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 2025-09-04.

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% 4 9% 6% 5% 5% 4 6% 7% 33%	126486
	Night	14% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	122171
Transparency	Day	48% 6% 13% 20% 11% 1	130082
	Night	47% 6% 13% 21% 12% 1	126662
Seeing	Day	18% 12% 26% 26% 13% 5%	15674
	Night	18% 11% 28% 29% 13% 2	24902

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% 14% 6% 13% 7% 5% 4 4 4 5% 26%	3597
	11th to 20th	Day	9% 18% 8% 19% 8% 7% 5% 3 4 5% 14%	4026
	21st to end	Day	17% 15% 6% 14% 7% 5% 5% 3 4 5% 18%	4466
February	1st to 10th	Day	14% 15% 6% 12% 7% 6% 6% 4 5% 5% 20%	1800
	11th to 20th	Day	12% 14% 6% 11% 8% 6% 4 4 5% 7% 23%	1753
	21st to end	Day	18% 14% 4 10% 6% 4 6% 3 5% 6% 24%	1560
March	1st to 10th	Day	17% 9% 4 10% 7% 4 4 3 4 6% 30%	4932
	11th to 20th	Day	10% 11% 4 9% 6% 5% 5% 4 6% 8% 30%	4980
	21st to end	Day	16% 10% 4 9% 6% 4 6% 4 5% 5% 32%	5465
April	1st to 10th	Day	16% 11% 4 9% 6% 5% 4 4 5% 6% 30%	2338
	11th to 20th	Day	17% 9% 3 8% 5% 4 4 4 4 6% 36%	2472
	21st to end	Day	18% 9% 3 8% 5% 4 4 4 5% 6% 33%	2600
May	1st to 10th	Day	19% 9% 3 8% 4 4 4 3 5% 7% 33%	5888
	11th to 20th	Day	15% 9% 4 8% 5% 5% 5% 4 6% 7% 32%	6170
	21st to end	Day	16% 6% 2 7% 5% 5% 5% 4 6% 6% 39%	6949
June	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 5% 5% 8% 40%	2800
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 8% 9% 39%	2800
	21st to end	Day	12% 5% 2 7% 5% 5% 5% 4 6% 9% 40%	2955
July	1st to 10th	Day	8% 5% 2 6% 5% 4 6% 5% 8% 10% 41%	6433
	11th to 20th	Day	7% 4 2 5% 5% 5% 5% 5% 7% 9% 45%	6273
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 6% 9% 47%	6424
August	1st to 10th	Day	16% 6% 2 6% 4 4 5% 5% 7% 8% 38%	4234
	11th to 20th	Day	9% 7% 3 7% 5% 5% 6% 5% 7% 9% 36%	4174
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	4520
September	1st to 10th	Day	12% 6% 2 6% 4 5% 5% 3 6% 9% 41%	2316
	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% 8% 4 8% 6% 5% 6% 5% 6% 8% 32%	1720
	11th to 20th	Day	13% 12% 5% 9% 6% 5% 6% 5% 7% 7% 26%	1600
	21st to end	Day	17% 14% 4 9% 6% 4 4 6% 6% 6% 22%	1600
December	1st to 10th	Day	15% 16% 6% 12% 8% 6% 5% 5% 7% 6% 15%	2600
	11th to 20th	Day	21% 16% 3 10% 5% 4 4 4 5% 6% 21%	2986
	21st to end	Day	24% 17% 7% 13% 6% 5% 4 4 4 4 12%	3960
January	1st to 10th	Night	13% 11% 4 11% 6% 6% 5% 4 5% 7% 27%	6331
	11th to 20th	Night	11% 12% 5% 14% 10% 7% 6% 5% 6% 7% 17%	5859
	21st to end	Night	16% 12% 6% 11% 6% 5% 5% 4 5% 7% 23%	6490
February	1st to 10th	Night	14% 13% 5% 10% 7% 5% 5% 4 6% 7% 25%	2600
	11th to 20th	Night	13% 12% 5% 11% 7% 5% 5% 4 6% 6% 27%	2533
	21st to end	Night	13% 12% 4 11% 6% 6% 4 4 5% 7% 27%	2067
March	1st to 10th	Night	15% 7% 3 8% 6% 5% 5% 4 6% 7% 33%	4908
	11th to 20th	Night	9% 7% 4 7% 5% 5% 5% 4 7% 8% 39%	4979
	21st to end	Night	18% 9% 2 7% 4 4 4 3 4 5% 39%	5467
April	1st to 10th	Night	17% 9% 3 8% 5% 5% 5% 4 5% 5% 33%	2060
	11th to 20th	Night	16% 8% 3 8% 5% 4 5% 3 4 6% 37%	1893
	21st to end	Night	15% 8% 3 7% 5% 4 4 4 5% 6% 39%	1800
May	1st to 10th	Night	20% 8% 3 8% 4 4 4 4 5% 6% 35%	4053
	11th to 20th	Night	13% 7% 3 8% 5% 5% 5% 4 7% 7% 36%	3720
	21st to end	Night	17% 5% 2 5% 4 4 4 3 5% 8% 44%	3926
June	1st to 10th	Night	10% 5% 2 6% 5% 5% 5% 5% 6% 8% 43%	1600
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 4 8% 8% 43%	1600
	21st to end	Night	10% 5% 2 5% 5% 5% 5% 4 5% 7% 48%	1428
July	1st to 10th	Night	6% 4 2 6% 5% 5% 4 4 6% 9% 48%	3502
	11th to 20th	Night	9% 4 2 6% 4 4 4 4 5% 8% 50%	3683
	21st to end	Night	7% 4 2 5% 4 4 4 4 6% 9% 52%	4428
August	1st to 10th	Night	9% 6% 3 6% 4 5% 5% 4 6% 7% 46%	2915
	11th to 20th	Night	10% 5% 2 5% 4 3 4 4 6% 8% 48%	2875
	21st to end	Night	9% 5% 2 5% 4 4 5% 4 6% 8% 47%	3345
September	1st to 10th	Night	11% 4 2 5% 5% 5% 4 3 7% 7% 47%	1931
	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% 4 4 4 6% 7% 40%	2575
	11th to 20th	Night	14% 11% 4 8% 5% 5% 6% 4 6% 7% 30%	2800
	21st to end	Night	19% 9% 4 8% 6% 5% 5% 5% 5% 8% 27%	2800
December	1st to 10th	Night	12% 13% 5% 10% 6% 7% 6% 6% 8% 8% 19%	4580
	11th to 20th	Night	17% 16% 4 9% 5% 5% 5% 4 5% 6% 23%	5208
	21st to end	Night	24% 17% 6% 12% 6% 4 4 3 4 6% 14%	6996

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	56% 7% 13% 12% 10% 2	3598
	11th to 20th	Day	71% 4 11% 10% 4	4060
	21st to end	Day	67% 6% 11% 9% 7%	4466
February	1st to 10th	Day	63% 6% 12% 14% 4 1	1800
	11th to 20th	Day	61% 5% 10% 15% 7% 2	1762
	21st to end	Day	59% 5% 13% 15% 7% 1	1560
March	1st to 10th	Day	55% 5% 14% 15% 11% 1	4935
	11th to 20th	Day	49% 5% 17% 19% 7% 2	4980
	21st to end	Day	51% 5% 11% 22% 11% 1	5465
April	1st to 10th	Day	54% 4 11% 19% 11% 1	2338
	11th to 20th	Day	49% 4 11% 21% 13% 1	2472
	21st to end	Day	51% 5% 11% 19% 12% 3	2600
May	1st to 10th	Day	49% 6% 13% 17% 10% 5%	5897
	11th to 20th	Day	49% 9% 11% 17% 13% 2	6174
	21st to end	Day	43% 6% 12% 21% 15% 3	6949
June	1st to 10th	Day	41% 7% 12% 22% 16% 2	2800
	11th to 20th	Day	39% 6% 14% 24% 15% 1	2800
	21st to end	Day	39% 7% 14% 25% 15% 1	2955
July	1st to 10th	Day	34% 7% 14% 27% 16% 2	6364
	11th to 20th	Day	32% 7% 14% 31% 16% 2	6198
	21st to end	Day	31% 8% 14% 28% 18% 1	6240
August	1st to 10th	Day	41% 7% 15% 23% 14% 1	4221
	11th to 20th	Day	40% 7% 17% 26% 10% 1	4213
	21st to end	Day	37% 6% 13% 34% 11% 1	4520
September	1st to 10th	Day	38% 7% 14% 28% 11% 1	2316
	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	48% 7% 14% 22% 8% 1	1720
	11th to 20th	Day	53% 7% 15% 17% 8% 1	1600
	21st to end	Day	58% 7% 15% 14% 5%	1600
December	1st to 10th	Day	65% 7% 15% 9% 4	2600
	11th to 20th	Day	62% 6% 13% 9% 10%	2986
	21st to end	Day	75% 6% 7% 8% 5%	3960
January	1st to 10th	Night	53% 7% 11% 20% 8% 1	6331
	11th to 20th	Night	62% 6% 14% 13% 4	5900
	21st to end	Night	59% 5% 11% 14% 10% 1	6490
February	1st to 10th	Night	58% 5% 13% 18% 5% 1	2600
	11th to 20th	Night	56% 5% 12% 17% 9% 1	2535
	21st to end	Night	54% 5% 14% 18% 8%	2067
March	1st to 10th	Night	47% 6% 15% 18% 12% 2	4918
	11th to 20th	Night	40% 5% 19% 23% 11% 1	4980
	21st to end	Night	47% 5% 8% 20% 17% 2	5467
April	1st to 10th	Night	51% 6% 10% 18% 13% 2	2060
	11th to 20th	Night	47% 4 9% 21% 16% 2	1893
	21st to end	Night	45% 6% 10% 21% 15% 3	1800
May	1st to 10th	Night	49% 7% 11% 14% 13% 7%	4060
	11th to 20th	Night	45% 11% 12% 19% 12% 2	3721
	21st to end	Night	38% 6% 14% 24% 17% 2	3927
June	1st to 10th	Night	38% 6% 11% 25% 18% 2	1600
	11th to 20th	Night	35% 6% 13% 26% 17% 2	1600
	21st to end	Night	35% 6% 14% 26% 18% 1	1428
July	1st to 10th	Night	32% 6% 13% 25% 21% 3	3462
	11th to 20th	Night	31% 5% 13% 28% 20% 2	3635
	21st to end	Night	29% 7% 13% 27% 23% 2	4297
August	1st to 10th	Night	36% 6% 14% 26% 16% 3	2904
	11th to 20th	Night	34% 6% 17% 31% 12% 1	2902
	21st to end	Night	32% 7% 13% 32% 15% 1	3345
September	1st to 10th	Night	34% 7% 13% 29% 15% 2	1931
	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% 6% 13% 24% 13% 1	2575
	11th to 20th	Night	50% 7% 15% 19% 8% 1	2800
	21st to end	Night	53% 7% 19% 16% 5%	2800
December	1st to 10th	Night	57% 8% 17% 14% 4	4580
	11th to 20th	Night	59% 7% 11% 13% 10% 1	5208
	21st to end	Night	70% 6% 10% 9% 5%	6996

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% 19% 34% 21% 8% 2	200
	11th to 20th	Day	25% 18% 37% 15% 5% 1	300
	21st to end	Day	24% 19% 31% 21% 5%	330
February	1st to 10th	Day	25% 18% 37% 16% 4	300
	11th to 20th	Day	23% 22% 36% 17% 2	300
	21st to end	Day	26% 26% 30% 16% 1	250
March	1st to 10th	Day	20% 15% 34% 23% 7%	500
	11th to 20th	Day	21% 18% 33% 20% 7% 1	500
	21st to end	Day	26% 13% 29% 23% 8% 1	550
April	1st to 10th	Day	20% 16% 28% 27% 8% 1	500
	11th to 20th	Day	25% 16% 25% 22% 9% 3	492
	21st to end	Day	22% 13% 31% 22% 8% 3	500
May	1st to 10th	Day	20% 12% 26% 26% 12% 4	503
	11th to 20th	Day	17% 9% 24% 28% 17% 5%	527
	21st to end	Day	14% 6% 24% 30% 17% 8%	573
June	1st to 10th	Day	15% 10% 27% 31% 14% 3	530
	11th to 20th	Day	13% 8% 24% 30% 20% 5%	530
	21st to end	Day	14% 10% 25% 32% 14% 5%	685
July	1st to 10th	Day	8% 4 21% 33% 22% 11%	560
	11th to 20th	Day	13% 5% 19% 29% 23% 10%	524
	21st to end	Day	9% 5% 22% 37% 20% 8%	576
August	1st to 10th	Day	10% 5% 17% 32% 22% 13%	526
	11th to 20th	Day	10% 7% 17% 31% 24% 13%	519
	21st to end	Day	11% 5% 21% 32% 20% 10%	583
September	1st to 10th	Day	11% 9% 19% 29% 21% 12%	502
	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	19% 21% 28% 20% 10% 2	295
	11th to 20th	Day	22% 21% 30% 20% 7% 2	200
	21st to end	Day	24% 18% 35% 18% 5% 1	200
December	1st to 10th	Day	20% 23% 32% 20% 5% 1	200
	11th to 20th	Day	26% 27% 27% 18% 3	200
	21st to end	Day	27% 17% 38% 14% 3 1	220
January	1st to 10th	Night	19% 18% 36% 20% 6% 1	800
	11th to 20th	Night	24% 18% 33% 20% 4	800
	21st to end	Night	19% 14% 37% 24% 6%	880
February	1st to 10th	Night	21% 17% 38% 21% 3	800
	11th to 20th	Night	20% 16% 35% 25% 3	783
	21st to end	Night	19% 17% 36% 24% 4	660
March	1st to 10th	Night	18% 13% 34% 28% 7%	800
	11th to 20th	Night	19% 14% 36% 24% 7%	800
	21st to end	Night	21% 11% 30% 29% 9% 1	880
April	1st to 10th	Night	21% 11% 32% 31% 6%	800
	11th to 20th	Night	20% 11% 30% 27% 10% 1	709
	21st to end	Night	19% 7% 26% 37% 11%	600
May	1st to 10th	Night	18% 5% 25% 34% 16% 3	600
	11th to 20th	Night	16% 5% 24% 39% 15% 2	461
	21st to end	Night	13% 5% 22% 33% 25% 2	433
June	1st to 10th	Night	13% 4 28% 39% 15% 2	400
	11th to 20th	Night	12% 6% 25% 40% 17% 1	400
	21st to end	Night	11% 5% 22% 42% 21%	400
July	1st to 10th	Night	6% 2 21% 47% 22% 2	390
	11th to 20th	Night	9% 3 21% 42% 22% 3	419
	21st to end	Night	8% 2 17% 46% 25% 2	657
August	1st to 10th	Night	9% 3 15% 42% 28% 4	597
	11th to 20th	Night	8% 3 14% 37% 35% 4	594
	21st to end	Night	12% 3 19% 36% 27% 3	730
September	1st to 10th	Night	11% 6% 18% 34% 25% 6%	773
	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	19% 13% 28% 29% 10% 2	780
	11th to 20th	Night	20% 15% 33% 24% 8% 2	800
	21st to end	Night	23% 14% 34% 23% 6%	800
December	1st to 10th	Night	24% 17% 33% 20% 6% 1	800
	11th to 20th	Night	23% 19% 36% 18% 5%	800
	21st to end	Night	28% 16% 31% 22% 4	880

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