 10% 5%	102106
Seeing	Day	19% 9% 25% 29% 16% 3	15261
	Night	21% 11% 26% 26% 13% 2	24473

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% 24% 13% 16% 8% 4 4 4 2 2 7%	2219
	11th to 20th	Day	11% 22% 11% 27% 10% 4 5% 2 2 2 4	2530
	21st to end	Day	19% 22% 10% 14% 7% 5% 5% 4 4 3 7%	3067
February	1st to 10th	Day	13% 22% 6% 16% 9% 5% 5% 3 4 4 15%	1710
	11th to 20th	Day	11% 17% 6% 12% 8% 6% 6% 3 6% 6% 19%	1663
	21st to end	Day	12% 19% 6% 12% 6% 6% 6% 3 5% 5% 21%	1541
March	1st to 10th	Day	17% 13% 4 8% 5% 5% 4 4 5% 6% 31%	3299
	11th to 20th	Day	10% 15% 7% 12% 7% 5% 4 3 5% 5% 27%	3480
	21st to end	Day	14% 12% 6% 8% 5% 4 5% 4 4 6% 32%	3815
April	1st to 10th	Day	18% 13% 4 7% 5% 4 4 4 5% 6% 31%	2210
	11th to 20th	Day	18% 9% 3 6% 4 4 3 3 5% 6% 39%	2442
	21st to end	Day	18% 10% 4 6% 4 3 4 3 5% 6% 37%	2470
May	1st to 10th	Day	16% 7% 3 6% 4 4 4 3 5% 7% 42%	4108
	11th to 20th	Day	13% 7% 2 6% 3 4 4 3 6% 8% 45%	4255
	21st to end	Day	14% 6% 2 5% 3 4 4 4 5% 7% 47%	4883
June	1st to 10th	Day	13% 7% 2 6% 5% 4 5% 4 6% 7% 41%	2850
	11th to 20th	Day	10% 6% 2 6% 4 5% 5% 3 6% 9% 44%	2850
	21st to end	Day	13% 7% 3 6% 4 4 3 4 6% 8% 42%	2850
July	1st to 10th	Day	12% 7% 3 6% 5% 4 4 4 6% 8% 40%	4760
	11th to 20th	Day	9% 5% 2 5% 3 4 4 3 6% 8% 52%	4671
	21st to end	Day	9% 5% 1 4 3 3 4 4 6% 8% 53%	4560
August	1st to 10th	Day	11% 6% 2 6% 4 4 4 4 7% 10% 44%	4104
	11th to 20th	Day	15% 6% 2 5% 3 4 4 4 6% 9% 43%	4044
	21st to end	Day	8% 6% 2 6% 4 5% 5% 4 6% 9% 46%	4532
September	1st to 10th	Day	12% 7% 2 7% 4 5% 5% 3 5% 7% 43%	2244
	11th to 20th	Day	12% 5% 2 6% 3 4 5% 4 6% 8% 46%	2090
	21st to end	Day	17% 8% 2 7% 4 5% 5% 3 5% 7% 37%	2090
October	1st to 10th	Day	16% 10% 4 7% 5% 5% 4 4 6% 6% 33%	1940
	11th to 20th	Day	24% 13% 4 9% 5% 5% 5% 3 4 6% 22%	1900
	21st to end	Day	25% 16% 5% 8% 5% 5% 4 2 5% 6% 19%	1981
November	1st to 10th	Day	17% 16% 5% 10% 5% 5% 6% 3 5% 7% 21%	1666
	11th to 20th	Day	17% 17% 5% 12% 7% 5% 5% 4 5% 6% 16%	1520
	21st to end	Day	22% 21% 7% 12% 7% 5% 5% 3 4 4 10%	1520
December	1st to 10th	Day	16% 30% 7% 16% 7% 5% 4 2 4 3 7%	1520
	11th to 20th	Day	19% 24% 7% 16% 7% 5% 3 3 4 3 9%	1707
	21st to end	Day	26% 25% 7% 18% 6% 4 2 2 3 3 5%	2431
January	1st to 10th	Night	16% 22% 9% 19% 10% 6% 4 2 3 2 7%	4734
	11th to 20th	Night	14% 18% 9% 23% 12% 6% 5% 3 3 3 6%	4440
	21st to end	Night	16% 18% 8% 18% 7% 7% 6% 3 3 4 10%	4588
February	1st to 10th	Night	11% 21% 6% 17% 8% 5% 3 3 5% 5% 15%	2470
	11th to 20th	Night	12% 17% 6% 13% 8% 5% 5% 3 4 5% 20%	2403
	21st to end	Night	14% 15% 6% 12% 6% 5% 5% 4 5% 4 24%	1908
March	1st to 10th	Night	12% 11% 4 9% 5% 5% 4 3 4 6% 38%	3541
	11th to 20th	Night	9% 13% 5% 9% 5% 5% 5% 3 5% 6% 36%	3479
	21st to end	Night	12% 9% 5% 10% 6% 5% 5% 4 6% 5% 32%	3817
April	1st to 10th	Night	20% 11% 3 7% 4 3 5% 3 5% 5% 33%	1986
	11th to 20th	Night	19% 8% 3 5% 4 3 5% 4 5% 7% 37%	1687
	21st to end	Night	19% 11% 3 8% 5% 4 4 4 5% 5% 33%	1710
May	1st to 10th	Night	14% 7% 3 8% 5% 5% 5% 4 5% 7% 38%	2833
	11th to 20th	Night	11% 7% 2 8% 4 5% 4 3 6% 8% 41%	2637
	21st to end	Night	12% 7% 2 6% 4 5% 6% 3 4 8% 43%	2690
June	1st to 10th	Night	12% 9% 2 6% 6% 4 5% 4 5% 8% 39%	1330
	11th to 20th	Night	9% 6% 2 6% 4 5% 5% 5% 6% 8% 44%	1330
	21st to end	Night	11% 7% 2 5% 5% 5% 5% 5% 5% 8% 41%	1330
July	1st to 10th	Night	10% 7% 2 6% 4 4 4 3 5% 8% 47%	2210
	11th to 20th	Night	11% 5% 2 6% 4 4 3 3 6% 7% 50%	2296
	21st to end	Night	7% 6% 3 5% 4 4 4 3 6% 8% 51%	2996
August	1st to 10th	Night	9% 7% 3 6% 4 4 4 4 6% 7% 45%	2825
	11th to 20th	Night	11% 7% 2 6% 4 4 5% 4 6% 8% 41%	2785
	21st to end	Night	9% 7% 2 6% 4 4 4 4 5% 7% 47%	3124
September	1st to 10th	Night	11% 5% 1 6% 4 4 5% 3 6% 7% 47%	1914
	11th to 20th	Night	11% 5% 2 5% 4 5% 4 3 6% 6% 48%	2090
	21st to end	Night	16% 9% 3 7% 4 3 4 4 5% 7% 37%	2090
October	1st to 10th	Night	17% 8% 4 9% 5% 5% 5% 3 4 6% 33%	2239
	11th to 20th	Night	22% 12% 3 10% 6% 5% 4 3 5% 6% 24%	2280
	21st to end	Night	23% 14% 5% 10% 6% 5% 5% 3 4 5% 20%	2612
November	1st to 10th	Night	19% 14% 4 10% 5% 5% 4 4 5% 5% 24%	2485
	11th to 20th	Night	16% 19% 6% 13% 7% 5% 5% 3 4 6% 17%	2660
	21st to end	Night	22% 21% 5% 12% 8% 5% 4 3 4 4 12%	2660
December	1st to 10th	Night	19% 25% 8% 15% 7% 4 4 3 3 3 8%	2660
	11th to 20th	Night	17% 22% 7% 16% 7% 6% 4 3 4 3 10%	3487
	21st to end	Night	24% 22% 8% 17% 7% 5% 3 2 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	55% 40% 4 1	2220
	11th to 20th	Day	60% 37% 1 2	2530
	21st to end	Day	49% 43% 5% 2 1	3067
February	1st to 10th	Day	26% 66% 3 5% 1	1710
	11th to 20th	Day	22% 64% 6% 4 3	1672
	21st to end	Day	22% 65% 4 6% 3	1541
March	1st to 10th	Day	34% 42% 10% 7% 6%	3302
	11th to 20th	Day	36% 41% 9% 11% 2	3480
	21st to end	Day	28% 44% 10% 12% 5% 1	3815
April	1st to 10th	Day	21% 65% 3 7% 4	2210
	11th to 20th	Day	17% 65% 3 10% 5% 1	2442
	21st to end	Day	17% 65% 5% 8% 5% 1	2470
May	1st to 10th	Day	25% 42% 7% 12% 13% 1	4120
	11th to 20th	Day	21% 45% 11% 16% 7% 1	4259
	21st to end	Day	24% 42% 8% 10% 15% 2	4883
June	1st to 10th	Day	14% 66% 3 10% 6% 1	2850
	11th to 20th	Day	16% 66% 3 9% 6% 1	2850
	21st to end	Day	15% 64% 3 9% 7% 1	2850
July	1st to 10th	Day	32% 36% 10% 13% 8% 1	4686
	11th to 20th	Day	20% 36% 9% 26% 9%	4590
	21st to end	Day	19% 35% 7% 24% 14% 1	4416
August	1st to 10th	Day	23% 36% 9% 21% 11% 1	4091
	11th to 20th	Day	29% 37% 11% 17% 6%	4083
	21st to end	Day	23% 37% 9% 25% 6%	4531
September	1st to 10th	Day	16% 62% 6% 10% 5%	2244
	11th to 20th	Day	12% 61% 4 15% 7%	2090
	21st to end	Day	16% 61% 7% 11% 4	2090
October	1st to 10th	Day	24% 37% 6% 19% 13%	3226
	11th to 20th	Day	31% 37% 8% 19% 5%	3140
	21st to end	Day	38% 36% 9% 13% 4	3244
November	1st to 10th	Day	30% 59% 5% 4 2	1666
	11th to 20th	Day	28% 62% 5% 4 1	1520
	21st to end	Day	30% 62% 4 3 1	1520
December	1st to 10th	Day	32% 62% 3 2 1	1520
	11th to 20th	Day	42% 50% 5% 3	1707
	21st to end	Day	55% 40% 4 1	2431
January	1st to 10th	Night	56% 39% 3 2	4734
	11th to 20th	Night	56% 39% 3 2	4440
	21st to end	Night	47% 42% 6% 4 1	4588
February	1st to 10th	Night	24% 66% 3 5% 2	2470
	11th to 20th	Night	20% 65% 3 9% 3	2405
	21st to end	Night	21% 66% 4 5% 4	1908
March	1st to 10th	Night	26% 43% 11% 11% 8%	3551
	11th to 20th	Night	27% 43% 10% 14% 5% 1	3480
	21st to end	Night	30% 47% 6% 11% 5% 1	3817
April	1st to 10th	Night	24% 65% 2 6% 3	1986
	11th to 20th	Night	18% 66% 3 7% 6% 1	1687
	21st to end	Night	19% 66% 3 5% 5% 1	1710
May	1st to 10th	Night	25% 44% 6% 12% 12%	2840
	11th to 20th	Night	24% 42% 8% 14% 11% 1	2638
	21st to end	Night	25% 43% 6% 12% 10% 3	2691
June	1st to 10th	Night	12% 65% 2 10% 9% 1	1330
	11th to 20th	Night	13% 65% 5% 10% 8% 1	1330
	21st to end	Night	13% 64% 4 10% 7% 1	1330
July	1st to 10th	Night	28% 36% 9% 14% 9% 4	2175
	11th to 20th	Night	22% 35% 5% 28% 10%	2254
	21st to end	Night	22% 35% 6% 16% 20% 1	2900
August	1st to 10th	Night	25% 36% 8% 20% 11% 1	2814
	11th to 20th	Night	31% 38% 10% 13% 7%	2812
	21st to end	Night	23% 37% 7% 24% 9%	3124
September	1st to 10th	Night	14% 61% 7% 13% 6%	1914
	11th to 20th	Night	13% 61% 5% 13% 8%	2090
	21st to end	Night	17% 61% 6% 11% 5%	2090
October	1st to 10th	Night	22% 36% 4 23% 14%	3733
	11th to 20th	Night	29% 37% 10% 18% 5%	3779
	21st to end	Night	33% 37% 11% 15% 3	4299
November	1st to 10th	Night	29% 59% 4 6% 1	2485
	11th to 20th	Night	27% 62% 6% 4 1	2660
	21st to end	Night	32% 63% 3 1 1	2660
December	1st to 10th	Night	31% 62% 3 2 1	2660
	11th to 20th	Night	40% 51% 6% 3	3486
	21st to end	Night	53% 39% 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% 10% 31% 29% 9% 2	460
	11th to 20th	Day	26% 10% 27% 22% 12% 5%	452
	21st to end	Day	23% 10% 25% 30% 11% 2	460
May	1st to 10th	Day	21% 4 26% 34% 13% 2	463
	11th to 20th	Day	17% 4 20% 32% 21% 5%	487
	21st to end	Day	12% 3 20% 37% 21% 6%	549
June	1st to 10th	Day	13% 3 24% 39% 19% 3	680
	11th to 20th	Day	12% 5% 24% 36% 19% 4	680
	21st to end	Day	17% 5% 20% 36% 19% 3	680
July	1st to 10th	Day	10% 3 17% 38% 28% 3	671
	11th to 20th	Day	12% 4 20% 35% 23% 6%	668
	21st to end	Day	12% 2 21% 34% 26% 4	587
August	1st to 10th	Day	13% 6% 17% 31% 27% 6%	486
	11th to 20th	Day	14% 3 13% 33% 29% 9%	479
	21st to end	Day	17% 6% 20% 28% 23% 6%	539
September	1st to 10th	Day	18% 5% 21% 27% 23% 7%	490
	11th to 20th	Day	11% 6% 18% 28% 29% 8%	490
	21st to end	Day	19% 8% 22% 25% 22% 5%	490
October	1st to 10th	Day	24% 12% 22% 21% 16% 4	490
	11th to 20th	Day	27% 15% 29% 22% 6% 1	490
	21st to end	Day	33% 15% 26% 20% 4 1	430
November	1st to 10th	Day	24% 13% 36% 18% 8%	264
	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	736
	11th to 20th	Night	22% 9% 25% 28% 12% 3	563
	21st to end	Night	24% 8% 23% 28% 15% 2	570
May	1st to 10th	Night	17% 2 23% 37% 19% 2	570
	11th to 20th	Night	17% 3 16% 39% 24% 2	464
	21st to end	Night	14% 3 19% 39% 22% 4	411
June	1st to 10th	Night	16% 6% 19% 37% 19% 3	380
	11th to 20th	Night	13% 4 18% 40% 21% 3	380
	21st to end	Night	13% 4 20% 33% 28% 2	380
July	1st to 10th	Night	8% 3 13% 47% 25% 4	370
	11th to 20th	Night	10% 2 19% 34% 31% 4	415
	21st to end	Night	9% 3 14% 41% 30% 3	624
August	1st to 10th	Night	11% 2 16% 36% 28% 6%	567
	11th to 20th	Night	12% 3 16% 29% 31% 9%	564
	21st to end	Night	14% 4 19% 31% 26% 5%	627
September	1st to 10th	Night	14% 5% 18% 32% 23% 9%	732
	11th to 20th	Night	13% 5% 20% 29% 26% 7%	760
	21st to end	Night	21% 8% 19% 22% 23% 7%	760
October	1st to 10th	Night	22% 9% 24% 24% 18% 3	760
	11th to 20th	Night	28% 13% 23% 25% 8% 2	760
	21st to end	Night	33% 13% 28% 21% 5% 1	836
November	1st to 10th	Night	27% 15% 25% 22% 9% 1	755
	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 2024-11-01 22:47:51. Version ac07

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