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 2024-11-25.

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 4 5% 7% 33%	101531
	Night	15% 12% 4 10% 6% 5% 4 4 5% 6% 30%	98041
Transparency	Day	48% 6% 13% 22% 11% 1	105038
	Night	55% 6% 12% 18% 9% 1	102623
Seeing	Day	17% 5% 19% 32% 22% 6%	15606
	Night	21% 7% 24% 30% 15% 3	25078

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	14% 19% 10% 19% 8% 6% 5% 3 4 5% 8%	2495
	11th to 20th	Day	13% 18% 6% 18% 15% 8% 4 3 4 3 7%	2530
	21st to end	Day	15% 22% 7% 15% 7% 6% 4 3 5% 5% 11%	2783
February	1st to 10th	Day	9% 17% 6% 14% 10% 7% 5% 4 4 6% 16%	1681
	11th to 20th	Day	11% 16% 7% 14% 9% 5% 4 3 4 6% 22%	1761
	21st to end	Day	11% 17% 6% 12% 8% 5% 5% 4 4 5% 23%	1570
March	1st to 10th	Day	12% 14% 3 7% 5% 4 4 3 5% 6% 36%	3118
	11th to 20th	Day	8% 14% 6% 12% 6% 6% 4 4 5% 6% 29%	3475
	21st to end	Day	14% 13% 7% 13% 6% 6% 5% 4 4 4 24%	3836
April	1st to 10th	Day	18% 14% 3 8% 5% 4 4 3 5% 6% 29%	2280
	11th to 20th	Day	18% 12% 4 7% 4 3 5% 3 5% 5% 33%	2418
	21st to end	Day	17% 10% 3 8% 4 4 4 4 5% 7% 33%	2470
May	1st to 10th	Day	14% 8% 3 6% 4 4 5% 4 6% 7% 39%	4108
	11th to 20th	Day	12% 7% 3 7% 4 4 4 4 5% 8% 43%	4135
	21st to end	Day	13% 6% 2 5% 4 4 4 4 5% 8% 46%	5054
June	1st to 10th	Day	13% 6% 2 7% 4 4 4 4 6% 8% 43%	2850
	11th to 20th	Day	12% 6% 2 7% 4 5% 5% 4 6% 9% 41%	2850
	21st to end	Day	13% 6% 2 6% 5% 4 4 4 6% 8% 41%	2850
July	1st to 10th	Day	17% 7% 2 7% 4 3 4 3 5% 7% 40%	4664
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 4 6% 8% 48%	4440
	21st to end	Day	9% 6% 2 6% 5% 4 4 4 5% 8% 48%	4814
August	1st to 10th	Day	9% 6% 2 6% 5% 5% 5% 4 7% 10% 42%	4152
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 6% 7% 45%	4044
	21st to end	Day	12% 6% 3 7% 5% 5% 4 3 5% 7% 44%	4509
September	1st to 10th	Day	14% 7% 3 6% 4 3 5% 4 6% 8% 41%	2206
	11th to 20th	Day	13% 6% 2 5% 4 4 4 4 6% 7% 44%	2090
	21st to end	Day	17% 8% 3 6% 5% 4 3 4 6% 7% 36%	2090
October	1st to 10th	Day	18% 10% 2 7% 5% 4 4 4 5% 7% 34%	2043
	11th to 20th	Day	23% 12% 4 10% 5% 5% 4 4 5% 7% 21%	1848
	21st to end	Day	22% 16% 5% 9% 6% 5% 5% 4 5% 6% 18%	1881
November	1st to 10th	Day	17% 16% 5% 9% 5% 4 4 3 5% 8% 24%	1762
	11th to 20th	Day	14% 19% 6% 12% 6% 6% 4 5% 6% 7% 16%	1734
	21st to end	Day	18% 22% 6% 11% 6% 5% 4 3 5% 4 15%	1552
December	1st to 10th	Day	13% 20% 9% 20% 7% 6% 5% 4 5% 3 10%	1336
	11th to 20th	Day	16% 17% 6% 15% 9% 7% 5% 4 4 5% 12%	1671
	21st to end	Day	21% 25% 9% 17% 6% 4 4 3 2 4 6%	2431
January	1st to 10th	Night	14% 17% 7% 20% 8% 5% 5% 3 4 5% 12%	4456
	11th to 20th	Night	13% 15% 8% 20% 11% 7% 5% 3 5% 4 10%	4440
	21st to end	Night	13% 22% 8% 15% 8% 5% 5% 4 4 4 12%	4884
February	1st to 10th	Night	10% 16% 5% 13% 9% 7% 5% 5% 6% 5% 20%	2498
	11th to 20th	Night	11% 15% 5% 13% 7% 5% 5% 4 4 6% 24%	2300
	21st to end	Night	15% 15% 5% 11% 6% 5% 5% 4 4 6% 24%	1884
March	1st to 10th	Night	15% 10% 4 6% 4 4 3 3 4 7% 41%	3722
	11th to 20th	Night	8% 15% 4 9% 5% 4 4 4 4 6% 37%	3484
	21st to end	Night	14% 11% 6% 10% 5% 5% 5% 4 5% 7% 30%	3817
April	1st to 10th	Night	19% 13% 3 7% 4 4 4 5% 5% 6% 29%	1909
	11th to 20th	Night	19% 9% 4 7% 4 3 3 3 6% 6% 35%	1711
	21st to end	Night	17% 10% 3 8% 5% 5% 5% 3 5% 6% 35%	1710
May	1st to 10th	Night	15% 8% 3 7% 5% 5% 4 4 5% 5% 38%	2833
	11th to 20th	Night	13% 7% 2 6% 5% 4 4 4 5% 8% 41%	2758
	21st to end	Night	16% 7% 3 5% 4 3 3 3 6% 7% 43%	2529
June	1st to 10th	Night	14% 7% 3 6% 4 4 5% 3 6% 8% 41%	1330
	11th to 20th	Night	10% 5% 2 6% 5% 5% 6% 5% 7% 8% 42%	1330
	21st to end	Night	14% 8% 2 6% 4 4 3 4 4 7% 45%	1330
July	1st to 10th	Night	13% 5% 3 6% 5% 4 4 3 6% 8% 44%	2301
	11th to 20th	Night	10% 6% 2 6% 4 3 4 4 5% 6% 51%	2530
	21st to end	Night	9% 5% 2 5% 4 4 4 4 5% 7% 50%	2750
August	1st to 10th	Night	10% 5% 2 5% 5% 4 4 4 6% 8% 46%	2764
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 4 8% 49%	2785
	21st to end	Night	11% 5% 1 6% 4 4 4 3 5% 7% 51%	3145
September	1st to 10th	Night	15% 6% 2 5% 4 3 3 3 4 7% 47%	1954
	11th to 20th	Night	15% 5% 2 6% 3 4 4 3 5% 7% 46%	2090
	21st to end	Night	18% 7% 2 7% 4 4 4 3 5% 8% 38%	2090
October	1st to 10th	Night	18% 8% 3 6% 5% 4 4 3 5% 6% 39%	2137
	11th to 20th	Night	22% 11% 4 7% 4 5% 4 4 5% 7% 26%	2327
	21st to end	Night	25% 15% 3 8% 5% 4 4 3 4 6% 23%	2717
November	1st to 10th	Night	15% 15% 4 8% 5% 4 5% 4 6% 7% 28%	2536
	11th to 20th	Night	15% 17% 5% 11% 6% 6% 5% 4 6% 5% 20%	2665
	21st to end	Night	18% 22% 6% 12% 5% 5% 5% 3 5% 4 15%	2723
December	1st to 10th	Night	13% 19% 8% 15% 6% 6% 5% 4 5% 6% 13%	2840
	11th to 20th	Night	15% 18% 6% 17% 8% 5% 5% 4 5% 5% 13%	3526
	21st to end	Night	22% 22% 9% 14% 6% 5% 4 3 3 3 8%	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	76% 11% 7% 6%	2496
	11th to 20th	Day	84% 8% 4 3 1	2530
	21st to end	Day	73% 10% 10% 5% 2	2783
February	1st to 10th	Day	66% 7% 11% 13% 4	1681
	11th to 20th	Day	61% 7% 12% 13% 7%	1770
	21st to end	Day	59% 6% 11% 18% 6%	1570
March	1st to 10th	Day	47% 5% 16% 21% 10% 1	3120
	11th to 20th	Day	52% 4 15% 21% 6% 1	3475
	21st to end	Day	55% 6% 15% 14% 9% 1	3836
April	1st to 10th	Day	53% 5% 12% 18% 11% 1	2280
	11th to 20th	Day	50% 4 11% 19% 14% 2	2418
	21st to end	Day	47% 5% 13% 16% 16% 4	2470
May	1st to 10th	Day	40% 7% 14% 21% 16% 2	4120
	11th to 20th	Day	37% 9% 11% 27% 14% 2	4139
	21st to end	Day	35% 6% 13% 22% 21% 3	5054
June	1st to 10th	Day	38% 6% 12% 28% 14% 2	2850
	11th to 20th	Day	38% 7% 14% 24% 16% 2	2850
	21st to end	Day	38% 6% 14% 25% 14% 3	2850
July	1st to 10th	Day	41% 5% 15% 23% 14% 1	4593
	11th to 20th	Day	28% 6% 15% 39% 12% 1	4365
	21st to end	Day	31% 5% 12% 33% 18% 1	4662
August	1st to 10th	Day	33% 7% 15% 31% 12% 1	4137
	11th to 20th	Day	36% 6% 16% 31% 11% 1	4083
	21st to end	Day	37% 6% 17% 32% 8% 1	4508
September	1st to 10th	Day	39% 6% 14% 27% 13% 1	2206
	11th to 20th	Day	35% 7% 16% 31% 11% 1	2090
	21st to end	Day	46% 6% 16% 23% 9%	2090
October	1st to 10th	Day	35% 5% 12% 31% 16%	3394
	11th to 20th	Day	54% 6% 10% 20% 10%	3051
	21st to end	Day	61% 5% 12% 17% 5%	3081
November	1st to 10th	Day	60% 5% 10% 20% 5%	1762
	11th to 20th	Day	69% 6% 12% 9% 4	1734
	21st to end	Day	74% 7% 10% 8% 2	1552
December	1st to 10th	Day	81% 4 8% 5% 2	1336
	11th to 20th	Day	77% 7% 8% 7% 1	1671
	21st to end	Day	86% 4 6% 3 2	2431
January	1st to 10th	Night	75% 7% 10% 7% 2	4456
	11th to 20th	Night	79% 9% 7% 5% 1	4440
	21st to end	Night	75% 9% 8% 6% 2	4884
February	1st to 10th	Night	65% 7% 10% 12% 5%	2498
	11th to 20th	Night	62% 7% 11% 15% 5%	2302
	21st to end	Night	61% 6% 11% 15% 7%	1884
March	1st to 10th	Night	45% 4 17% 22% 11%	3733
	11th to 20th	Night	47% 6% 15% 23% 8% 2	3485
	21st to end	Night	51% 7% 14% 19% 8% 1	3817
April	1st to 10th	Night	55% 5% 10% 18% 11% 1	1909
	11th to 20th	Night	48% 4 8% 24% 13% 3	1711
	21st to end	Night	50% 5% 9% 18% 14% 4	1710
May	1st to 10th	Night	45% 6% 11% 20% 15% 2	2840
	11th to 20th	Night	40% 8% 9% 27% 15% 1	2759
	21st to end	Night	40% 6% 13% 19% 15% 6%	2530
June	1st to 10th	Night	40% 4 11% 26% 17% 2	1330
	11th to 20th	Night	38% 6% 11% 26% 15% 3	1330
	21st to end	Night	40% 5% 12% 26% 13% 3	1330
July	1st to 10th	Night	39% 5% 14% 24% 16% 2	2264
	11th to 20th	Night	31% 5% 15% 31% 17% 1	2482
	21st to end	Night	33% 6% 14% 25% 22%	2662
August	1st to 10th	Night	35% 6% 15% 29% 14% 1	2755
	11th to 20th	Night	34% 6% 16% 31% 12% 1	2812
	21st to end	Night	33% 6% 13% 32% 15% 1	3145
September	1st to 10th	Night	39% 5% 12% 28% 15% 1	1954
	11th to 20th	Night	39% 6% 14% 30% 11% 1	2090
	21st to end	Night	45% 6% 15% 24% 9%	2090
October	1st to 10th	Night	40% 5% 11% 29% 14%	3566
	11th to 20th	Night	46% 5% 17% 20% 12%	3863
	21st to end	Night	57% 6% 13% 18% 5%	4467
November	1st to 10th	Night	58% 6% 12% 18% 6%	2536
	11th to 20th	Night	67% 7% 10% 10% 5%	2665
	21st to end	Night	75% 6% 9% 8% 2	2723
December	1st to 10th	Night	77% 7% 8% 7% 1	2840
	11th to 20th	Night	76% 6% 10% 6% 2	3525
	21st to end	Night	85% 5% 5% 4 2	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	18% 9% 29% 29% 14% 1	261
	11th to 20th	Day	20% 8% 31% 30% 9% 3	270
	21st to end	Day	24% 6% 24% 31% 13% 2	297
February	1st to 10th	Day	19% 9% 34% 28% 10% 1	270
	11th to 20th	Day	14% 9% 41% 25% 10% 1	270
	21st to end	Day	13% 14% 32% 36% 5%	225
March	1st to 10th	Day	18% 6% 27% 36% 13%	270
	11th to 20th	Day	16% 8% 30% 31% 12% 2	455
	21st to end	Day	25% 4 25% 31% 14% 2	506
April	1st to 10th	Day	21% 5% 25% 37% 10% 2	460
	11th to 20th	Day	24% 6% 24% 28% 14% 3	452
	21st to end	Day	20% 4 25% 29% 18% 4	460
May	1st to 10th	Day	21% 2 22% 33% 18% 4	463
	11th to 20th	Day	16% 2 15% 38% 23% 6%	515
	21st to end	Day	14% 1 14% 37% 26% 8%	734
June	1st to 10th	Day	12% 2 16% 37% 28% 5%	680
	11th to 20th	Day	14% 3 13% 40% 26% 4	680
	21st to end	Day	17% 3 9% 38% 28% 5%	680
July	1st to 10th	Day	10% 2 7% 37% 37% 7%	671
	11th to 20th	Day	11% 2 12% 33% 34% 9%	668
	21st to end	Day	11% 2 10% 34% 34% 9%	739
August	1st to 10th	Day	11% 1 10% 33% 34% 10%	522
	11th to 20th	Day	11% 2 7% 29% 36% 15%	479
	21st to end	Day	14% 4 11% 33% 30% 8%	539
September	1st to 10th	Day	14% 3 15% 30% 26% 12%	490
	11th to 20th	Day	14% 1 12% 27% 32% 14%	490
	21st to end	Day	21% 3 13% 27% 27% 8%	490
October	1st to 10th	Day	24% 4 21% 24% 20% 6%	490
	11th to 20th	Day	24% 5% 24% 31% 13% 2	438
	21st to end	Day	30% 7% 22% 28% 12% 1	330
November	1st to 10th	Day	21% 10% 22% 30% 13% 3	300
	11th to 20th	Day	21% 13% 31% 27% 8% 1	266
	21st to end	Day	29% 14% 24% 24% 9%	188
December	1st to 10th	Day	22% 8% 31% 29% 9% 1	180
	11th to 20th	Day	24% 18% 30% 20% 7% 1	180
	21st to end	Day	30% 11% 32% 21% 4 3	198
January	1st to 10th	Night	23% 11% 31% 26% 7% 1	950
	11th to 20th	Night	25% 10% 30% 25% 9% 1	950
	21st to end	Night	19% 8% 31% 32% 9% 2	1045
February	1st to 10th	Night	17% 9% 37% 28% 9% 1	788
	11th to 20th	Night	20% 9% 33% 27% 10% 1	743
	21st to end	Night	23% 6% 32% 32% 7%	628
March	1st to 10th	Night	18% 7% 29% 33% 11% 1	760
	11th to 20th	Night	17% 8% 31% 32% 10% 2	760
	21st to end	Night	20% 5% 26% 35% 13% 1	836
April	1st to 10th	Night	25% 5% 24% 35% 9% 1	579
	11th to 20th	Night	23% 4 21% 38% 12% 2	563
	21st to end	Night	20% 4 23% 34% 17% 2	570
May	1st to 10th	Night	17% 4 18% 39% 21% 2	570
	11th to 20th	Night	20% 3 14% 37% 23% 2	561
	21st to end	Night	15% 2 14% 37% 28% 5%	487
June	1st to 10th	Night	17% 1 17% 36% 27% 3	380
	11th to 20th	Night	12% 2 12% 43% 27% 4	380
	21st to end	Night	16% 3 12% 38% 27% 5%	380
July	1st to 10th	Night	11% 1 10% 40% 33% 5%	420
	11th to 20th	Night	9% 2 11% 37% 33% 9%	552
	21st to end	Night	11% 3 10% 38% 33% 6%	624
August	1st to 10th	Night	12% 2 11% 40% 31% 4	567
	11th to 20th	Night	13% 2 12% 30% 34% 9%	564
	21st to end	Night	16% 3 11% 35% 32% 3	627
September	1st to 10th	Night	18% 4 16% 30% 25% 8%	624
	11th to 20th	Night	17% 2 14% 31% 27% 8%	760
	21st to end	Night	22% 4 14% 29% 24% 7%	760
October	1st to 10th	Night	24% 6% 19% 31% 17% 3	760
	11th to 20th	Night	28% 8% 24% 28% 10% 1	760
	21st to end	Night	35% 10% 21% 26% 7% 1	836
November	1st to 10th	Night	23% 9% 27% 29% 10% 2	780
	11th to 20th	Night	27% 11% 30% 24% 8%	800
	21st to end	Night	33% 11% 28% 23% 4	779
December	1st to 10th	Night	23% 13% 34% 23% 6% 1	940
	11th to 20th	Night	26% 15% 34% 19% 6% 1	950
	21st to end	Night	33% 10% 32% 20% 5%	1045

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