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 2024-03-28.

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% 4 7% 7% 4 5% 5% 6% 7% 32%	78921
	Night	15% 9% 4 7% 6% 4 5% 5% 5% 7% 33%	81037
Transparency	Day	50% 6% 13% 19% 10% 1	78638
	Night	49% 6% 13% 20% 10% 1	80854
Seeing	Day	17% 11% 26% 27% 14% 6%	13739
	Night	18% 10% 28% 29% 13% 2	23201

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	15% 16% 6% 9% 8% 3 4 4 4 6% 26%	2517
	11th to 20th	Day	11% 17% 8% 15% 9% 4 5% 4 4 5% 18%	2840
	21st to end	Day	22% 17% 7% 11% 8% 3 5% 4 3 4 16%	3124
February	1st to 10th	Day	14% 15% 6% 10% 9% 4 6% 6% 5% 5% 21%	1710
	11th to 20th	Day	12% 12% 7% 9% 8% 5% 5% 4 5% 7% 24%	1663
	21st to end	Day	18% 12% 5% 10% 6% 3 5% 5% 5% 7% 25%	1484
March	1st to 10th	Day	14% 10% 6% 9% 7% 4 5% 5% 5% 6% 29%	2087
	11th to 20th	Day	13% 12% 5% 8% 7% 4 5% 4 5% 5% 32%	2090
	21st to end	Day	16% 10% 5% 7% 6% 4 5% 5% 5% 5% 31%	2255
April	1st to 10th	Day	16% 9% 5% 9% 6% 4 5% 5% 4 6% 30%	2104
	11th to 20th	Day	17% 8% 3 7% 6% 3 5% 4 4 6% 36%	2222
	21st to end	Day	19% 8% 3 7% 6% 3 5% 5% 5% 6% 33%	2340
May	1st to 10th	Day	15% 8% 4 7% 6% 3 4 5% 6% 7% 34%	2340
	11th to 20th	Day	14% 6% 3 6% 6% 4 5% 5% 6% 6% 38%	2414
	21st to end	Day	10% 5% 2 6% 6% 3 5% 6% 6% 8% 44%	2725
June	1st to 10th	Day	13% 5% 3 6% 5% 4 5% 5% 5% 8% 41%	2520
	11th to 20th	Day	11% 5% 3 6% 5% 3 6% 5% 7% 9% 40%	2520
	21st to end	Day	12% 5% 2 5% 5% 3 6% 5% 6% 9% 41%	2659
July	1st to 10th	Day	7% 4 2 4 4 4 6% 6% 8% 9% 46%	2552
	11th to 20th	Day	11% 5% 2 4 5% 4 6% 7% 8% 10% 38%	2489
	21st to end	Day	8% 5% 2 5% 6% 5% 6% 6% 7% 10% 39%	2547
August	1st to 10th	Day	8% 5% 2 6% 5% 4 6% 7% 7% 11% 40%	2327
	11th to 20th	Day	7% 5% 3 6% 7% 4 6% 7% 7% 9% 39%	2303
	21st to end	Day	11% 6% 2 5% 6% 4 5% 5% 7% 9% 41%	2494
September	1st to 10th	Day	13% 6% 2 6% 5% 4 6% 4 5% 9% 39%	2160
	11th to 20th	Day	10% 5% 3 4 6% 3 4 5% 5% 9% 45%	2022
	21st to end	Day	13% 6% 3 6% 7% 4 6% 6% 6% 8% 36%	1962
October	1st to 10th	Day	18% 9% 3 5% 5% 4 5% 4 6% 7% 34%	1800
	11th to 20th	Day	19% 7% 4 8% 7% 3 4 5% 5% 7% 29%	1800
	21st to end	Day	23% 10% 4 7% 7% 5% 4 4 6% 5% 24%	1980
November	1st to 10th	Day	13% 7% 4 8% 7% 4 6% 6% 6% 8% 32%	1633
	11th to 20th	Day	13% 11% 5% 8% 8% 4 6% 6% 7% 8% 26%	1520
	21st to end	Day	18% 13% 5% 7% 7% 3 5% 7% 6% 7% 23%	1520
December	1st to 10th	Day	14% 16% 6% 12% 9% 4 5% 5% 5% 7% 19%	1520
	11th to 20th	Day	17% 13% 4 9% 8% 5% 6% 5% 6% 6% 21%	1906
	21st to end	Day	27% 17% 7% 12% 7% 4 4 4 4 4 10%	2772
January	1st to 10th	Night	16% 13% 5% 10% 8% 4 5% 5% 5% 7% 22%	4425
	11th to 20th	Night	14% 12% 6% 11% 9% 5% 7% 6% 6% 7% 17%	4120
	21st to end	Night	21% 13% 7% 9% 7% 4 5% 4 4 6% 21%	4532
February	1st to 10th	Night	14% 12% 5% 9% 8% 4 6% 5% 5% 7% 25%	2470
	11th to 20th	Night	13% 11% 6% 9% 8% 3 6% 5% 6% 6% 27%	2403
	21st to end	Night	14% 11% 4 9% 7% 4 5% 5% 5% 7% 28%	1967
March	1st to 10th	Night	14% 7% 4 8% 6% 4 6% 6% 6% 6% 34%	2080
	11th to 20th	Night	14% 10% 5% 8% 6% 3 4 5% 4 7% 34%	2089
	21st to end	Night	16% 9% 3 8% 6% 3 4 4 4 5% 38%	2256
April	1st to 10th	Night	17% 7% 4 7% 6% 4 5% 5% 5% 6% 35%	1854
	11th to 20th	Night	15% 7% 4 7% 6% 3 5% 4 4 7% 39%	1702
	21st to end	Night	16% 8% 4 6% 6% 3 4 4 5% 7% 38%	1620
May	1st to 10th	Night	16% 6% 3 6% 5% 3 4 5% 6% 7% 39%	1620
	11th to 20th	Night	14% 5% 3 6% 4 3 5% 5% 7% 6% 42%	1476
	21st to end	Night	9% 5% 2 4 4 3 5% 4 6% 8% 51%	1550
June	1st to 10th	Night	10% 5% 2 6% 6% 4 5% 5% 6% 8% 43%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 3 5% 5% 7% 8% 44%	1440
	21st to end	Night	10% 4 2 4 5% 3 4 5% 5% 7% 49%	1286
July	1st to 10th	Night	6% 3 2 3 4 3 4 4 5% 9% 57%	1394
	11th to 20th	Night	9% 4 1 4 4 3 4 4 5% 10% 50%	1467
	21st to end	Night	7% 3 2 4 5% 3 4 5% 7% 9% 51%	1771
August	1st to 10th	Night	7% 5% 2 4 5% 4 6% 5% 6% 8% 48%	1611
	11th to 20th	Night	7% 4 2 4 5% 3 4 5% 6% 9% 52%	1598
	21st to end	Night	10% 4 2 4 6% 4 5% 5% 6% 7% 47%	1844
September	1st to 10th	Night	12% 4 2 5% 6% 5% 3 4 7% 7% 45%	1800
	11th to 20th	Night	10% 5% 2 4 4 2 3 4 5% 7% 54%	1934
	21st to end	Night	13% 6% 3 7% 5% 3 4 5% 6% 6% 41%	1998
October	1st to 10th	Night	17% 7% 3 6% 5% 3 3 5% 6% 7% 41%	2160
	11th to 20th	Night	17% 8% 3 6% 6% 4 5% 5% 5% 7% 35%	2160
	21st to end	Night	20% 9% 4 7% 6% 4 5% 5% 5% 6% 30%	2376
November	1st to 10th	Night	15% 6% 3 5% 6% 3 5% 5% 6% 7% 40%	2442
	11th to 20th	Night	14% 10% 4 7% 6% 3 6% 5% 6% 8% 30%	2660
	21st to end	Night	18% 8% 4 8% 6% 3 6% 6% 5% 8% 27%	2660
December	1st to 10th	Night	18% 12% 4 9% 6% 4 7% 5% 6% 6% 23%	2660
	11th to 20th	Night	16% 14% 5% 10% 6% 4 5% 6% 5% 6% 23%	3288
	21st to end	Night	28% 17% 7% 10% 6% 3 5% 4 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	55% 6% 14% 16% 8% 1	2090
	11th to 20th	Day	54% 4 11% 18% 10% 2	2090
	21st to end	Day	53% 4 11% 19% 11% 2	2255
April	1st to 10th	Day	54% 4 12% 18% 11% 1	2104
	11th to 20th	Day	48% 4 12% 21% 14% 1	2222
	21st to end	Day	51% 5% 11% 19% 12% 3	2340
May	1st to 10th	Day	48% 5% 12% 22% 12% 1	2340
	11th to 20th	Day	46% 7% 10% 22% 13% 2	2418
	21st to end	Day	37% 7% 16% 23% 15% 2	2725
June	1st to 10th	Day	40% 7% 13% 23% 16% 2	2520
	11th to 20th	Day	39% 6% 14% 24% 16% 1	2520
	21st to end	Day	38% 6% 14% 25% 16% 1	2659
July	1st to 10th	Day	30% 8% 14% 26% 18% 3	2482
	11th to 20th	Day	36% 8% 17% 27% 12% 1	2414
	21st to end	Day	38% 9% 15% 21% 15% 2	2405
August	1st to 10th	Day	36% 7% 18% 25% 12% 2	2314
	11th to 20th	Day	38% 7% 19% 24% 11% 1	2303
	21st to end	Day	38% 6% 14% 28% 13% 1	2494
September	1st to 10th	Day	40% 8% 14% 26% 11% 1	2160
	11th to 20th	Day	35% 5% 15% 29% 15%	2022
	21st to end	Day	43% 6% 16% 24% 9% 1	1962
October	1st to 10th	Day	48% 6% 14% 19% 11% 2	1800
	11th to 20th	Day	52% 6% 14% 21% 7% 1	1800
	21st to end	Day	60% 6% 12% 15% 6% 1	1980
November	1st to 10th	Day	48% 7% 14% 22% 8% 1	1633
	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	49% 6% 14% 20% 9% 2	2090
	11th to 20th	Night	50% 5% 11% 21% 11% 2	2090
	21st to end	Night	49% 5% 7% 22% 15% 3	2256
April	1st to 10th	Night	49% 6% 11% 19% 14% 1	1854
	11th to 20th	Night	46% 4 9% 21% 16% 3	1702
	21st to end	Night	46% 6% 9% 21% 15% 3	1620
May	1st to 10th	Night	43% 6% 10% 22% 16% 3	1620
	11th to 20th	Night	40% 6% 10% 25% 15% 4	1477
	21st to end	Night	31% 7% 14% 25% 19% 4	1551
June	1st to 10th	Night	38% 6% 11% 25% 18% 1	1440
	11th to 20th	Night	35% 6% 13% 25% 18% 2	1440
	21st to end	Night	35% 6% 14% 27% 17% 1	1286
July	1st to 10th	Night	26% 6% 10% 26% 28% 4	1354
	11th to 20th	Night	31% 6% 13% 29% 19% 2	1419
	21st to end	Night	29% 8% 15% 26% 19% 3	1672
August	1st to 10th	Night	32% 7% 17% 30% 12% 2	1600
	11th to 20th	Night	28% 7% 17% 33% 14% 2	1598
	21st to end	Night	34% 7% 13% 29% 15% 2	1844
September	1st to 10th	Night	36% 7% 14% 27% 15% 2	1800
	11th to 20th	Night	29% 6% 12% 34% 19% 1	1934
	21st to end	Night	41% 6% 12% 27% 12%	1998
October	1st to 10th	Night	42% 6% 13% 24% 15% 1	2160
	11th to 20th	Night	48% 6% 14% 23% 9%	2160
	21st to end	Night	54% 6% 11% 18% 10% 1	2376
November	1st to 10th	Night	42% 7% 13% 24% 13% 1	2442
	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	25% 13% 30% 23% 9% 1	490
April	1st to 10th	Day	16% 16% 30% 28% 9% 1	420
	11th to 20th	Day	23% 15% 25% 23% 11% 4	412
	21st to end	Day	21% 12% 31% 24% 9% 3	420
May	1st to 10th	Day	20% 10% 27% 25% 13% 5%	423
	11th to 20th	Day	17% 8% 25% 29% 16% 6%	447
	21st to end	Day	13% 5% 23% 32% 18% 9%	485
June	1st to 10th	Day	14% 7% 27% 34% 15% 3	450
	11th to 20th	Day	12% 7% 22% 31% 22% 5%	450
	21st to end	Day	13% 9% 22% 34% 16% 5%	589
July	1st to 10th	Day	8% 3 19% 35% 24% 12%	477
	11th to 20th	Day	14% 4 17% 31% 23% 11%	444
	21st to end	Day	9% 4 19% 39% 20% 8%	488
August	1st to 10th	Day	7% 5% 17% 33% 24% 14%	446
	11th to 20th	Day	9% 5% 15% 32% 26% 13%	439
	21st to end	Day	11% 5% 20% 32% 20% 12%	495
September	1st to 10th	Day	12% 7% 18% 30% 21% 12%	450
	11th to 20th	Day	11% 4 16% 28% 26% 15%	450
	21st to end	Day	14% 11% 24% 27% 17% 8%	450
October	1st to 10th	Day	23% 12% 19% 21% 16% 9%	450
	11th to 20th	Day	19% 16% 28% 25% 10% 1	450
	21st to end	Day	29% 11% 28% 21% 9% 1	495
November	1st to 10th	Day	17% 22% 27% 22% 11% 2	268
	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% 29% 9% 1	821
April	1st to 10th	Night	19% 11% 32% 31% 6%	720
	11th to 20th	Night	18% 11% 32% 27% 11% 2	638
	21st to end	Night	19% 6% 27% 36% 11%	540
May	1st to 10th	Night	17% 4 25% 35% 16% 3	540
	11th to 20th	Night	15% 4 25% 39% 14% 2	415
	21st to end	Night	12% 4 22% 34% 26% 2	389
June	1st to 10th	Night	12% 4 27% 39% 16% 2	360
	11th to 20th	Night	12% 6% 24% 39% 18% 1	360
	21st to end	Night	10% 4 21% 43% 22%	360
July	1st to 10th	Night	6% 2 19% 47% 24% 2	350
	11th to 20th	Night	9% 2 20% 43% 22% 3	375
	21st to end	Night	8% 2 17% 46% 24% 3	591
August	1st to 10th	Night	9% 3 14% 42% 29% 4	537
	11th to 20th	Night	7% 2 13% 37% 36% 4	534
	21st to end	Night	12% 3 19% 35% 28% 4	656
September	1st to 10th	Night	12% 6% 17% 35% 25% 6%	720
	11th to 20th	Night	11% 4 18% 33% 27% 6%	720
	21st to end	Night	15% 6% 20% 29% 25% 5%	720
October	1st to 10th	Night	19% 8% 23% 27% 20% 3	720
	11th to 20th	Night	20% 12% 30% 28% 10% 2	720
	21st to end	Night	26% 12% 27% 29% 6%	792
November	1st to 10th	Night	18% 12% 29% 29% 11% 2	740
	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

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