Hamilton Centre Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hamilton Centre Observatory 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% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	101604
	Night	14% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	97371
Transparency	Day	47% 6% 14% 21% 11% 1	105210
	Night	46% 6% 13% 21% 12% 1	101853
Seeing	Day	17% 12% 26% 26% 14% 5%	14630
	Night	17% 10% 28% 29% 13% 2	23887

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% 16% 6% 10% 7% 4 3 3 4 6% 26%	2517
	11th to 20th	Day	11% 18% 8% 16% 7% 6% 4 3 4 5% 18%	2840
	21st to end	Day	21% 19% 6% 12% 7% 5% 4 3 3 4 16%	3124
February	1st to 10th	Day	14% 16% 5% 11% 8% 6% 6% 4 5% 5% 20%	1710
	11th to 20th	Day	12% 14% 6% 11% 8% 6% 4 4 5% 7% 24%	1663
	21st to end	Day	18% 13% 4 10% 6% 4 6% 3 5% 6% 25%	1484
March	1st to 10th	Day	18% 10% 5% 9% 7% 5% 5% 4 4 5% 28%	3432
	11th to 20th	Day	11% 13% 5% 10% 6% 5% 5% 4 6% 7% 29%	3480
	21st to end	Day	15% 10% 4 8% 5% 4 6% 4 6% 5% 33%	3815
April	1st to 10th	Day	16% 11% 4 9% 6% 6% 5% 4 5% 6% 30%	2221
	11th to 20th	Day	17% 9% 3 8% 5% 4 4 4 4 6% 36%	2347
	21st to end	Day	18% 9% 3 8% 5% 4 4 4 5% 6% 33%	2470
May	1st to 10th	Day	17% 8% 3 7% 5% 5% 4 3 5% 8% 34%	4108
	11th to 20th	Day	15% 10% 3 8% 5% 5% 5% 4 6% 7% 31%	4292
	21st to end	Day	10% 6% 2 6% 5% 4 4 3 6% 7% 47%	4837
June	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 5% 5% 8% 40%	2660
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 4 8% 9% 40%	2660
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 4 6% 9% 40%	2807
July	1st to 10th	Day	9% 4 2 6% 4 4 5% 5% 9% 11% 41%	4486
	11th to 20th	Day	9% 5% 2 5% 5% 5% 5% 5% 8% 9% 43%	4381
	21st to end	Day	6% 5% 2 6% 5% 6% 6% 5% 7% 10% 43%	4466
August	1st to 10th	Day	16% 7% 2 6% 4 4 5% 5% 6% 8% 37%	4104
	11th to 20th	Day	9% 7% 3 8% 5% 5% 6% 5% 7% 9% 36%	4044
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	4382
September	1st to 10th	Day	12% 6% 2 7% 4 5% 6% 3 6% 9% 41%	2280
	11th to 20th	Day	9% 6% 2 5% 5% 4 5% 4 5% 8% 46%	2134
	21st to end	Day	14% 6% 2 8% 6% 5% 5% 5% 7% 8% 34%	2071
October	1st to 10th	Day	17% 9% 3 6% 4 5% 5% 3 6% 7% 35%	1900
	11th to 20th	Day	19% 8% 4 9% 6% 4 4 4 5% 7% 31%	1900
	21st to end	Day	22% 11% 4 8% 6% 6% 5% 4 6% 5% 25%	2090
November	1st to 10th	Day	13% 7% 3 8% 6% 5% 6% 4 6% 8% 32%	1661
	11th to 20th	Day	13% 12% 5% 9% 6% 5% 6% 5% 7% 8% 25%	1520
	21st to end	Day	17% 14% 4 8% 6% 4 4 6% 6% 6% 23%	1520
December	1st to 10th	Day	13% 16% 6% 13% 7% 5% 5% 4 6% 6% 19%	1520
	11th to 20th	Day	17% 14% 4 11% 6% 6% 6% 5% 6% 6% 21%	1906
	21st to end	Day	27% 18% 7% 13% 6% 5% 4 3 4 4 10%	2772
January	1st to 10th	Night	15% 14% 5% 12% 7% 5% 5% 4 5% 7% 22%	4425
	11th to 20th	Night	14% 12% 5% 12% 8% 7% 6% 5% 6% 7% 17%	4120
	21st to end	Night	21% 14% 6% 10% 6% 5% 5% 3 4 6% 21%	4532
February	1st to 10th	Night	14% 13% 5% 10% 7% 5% 5% 4 5% 7% 25%	2470
	11th to 20th	Night	12% 12% 5% 11% 7% 5% 5% 4 6% 6% 27%	2403
	21st to end	Night	14% 12% 4 11% 6% 6% 4 4 5% 7% 27%	1967
March	1st to 10th	Night	19% 8% 3 8% 5% 5% 5% 4 5% 6% 32%	3408
	11th to 20th	Night	12% 9% 4 8% 4 4 4 3 6% 7% 39%	3479
	21st to end	Night	16% 9% 3 7% 4 4 4 3 4 5% 41%	3817
April	1st to 10th	Night	17% 9% 3 8% 5% 5% 5% 4 5% 5% 34%	1957
	11th to 20th	Night	15% 8% 3 8% 6% 4 5% 2 4 7% 38%	1797
	21st to end	Night	16% 8% 3 6% 5% 4 4 4 5% 6% 38%	1710
May	1st to 10th	Night	17% 7% 3 7% 4 4 4 4 5% 6% 39%	2833
	11th to 20th	Night	14% 8% 3 8% 5% 5% 5% 4 7% 7% 34%	2598
	21st to end	Night	9% 5% 2 5% 4 4 4 3 6% 8% 50%	2738
June	1st to 10th	Night	11% 5% 2 6% 5% 6% 5% 5% 6% 8% 43%	1520
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 4 8% 8% 43%	1520
	21st to end	Night	10% 5% 2 5% 4 5% 5% 4 5% 7% 47%	1357
July	1st to 10th	Night	6% 4 2 6% 5% 4 4 4 6% 9% 52%	2448
	11th to 20th	Night	10% 4 1 5% 3 4 4 3 6% 9% 51%	2575
	21st to end	Night	6% 4 2 5% 4 4 5% 4 7% 9% 50%	3087
August	1st to 10th	Night	9% 6% 3 6% 4 5% 5% 4 6% 7% 45%	2825
	11th to 20th	Night	10% 6% 2 5% 4 3 4 4 6% 8% 48%	2785
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 6% 8% 48%	3242
September	1st to 10th	Night	11% 4 2 6% 5% 5% 4 3 7% 7% 47%	1900
	11th to 20th	Night	9% 5% 1 5% 3 3 4 3 5% 7% 55%	2042
	21st to end	Night	14% 6% 3 8% 4 5% 4 4 6% 6% 39%	2109
October	1st to 10th	Night	16% 7% 2 6% 4 3 4 4 6% 6% 42%	2280
	11th to 20th	Night	17% 8% 3 7% 5% 5% 4 4 5% 7% 36%	2280
	21st to end	Night	19% 9% 4 7% 5% 5% 5% 4 5% 6% 31%	2508
November	1st to 10th	Night	14% 7% 2 6% 5% 5% 4 4 6% 7% 40%	2487
	11th to 20th	Night	14% 11% 4 7% 5% 5% 6% 4 7% 7% 30%	2660
	21st to end	Night	18% 9% 4 8% 6% 5% 5% 5% 5% 8% 27%	2660
December	1st to 10th	Night	17% 13% 4 9% 5% 7% 5% 5% 6% 6% 23%	2660
	11th to 20th	Night	15% 15% 5% 10% 6% 5% 5% 5% 5% 6% 23%	3288
	21st to end	Night	27% 18% 6% 12% 5% 5% 4 3 4 5% 11%	4884

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	58% 7% 15% 11% 8%	2518
	11th to 20th	Day	68% 5% 12% 11% 4	2840
	21st to end	Day	72% 6% 10% 8% 4	3124
February	1st to 10th	Day	64% 6% 12% 14% 3 1	1710
	11th to 20th	Day	60% 5% 10% 16% 7% 2	1672
	21st to end	Day	59% 5% 13% 15% 7% 1	1484
March	1st to 10th	Day	58% 6% 12% 17% 6% 1	3435
	11th to 20th	Day	52% 5% 13% 20% 8% 1	3480
	21st to end	Day	48% 5% 13% 22% 11% 2	3815
April	1st to 10th	Day	53% 4 12% 18% 11% 1	2221
	11th to 20th	Day	49% 4 12% 21% 13% 1	2347
	21st to end	Day	51% 5% 11% 19% 12% 2	2470
May	1st to 10th	Day	46% 6% 14% 19% 12% 3	4120
	11th to 20th	Day	49% 10% 13% 19% 8% 1	4296
	21st to end	Day	35% 6% 13% 23% 19% 4	4837
June	1st to 10th	Day	41% 7% 12% 22% 16% 2	2660
	11th to 20th	Day	39% 6% 14% 24% 16% 1	2660
	21st to end	Day	39% 7% 14% 25% 15% 1	2807
July	1st to 10th	Day	34% 7% 16% 27% 15% 2	4416
	11th to 20th	Day	32% 7% 15% 34% 12% 1	4306
	21st to end	Day	34% 9% 15% 22% 18% 1	4324
August	1st to 10th	Day	42% 6% 14% 23% 14% 1	4091
	11th to 20th	Day	40% 7% 17% 25% 10% 1	4083
	21st to end	Day	36% 6% 13% 33% 11% 1	4382
September	1st to 10th	Day	39% 7% 14% 27% 11% 1	2280
	11th to 20th	Day	34% 5% 15% 30% 15%	2134
	21st to end	Day	44% 6% 15% 24% 9% 1	2071
October	1st to 10th	Day	39% 6% 16% 25% 14% 1	3160
	11th to 20th	Day	40% 5% 14% 24% 16%	3140
	21st to end	Day	51% 6% 14% 22% 7%	3428
November	1st to 10th	Day	47% 7% 15% 22% 8% 1	1661
	11th to 20th	Day	53% 7% 16% 16% 7% 1	1520
	21st to end	Day	58% 7% 16% 15% 5%	1520
December	1st to 10th	Day	64% 6% 14% 12% 4	1520
	11th to 20th	Day	62% 8% 14% 10% 7%	1906
	21st to end	Day	77% 6% 8% 8% 2	2772
January	1st to 10th	Night	60% 7% 14% 13% 5%	4425
	11th to 20th	Night	62% 7% 18% 10% 3	4120
	21st to end	Night	64% 6% 11% 14% 4 1	4532
February	1st to 10th	Night	59% 5% 12% 19% 4 1	2470
	11th to 20th	Night	56% 6% 12% 16% 9% 1	2405
	21st to end	Night	54% 5% 14% 18% 8%	1967
March	1st to 10th	Night	51% 6% 14% 21% 6% 1	3418
	11th to 20th	Night	43% 6% 17% 21% 12% 2	3480
	21st to end	Night	44% 6% 8% 24% 17% 2	3817
April	1st to 10th	Night	50% 6% 10% 19% 14% 1	1957
	11th to 20th	Night	47% 4 9% 21% 15% 3	1797
	21st to end	Night	46% 6% 10% 20% 15% 3	1710
May	1st to 10th	Night	44% 8% 12% 18% 16% 4	2840
	11th to 20th	Night	46% 12% 13% 19% 9% 2	2599
	21st to end	Night	29% 7% 14% 24% 23% 3	2739
June	1st to 10th	Night	38% 7% 11% 25% 18% 1	1520
	11th to 20th	Night	34% 7% 13% 26% 18% 2	1520
	21st to end	Night	35% 6% 14% 27% 17% 1	1357
July	1st to 10th	Night	30% 5% 13% 25% 23% 4	2408
	11th to 20th	Night	30% 5% 13% 32% 18% 1	2527
	21st to end	Night	28% 8% 16% 27% 18% 2	2988
August	1st to 10th	Night	36% 6% 13% 27% 15% 3	2814
	11th to 20th	Night	34% 6% 17% 31% 12% 1	2812
	21st to end	Night	32% 7% 12% 32% 15% 1	3242
September	1st to 10th	Night	35% 7% 13% 28% 15% 2	1900
	11th to 20th	Night	29% 6% 12% 34% 20%	2042
	21st to end	Night	43% 6% 12% 27% 12%	2109
October	1st to 10th	Night	32% 4 11% 27% 25% 1	3800
	11th to 20th	Night	43% 5% 12% 25% 15%	3779
	21st to end	Night	40% 6% 15% 26% 12% 1	4120
November	1st to 10th	Night	42% 7% 13% 24% 13% 1	2487
	11th to 20th	Night	49% 7% 15% 19% 8% 1	2660
	21st to end	Night	53% 7% 19% 16% 5%	2660
December	1st to 10th	Night	59% 8% 15% 15% 4	2660
	11th to 20th	Night	60% 8% 14% 14% 4	3288
	21st to end	Night	75% 6% 9% 7% 2	4884

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	17% 17% 34% 22% 8% 2	180
	11th to 20th	Day	23% 17% 37% 16% 5% 1	270
	21st to end	Day	23% 18% 31% 23% 5%	297
February	1st to 10th	Day	21% 18% 39% 17% 5%	270
	11th to 20th	Day	18% 23% 38% 19% 2	270
	21st to end	Day	24% 27% 31% 17% 1	226
March	1st to 10th	Day	20% 14% 34% 25% 7%	460
	11th to 20th	Day	19% 18% 33% 22% 7% 1	460
	21st to end	Day	24% 12% 30% 24% 9% 1	506
April	1st to 10th	Day	18% 15% 29% 28% 8% 1	460
	11th to 20th	Day	24% 16% 25% 22% 10% 4	452
	21st to end	Day	21% 13% 31% 23% 9% 3	460
May	1st to 10th	Day	19% 11% 25% 27% 13% 5%	463
	11th to 20th	Day	17% 8% 23% 29% 18% 5%	487
	21st to end	Day	12% 5% 24% 32% 18% 9%	529
June	1st to 10th	Day	14% 8% 26% 33% 15% 3	490
	11th to 20th	Day	12% 8% 23% 31% 21% 5%	490
	21st to end	Day	14% 10% 24% 32% 15% 5%	637
July	1st to 10th	Day	8% 4 20% 34% 23% 11%	518
	11th to 20th	Day	14% 5% 18% 30% 23% 11%	484
	21st to end	Day	9% 5% 20% 38% 21% 8%	532
August	1st to 10th	Day	11% 5% 17% 31% 23% 13%	486
	11th to 20th	Day	9% 6% 16% 30% 25% 13%	479
	21st to end	Day	11% 5% 20% 32% 21% 11%	539
September	1st to 10th	Day	11% 9% 19% 28% 21% 12%	490
	11th to 20th	Day	10% 4 15% 28% 27% 16%	490
	21st to end	Day	15% 11% 24% 26% 16% 8%	490
October	1st to 10th	Day	22% 12% 20% 22% 16% 8%	490
	11th to 20th	Day	19% 15% 29% 25% 11% 2	490
	21st to end	Day	29% 12% 28% 21% 9% 1	539
November	1st to 10th	Day	17% 22% 27% 22% 11% 2	278
	11th to 20th	Day	21% 21% 28% 21% 7% 2	180
	21st to end	Day	22% 16% 37% 19% 6% 1	180
December	1st to 10th	Day	17% 23% 33% 21% 5% 1	180
	11th to 20th	Day	23% 26% 29% 19% 3	180
	21st to end	Day	27% 18% 39% 13% 3 1	198
January	1st to 10th	Night	19% 18% 35% 21% 7% 1	760
	11th to 20th	Night	24% 18% 33% 21% 4	760
	21st to end	Night	18% 13% 38% 25% 7%	836
February	1st to 10th	Night	21% 16% 38% 22% 3	760
	11th to 20th	Night	19% 16% 36% 26% 3	743
	21st to end	Night	19% 17% 36% 24% 4	628
March	1st to 10th	Night	17% 12% 34% 29% 7%	760
	11th to 20th	Night	19% 14% 36% 24% 7%	760
	21st to end	Night	20% 10% 30% 30% 9% 1	836
April	1st to 10th	Night	20% 11% 32% 31% 6%	760
	11th to 20th	Night	19% 11% 31% 27% 10% 1	673
	21st to end	Night	19% 6% 26% 37% 11%	570
May	1st to 10th	Night	17% 4 25% 35% 16% 3	570
	11th to 20th	Night	16% 5% 24% 39% 15% 2	438
	21st to end	Night	12% 5% 21% 34% 26% 2	411
June	1st to 10th	Night	13% 4 27% 39% 16% 2	380
	11th to 20th	Night	12% 6% 24% 40% 17% 1	380
	21st to end	Night	11% 4 21% 42% 21%	380
July	1st to 10th	Night	6% 2 20% 46% 23% 2	370
	11th to 20th	Night	9% 3 20% 42% 23% 3	397
	21st to end	Night	8% 2 16% 46% 25% 3	624
August	1st to 10th	Night	9% 3 15% 42% 28% 4	567
	11th to 20th	Night	8% 2 14% 37% 35% 4	564
	21st to end	Night	12% 3 19% 36% 28% 4	693
September	1st to 10th	Night	12% 6% 18% 34% 25% 6%	760
	11th to 20th	Night	11% 4 17% 32% 29% 6%	760
	21st to end	Night	17% 6% 20% 29% 24% 5%	760
October	1st to 10th	Night	19% 8% 24% 26% 20% 3	760
	11th to 20th	Night	19% 12% 29% 29% 10% 2	760
	21st to end	Night	25% 13% 27% 29% 6%	836
November	1st to 10th	Night	18% 12% 29% 29% 10% 2	755
	11th to 20th	Night	18% 15% 33% 24% 8% 2	760
	21st to end	Night	22% 13% 34% 23% 7%	760
December	1st to 10th	Night	24% 17% 32% 20% 6% 1	760
	11th to 20th	Night	22% 18% 36% 19% 5%	760
	21st to end	Night	28% 16% 32% 21% 3	836

Page generated at 2024-11-01 22:47:30. Version ac07

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