Hammond Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hammond Bay Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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% 5% 4 3 5% 7% 33%	126966
	Night	14% 12% 5% 11% 6% 5% 4 3 5% 6% 29%	124913
Transparency	Day	49% 6% 12% 21% 10% 1	130456
	Night	56% 6% 11% 17% 9% 1	129511
Seeing	Day	17% 5% 19% 32% 22% 5%	17073
	Night	21% 7% 24% 30% 15% 3	26654

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	11% 16% 9% 24% 10% 5% 5% 3 4 5% 9%	3650
	11th to 20th	Day	11% 18% 6% 23% 14% 8% 4 3 4 3 6%	3637
	21st to end	Day	12% 18% 7% 19% 8% 6% 5% 4 5% 5% 11%	3982
February	1st to 10th	Day	9% 17% 6% 15% 10% 7% 5% 4 4 6% 17%	1770
	11th to 20th	Day	11% 16% 7% 14% 9% 5% 5% 3 4 6% 22%	1857
	21st to end	Day	11% 18% 7% 12% 8% 5% 5% 4 4 5% 22%	1650
March	1st to 10th	Day	13% 12% 3 9% 5% 5% 5% 3 6% 7% 33%	4478
	11th to 20th	Day	11% 13% 5% 11% 6% 6% 5% 4 6% 6% 28%	4975
	21st to end	Day	19% 12% 7% 12% 5% 5% 4 3 4 4 23%	5500
April	1st to 10th	Day	18% 14% 4 8% 5% 4 5% 3 5% 6% 28%	2400
	11th to 20th	Day	18% 12% 4 7% 4 3 5% 3 5% 5% 33%	2547
	21st to end	Day	16% 10% 3 8% 4 5% 4 4 6% 7% 33%	2600
May	1st to 10th	Day	16% 9% 3 6% 4 3 4 3 5% 7% 40%	5888
	11th to 20th	Day	11% 8% 3 8% 5% 4 4 4 5% 7% 42%	5943
	21st to end	Day	14% 8% 2 6% 4 4 4 3 5% 7% 44%	7264
June	1st to 10th	Day	13% 6% 3 7% 4 4 4 4 6% 8% 43%	3000
	11th to 20th	Day	12% 6% 2 7% 4 5% 5% 4 6% 9% 40%	3000
	21st to end	Day	13% 7% 2 6% 5% 4 4 4 6% 8% 40%	3000
July	1st to 10th	Day	15% 6% 2 6% 3 3 4 3 5% 8% 45%	6681
	11th to 20th	Day	10% 4 2 5% 4 4 5% 4 6% 8% 49%	6360
	21st to end	Day	8% 5% 2 6% 4 4 4 4 5% 9% 50%	6926
August	1st to 10th	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 10% 42%	4284
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 6% 7% 45%	4174
	21st to end	Day	12% 6% 3 7% 5% 5% 4 3 5% 7% 44%	4651
September	1st to 10th	Day	13% 7% 3 6% 5% 4 5% 4 6% 8% 40%	2322
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 6% 7% 45%	2200
	21st to end	Day	17% 8% 3 6% 5% 4 3 4 6% 7% 36%	2200
October	1st to 10th	Day	18% 10% 2 6% 5% 4 4 4 6% 7% 35%	2150
	11th to 20th	Day	23% 12% 4 10% 5% 5% 5% 4 5% 7% 21%	1945
	21st to end	Day	21% 16% 5% 9% 6% 5% 5% 4 5% 6% 19%	1980
November	1st to 10th	Day	16% 16% 5% 9% 5% 4 4 3 5% 8% 24%	1852
	11th to 20th	Day	14% 18% 6% 12% 6% 6% 4 5% 6% 7% 16%	1820
	21st to end	Day	18% 22% 7% 12% 6% 6% 4 2 5% 4 14%	1680
December	1st to 10th	Day	12% 24% 9% 17% 7% 6% 5% 4 5% 3 8%	2356
	11th to 20th	Day	14% 19% 8% 16% 9% 6% 5% 3 4 4 12%	2691
	21st to end	Day	22% 23% 9% 14% 5% 6% 4 2 3 4 7%	3553
January	1st to 10th	Night	11% 14% 6% 26% 10% 6% 4 3 4 4 10%	6517
	11th to 20th	Night	12% 15% 7% 20% 10% 7% 5% 3 5% 4 10%	6401
	21st to end	Night	10% 18% 7% 16% 8% 6% 6% 4 5% 5% 16%	6996
February	1st to 10th	Night	9% 16% 5% 13% 9% 7% 5% 5% 6% 5% 20%	2629
	11th to 20th	Night	11% 15% 5% 13% 7% 5% 5% 4 4 6% 23%	2424
	21st to end	Night	14% 15% 5% 12% 6% 5% 4 4 4 6% 24%	1980
March	1st to 10th	Night	15% 10% 4 8% 6% 4 3 3 5% 7% 35%	5362
	11th to 20th	Night	11% 13% 4 9% 5% 4 5% 4 5% 7% 33%	4984
	21st to end	Night	20% 12% 6% 9% 5% 4 4 3 4 5% 29%	5467
April	1st to 10th	Night	19% 13% 3 8% 4 4 4 5% 5% 6% 30%	2009
	11th to 20th	Night	19% 10% 4 7% 4 3 3 3 6% 6% 35%	1802
	21st to end	Night	17% 10% 3 8% 5% 5% 5% 3 5% 5% 34%	1800
May	1st to 10th	Night	15% 9% 3 7% 5% 5% 4 4 5% 5% 39%	4053
	11th to 20th	Night	14% 9% 2 7% 4 4 5% 4 5% 7% 41%	3952
	21st to end	Night	19% 9% 3 5% 3 3 3 3 5% 6% 41%	3630
June	1st to 10th	Night	13% 7% 3 6% 4 4 5% 3 6% 8% 42%	1400
	11th to 20th	Night	10% 5% 3 6% 5% 5% 6% 5% 7% 8% 40%	1400
	21st to end	Night	15% 8% 2 6% 4 4 3 4 4 7% 44%	1400
July	1st to 10th	Night	12% 6% 3 6% 4 4 4 3 6% 8% 45%	3293
	11th to 20th	Night	10% 6% 2 5% 3 3 4 4 5% 6% 52%	3620
	21st to end	Night	8% 4 2 5% 3 4 4 4 5% 8% 53%	3948
August	1st to 10th	Night	10% 5% 2 5% 5% 4 4 4 6% 8% 46%	2852
	11th to 20th	Night	15% 5% 1 5% 3 3 3 3 4 8% 49%	2875
	21st to end	Night	11% 5% 1 6% 4 4 4 3 5% 7% 52%	3245
September	1st to 10th	Night	15% 5% 2 5% 4 3 3 3 4 8% 47%	2057
	11th to 20th	Night	14% 5% 2 6% 3 4 4 3 5% 7% 46%	2200
	21st to end	Night	17% 7% 2 7% 4 4 4 4 5% 8% 39%	2200
October	1st to 10th	Night	17% 8% 3 6% 4 4 4 3 5% 6% 40%	2250
	11th to 20th	Night	22% 11% 4 8% 4 5% 4 4 5% 7% 26%	2450
	21st to end	Night	24% 14% 3 7% 5% 4 4 3 4 6% 24%	2860
November	1st to 10th	Night	15% 15% 4 9% 5% 4 5% 4 6% 7% 28%	2666
	11th to 20th	Night	14% 16% 5% 11% 7% 6% 5% 4 5% 5% 21%	2798
	21st to end	Night	19% 21% 6% 12% 5% 5% 4 3 5% 5% 16%	2940
December	1st to 10th	Night	12% 20% 8% 15% 7% 6% 6% 4 5% 6% 12%	5050
	11th to 20th	Night	13% 20% 7% 17% 7% 5% 4 3 4 5% 14%	5736
	21st to end	Night	23% 21% 11% 14% 7% 5% 4 2 3 3 7%	7667

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	77% 9% 6% 6% 1 1	3651
	11th to 20th	Day	84% 7% 5% 4 1	3668
	21st to end	Day	71% 9% 11% 6% 3	3982
February	1st to 10th	Day	66% 7% 11% 13% 4	1770
	11th to 20th	Day	62% 7% 12% 12% 7%	1866
	21st to end	Day	59% 6% 11% 18% 6%	1650
March	1st to 10th	Day	48% 5% 17% 16% 11% 2	4480
	11th to 20th	Day	54% 5% 15% 18% 7% 1	4975
	21st to end	Day	57% 5% 13% 16% 8% 1	5500
April	1st to 10th	Day	53% 5% 12% 18% 11% 1	2400
	11th to 20th	Day	49% 4 10% 20% 15% 2	2547
	21st to end	Day	46% 5% 13% 16% 16% 3	2600
May	1st to 10th	Day	40% 6% 12% 21% 18% 3	5897
	11th to 20th	Day	41% 8% 9% 24% 17% 2	5947
	21st to end	Day	39% 5% 12% 22% 17% 4	7264
June	1st to 10th	Day	38% 6% 12% 28% 14% 2	3000
	11th to 20th	Day	39% 7% 14% 24% 16% 2	3000
	21st to end	Day	39% 7% 13% 25% 13% 3	3000
July	1st to 10th	Day	37% 5% 15% 29% 14% 1	6611
	11th to 20th	Day	28% 6% 15% 39% 10% 1	6285
	21st to end	Day	30% 5% 13% 35% 16% 1	6728
August	1st to 10th	Day	33% 7% 16% 31% 12% 1	4269
	11th to 20th	Day	36% 6% 15% 31% 11% 1	4213
	21st to end	Day	37% 6% 17% 32% 8% 1	4650
September	1st to 10th	Day	39% 6% 14% 27% 12% 1	2322
	11th to 20th	Day	34% 7% 17% 30% 11% 1	2200
	21st to end	Day	46% 6% 16% 22% 10%	2200
October	1st to 10th	Day	35% 5% 12% 31% 16%	3501
	11th to 20th	Day	54% 6% 10% 20% 10%	3148
	21st to end	Day	60% 5% 12% 18% 5%	3180
November	1st to 10th	Day	60% 5% 10% 20% 5%	1852
	11th to 20th	Day	68% 7% 12% 9% 4	1820
	21st to end	Day	74% 7% 10% 8% 2	1680
December	1st to 10th	Day	78% 6% 10% 5% 1	2356
	11th to 20th	Day	77% 8% 9% 5% 1	2691
	21st to end	Day	84% 3 5% 5% 3	3553
January	1st to 10th	Night	75% 6% 10% 6% 2	6517
	11th to 20th	Night	80% 8% 7% 4 1	6445
	21st to end	Night	70% 8% 10% 9% 3	6996
February	1st to 10th	Night	64% 7% 11% 12% 6%	2629
	11th to 20th	Night	63% 7% 10% 14% 5%	2426
	21st to end	Night	61% 6% 11% 15% 6%	1980
March	1st to 10th	Night	50% 4 16% 18% 11% 1	5373
	11th to 20th	Night	50% 5% 19% 18% 7% 1	4985
	21st to end	Night	56% 6% 11% 17% 9% 1	5467
April	1st to 10th	Night	55% 5% 9% 18% 11% 1	2009
	11th to 20th	Night	49% 4 8% 24% 13% 3	1802
	21st to end	Night	51% 5% 9% 17% 14% 4	1800
May	1st to 10th	Night	45% 5% 10% 20% 16% 3	4060
	11th to 20th	Night	43% 7% 7% 25% 16% 1	3953
	21st to end	Night	44% 5% 11% 21% 12% 6%	3631
June	1st to 10th	Night	39% 4 11% 27% 17% 2	1400
	11th to 20th	Night	39% 6% 11% 26% 15% 3	1400
	21st to end	Night	41% 5% 12% 26% 13% 3	1400
July	1st to 10th	Night	38% 5% 14% 25% 17% 2	3256
	11th to 20th	Night	31% 5% 15% 32% 17%	3572
	21st to end	Night	30% 5% 14% 32% 19%	3832
August	1st to 10th	Night	34% 6% 15% 30% 14% 1	2843
	11th to 20th	Night	34% 6% 16% 31% 12% 1	2902
	21st to end	Night	33% 6% 14% 32% 14% 1	3245
September	1st to 10th	Night	38% 5% 13% 28% 14% 1	2057
	11th to 20th	Night	38% 6% 14% 30% 11% 1	2200
	21st to end	Night	45% 6% 15% 24% 10%	2200
October	1st to 10th	Night	39% 5% 11% 30% 14%	3679
	11th to 20th	Night	47% 5% 16% 19% 12%	3986
	21st to end	Night	56% 6% 13% 19% 5%	4610
November	1st to 10th	Night	58% 6% 12% 18% 6%	2666
	11th to 20th	Night	66% 7% 11% 10% 5% 1	2798
	21st to end	Night	75% 6% 9% 8% 2	2940
December	1st to 10th	Night	77% 8% 8% 7% 1	5050
	11th to 20th	Night	75% 6% 10% 7% 3	5735
	21st to end	Night	85% 5% 5% 4 1	7667

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	18% 12% 29% 30% 12% 1	319
	11th to 20th	Day	19% 9% 32% 30% 8% 2	318
	21st to end	Day	22% 8% 25% 31% 13% 2	330
February	1st to 10th	Day	19% 9% 34% 28% 9% 1	300
	11th to 20th	Day	15% 9% 40% 25% 10% 1	300
	21st to end	Day	15% 13% 31% 35% 6%	249
March	1st to 10th	Day	18% 6% 27% 36% 13%	300
	11th to 20th	Day	16% 9% 29% 32% 11% 2	495
	21st to end	Day	26% 4 24% 30% 14% 1	550
April	1st to 10th	Day	21% 6% 25% 36% 10% 2	500
	11th to 20th	Day	24% 6% 24% 29% 14% 3	492
	21st to end	Day	20% 4 25% 29% 18% 4	500
May	1st to 10th	Day	21% 2 21% 34% 18% 4	503
	11th to 20th	Day	16% 2 15% 39% 22% 6%	557
	21st to end	Day	15% 2 13% 36% 26% 7%	789
June	1st to 10th	Day	12% 2 16% 37% 28% 5%	730
	11th to 20th	Day	14% 3 13% 40% 25% 4	730
	21st to end	Day	17% 4 9% 38% 27% 4	730
July	1st to 10th	Day	11% 2 7% 38% 36% 7%	721
	11th to 20th	Day	11% 2 12% 33% 33% 9%	718
	21st to end	Day	10% 2 9% 35% 34% 9%	794
August	1st to 10th	Day	11% 1 10% 32% 36% 11%	564
	11th to 20th	Day	12% 2 7% 29% 35% 15%	519
	21st to end	Day	14% 4 11% 33% 31% 8%	583
September	1st to 10th	Day	14% 4 15% 29% 25% 13%	530
	11th to 20th	Day	14% 1 11% 26% 32% 16%	530
	21st to end	Day	21% 4 12% 27% 28% 8%	530
October	1st to 10th	Day	23% 4 20% 25% 21% 6%	530
	11th to 20th	Day	24% 5% 23% 33% 13% 2	475
	21st to end	Day	28% 7% 22% 29% 13% 2	363
November	1st to 10th	Day	21% 11% 23% 30% 13% 3	330
	11th to 20th	Day	21% 14% 31% 26% 8% 1	292
	21st to end	Day	28% 14% 26% 24% 8%	220
December	1st to 10th	Day	23% 10% 30% 25% 10%	220
	11th to 20th	Day	24% 20% 31% 18% 6%	220
	21st to end	Day	31% 10% 32% 21% 5% 2	242
January	1st to 10th	Night	22% 12% 32% 27% 7% 1	1050
	11th to 20th	Night	25% 10% 31% 25% 8%	1031
	21st to end	Night	19% 9% 31% 31% 8% 1	1100
February	1st to 10th	Night	16% 9% 38% 27% 8% 1	829
	11th to 20th	Night	20% 10% 33% 27% 10% 1	783
	21st to end	Night	22% 6% 31% 33% 7%	660
March	1st to 10th	Night	19% 8% 29% 32% 11% 1	800
	11th to 20th	Night	18% 8% 31% 32% 10% 2	800
	21st to end	Night	21% 5% 26% 34% 12% 1	880
April	1st to 10th	Night	25% 5% 25% 35% 9% 1	609
	11th to 20th	Night	23% 5% 21% 37% 12% 2	593
	21st to end	Night	21% 4 23% 34% 16% 2	600
May	1st to 10th	Night	17% 4 18% 39% 21% 2	600
	11th to 20th	Night	20% 3 15% 36% 24% 2	591
	21st to end	Night	15% 2 14% 38% 27% 4	513
June	1st to 10th	Night	17% 1 17% 36% 27% 3	400
	11th to 20th	Night	13% 2 12% 43% 27% 4	400
	21st to end	Night	16% 3 12% 38% 26% 5%	400
July	1st to 10th	Night	10% 2 10% 40% 34% 5%	443
	11th to 20th	Night	9% 2 10% 37% 33% 8%	582
	21st to end	Night	10% 2 10% 39% 33% 5%	657
August	1st to 10th	Night	11% 2 11% 39% 32% 5%	597
	11th to 20th	Night	12% 2 12% 31% 34% 9%	594
	21st to end	Night	16% 3 11% 35% 32% 3	660
September	1st to 10th	Night	18% 4 16% 30% 25% 8%	657
	11th to 20th	Night	17% 2 14% 30% 28% 10%	800
	21st to end	Night	21% 4 14% 30% 25% 7%	800
October	1st to 10th	Night	23% 6% 19% 31% 19% 3	800
	11th to 20th	Night	29% 9% 24% 29% 10% 1	800
	21st to end	Night	34% 9% 21% 26% 9% 1	880
November	1st to 10th	Night	23% 10% 26% 29% 10% 2	820
	11th to 20th	Night	26% 11% 31% 24% 8%	840
	21st to end	Night	33% 12% 29% 22% 4	840
December	1st to 10th	Night	22% 14% 34% 22% 7% 1	1040
	11th to 20th	Night	25% 15% 34% 18% 6% 1	1050
	21st to end	Night	32% 11% 31% 19% 6%	1155

Page generated at 2026-01-14 01:14:29. Version ac07

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