Hamilton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hamilton clear sky chart from 2005-11-06 to 2026-09-29.

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	4 5% 2 6% 4 5% 5% 4 7% 8% 50%	158119
	Night	3 6% 2 6% 4 5% 5% 4 7% 8% 48%	150641
Transparency	Day	32% 9% 14% 24% 16% 6%	152773
	Night	33% 10% 17% 24% 12% 4	146996
Seeing	Day	12% 18% 29% 26% 13% 3	17627
	Night	9% 12% 26% 35% 15% 2	26846

By Month

Percentage Cloud Cover SW (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	1 5% 4 9% 8% 9% 8% 8% 11% 11% 27%	4519
	11th to 20th	Day	1 3 2 9% 5% 5% 7% 5% 6% 9% 48%	4740
	21st to end	Day	1 4 2 5% 5% 6% 6% 6% 9% 10% 46%	5187
February	1st to 10th	Day	2 10% 4 10% 8% 8% 9% 7% 7% 9% 26%	1827
	11th to 20th	Day	1 9% 4 10% 8% 7% 7% 5% 7% 9% 32%	1920
	21st to end	Day	2 9% 5% 11% 8% 7% 7% 7% 7% 9% 30%	1730
March	1st to 10th	Day	2 7% 3 8% 5% 5% 6% 6% 9% 10% 39%	4618
	11th to 20th	Day	2 8% 3 10% 6% 5% 6% 5% 7% 8% 39%	5099
	21st to end	Day	3 10% 4 11% 10% 8% 8% 6% 7% 9% 24%	5607
April	1st to 10th	Day	5% 10% 4 9% 7% 6% 7% 5% 8% 10% 29%	2520
	11th to 20th	Day	6% 10% 3 9% 6% 6% 6% 4 6% 8% 34%	2681
	21st to end	Day	9% 11% 4 9% 6% 5% 5% 5% 6% 7% 33%	2730
May	1st to 10th	Day	7% 8% 3 6% 4 4 4 3 6% 7% 49%	7626
	11th to 20th	Day	7% 6% 3 7% 5% 5% 6% 5% 7% 9% 40%	7817
	21st to end	Day	10% 7% 2 6% 4 4 4 4 6% 7% 47%	9566
June	1st to 10th	Day	12% 7% 2 8% 6% 4 6% 5% 6% 7% 36%	3150
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 4 6% 7% 41%	3150
	21st to end	Day	5% 3 2 4 3 4 4 4 6% 7% 58%	3150
July	1st to 10th	Day	2 1 1 2 2 3 3 3 5% 8% 69%	8819
	11th to 20th	Day	2 2 1 3 2 3 3 3 4 5% 72%	8224
	21st to end	Day	1 1 1 2 2 3 3 3 6% 8% 69%	8922
August	1st to 10th	Day	2 1 1 3 3 3 4 4 6% 8% 65%	7742
	11th to 20th	Day	2 1 1 3 2 3 4 4 7% 9% 64%	7565
	21st to end	Day	4 3 1 3 3 3 4 4 6% 8% 62%	8361
September	1st to 10th	Day	4 3 1 3 3 3 4 4 5% 7% 63%	2467
	11th to 20th	Day	6% 3 2 5% 4 4 5% 5% 7% 7% 50%	2310
	21st to end	Day	7% 5% 2 4 3 3 4 3 5% 7% 58%	2289
October	1st to 10th	Day	5% 4 2 5% 3 4 4 3 4 5% 60%	3537
	11th to 20th	Day	5% 5% 2 5% 4 4 4 4 5% 7% 56%	3130
	21st to end	Day	3 7% 2 8% 6% 5% 5% 5% 7% 8% 45%	3200
November	1st to 10th	Day	5% 8% 3 9% 6% 6% 7% 5% 7% 8% 34%	1854
	11th to 20th	Day	3 6% 2 9% 7% 5% 6% 6% 9% 9% 37%	1775
	21st to end	Day	3 7% 3 6% 5% 5% 6% 4 9% 10% 42%	1637
December	1st to 10th	Day	1 4 2 6% 7% 7% 6% 4 8% 9% 48%	2360
	11th to 20th	Day	1 6% 3 9% 8% 7% 8% 6% 10% 10% 31%	2715
	21st to end	Day	2 7% 4 10% 6% 6% 7% 6% 6% 8% 39%	3575
January	1st to 10th	Night	1 6% 3 10% 7% 8% 10% 7% 9% 10% 29%	8431
	11th to 20th	Night	1 4 3 9% 6% 6% 6% 4 6% 10% 45%	8220
	21st to end	Night	1 5% 2 6% 5% 5% 6% 5% 8% 10% 47%	8994
February	1st to 10th	Night	3 11% 5% 11% 7% 6% 7% 6% 7% 10% 27%	2793
	11th to 20th	Night	1 8% 3 11% 7% 6% 7% 6% 8% 9% 33%	2584
	21st to end	Night	2 7% 5% 9% 6% 7% 8% 6% 8% 9% 33%	2076
March	1st to 10th	Night	2 6% 3 7% 5% 5% 6% 5% 8% 8% 47%	5496
	11th to 20th	Night	2 8% 4 9% 6% 6% 6% 5% 7% 8% 39%	5121
	21st to end	Night	5% 12% 4 10% 6% 7% 8% 6% 8% 9% 26%	5595
April	1st to 10th	Night	5% 12% 4 7% 5% 5% 6% 5% 7% 9% 36%	2100
	11th to 20th	Night	6% 9% 3 9% 5% 5% 6% 5% 7% 8% 37%	1887
	21st to end	Night	8% 10% 2 8% 5% 5% 5% 4 7% 9% 38%	1890
May	1st to 10th	Night	8% 9% 2 5% 3 3 3 3 5% 7% 53%	5321
	11th to 20th	Night	8% 7% 2 7% 4 4 4 4 6% 7% 47%	5058
	21st to end	Night	11% 8% 2 5% 4 4 4 3 5% 7% 47%	4589
June	1st to 10th	Night	11% 7% 3 7% 5% 4 5% 4 6% 8% 41%	1470
	11th to 20th	Night	12% 7% 2 4 4 3 5% 4 5% 7% 45%	1470
	21st to end	Night	5% 2 1 4 3 3 4 4 6% 8% 61%	1470
July	1st to 10th	Night	2 1 1 1 2 1 3 3 6% 8% 73%	4140
	11th to 20th	Night	1 1 2 2 2 4 3 5% 7% 71%	4731
	21st to end	Night	1 1 1 2 2 2 3 3 4 7% 75%	5153
August	1st to 10th	Night	2 1 1 3 2 3 2 3 5% 8% 70%	5070
	11th to 20th	Night	2 2 1 2 2 3 4 3 6% 7% 69%	5291
	21st to end	Night	5% 3 1 3 3 3 3 3 5% 7% 64%	5842
September	1st to 10th	Night	4 3 1 4 3 3 4 3 5% 6% 65%	2150
	11th to 20th	Night	6% 3 1 5% 3 4 5% 4 7% 9% 54%	2310
	21st to end	Night	6% 4 2 4 3 3 4 3 5% 6% 60%	2288
October	1st to 10th	Night	5% 5% 2 4 3 3 4 3 5% 6% 61%	3660
	11th to 20th	Night	5% 5% 2 5% 4 4 5% 4 5% 7% 55%	4014
	21st to end	Night	5% 7% 2 7% 5% 5% 4 4 6% 9% 47%	4600
November	1st to 10th	Night	5% 7% 2 7% 5% 6% 6% 5% 7% 10% 39%	2665
	11th to 20th	Night	5% 9% 2 8% 6% 5% 7% 6% 9% 10% 34%	2821
	21st to end	Night	2 7% 3 6% 4 5% 6% 6% 8% 10% 43%	2982
December	1st to 10th	Night	1 6% 2 6% 4 4 6% 4 7% 8% 52%	5040
	11th to 20th	Night	2 6% 2 8% 6% 6% 8% 7% 10% 11% 34%	5696
	21st to end	Night	2 7% 3 10% 7% 7% 8% 6% 8% 9% 35%	7623

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	42% 18% 17% 18% 4	4520
	11th to 20th	Day	34% 8% 24% 23% 9% 1	4740
	21st to end	Day	31% 11% 20% 27% 10%	5187
February	1st to 10th	Day	58% 12% 13% 13% 4 1	1827
	11th to 20th	Day	54% 12% 15% 12% 6% 1	1929
	21st to end	Day	55% 10% 12% 13% 8% 1	1730
March	1st to 10th	Day	39% 9% 14% 20% 17% 1	4618
	11th to 20th	Day	39% 10% 16% 29% 6%	5099
	21st to end	Day	53% 14% 16% 16% 2	5607
April	1st to 10th	Day	54% 11% 15% 16% 3	2520
	11th to 20th	Day	49% 9% 15% 19% 7% 1	2684
	21st to end	Day	51% 11% 13% 18% 7%	2730
May	1st to 10th	Day	34% 6% 13% 31% 15% 1	7626
	11th to 20th	Day	36% 11% 15% 23% 14% 1	7820
	21st to end	Day	36% 8% 16% 28% 12%	9568
June	1st to 10th	Day	47% 10% 16% 19% 8% 1	3150
	11th to 20th	Day	42% 8% 13% 18% 15% 4	3150
	21st to end	Day	27% 8% 12% 29% 21% 3	3150
July	1st to 10th	Day	13% 5% 10% 24% 31% 17%	8746
	11th to 20th	Day	14% 4 5% 19% 28% 29%	8148
	21st to end	Day	12% 5% 11% 21% 32% 18%	8723
August	1st to 10th	Day	13% 5% 7% 21% 30% 24%	6133
	11th to 20th	Day	17% 8% 17% 28% 25% 5%	5964
	21st to end	Day	18% 6% 11% 36% 24% 6%	6556
September	1st to 10th	Day	22% 6% 11% 28% 24% 8%	2467
	11th to 20th	Day	31% 8% 12% 27% 17% 4	2310
	21st to end	Day	28% 6% 17% 29% 17% 2	2289
October	1st to 10th	Day	28% 6% 9% 36% 20% 1	3537
	11th to 20th	Day	31% 7% 19% 30% 12% 1	3130
	21st to end	Day	35% 10% 14% 26% 14%	3200
November	1st to 10th	Day	49% 11% 17% 17% 5% 1	1854
	11th to 20th	Day	46% 13% 19% 17% 4 1	1775
	21st to end	Day	42% 12% 17% 19% 8% 1	1637
December	1st to 10th	Day	31% 13% 26% 23% 7% 1	2360
	11th to 20th	Day	45% 15% 20% 15% 4	2714
	21st to end	Day	43% 10% 18% 20% 7% 3	3575
January	1st to 10th	Night	44% 16% 20% 17% 2	8432
	11th to 20th	Night	35% 9% 27% 20% 7% 2	8220
	21st to end	Night	30% 14% 20% 27% 8%	8994
February	1st to 10th	Night	57% 13% 14% 12% 4	2793
	11th to 20th	Night	51% 13% 15% 14% 6%	2585
	21st to end	Night	50% 13% 13% 16% 7% 1	2076
March	1st to 10th	Night	35% 10% 14% 24% 16% 1	5496
	11th to 20th	Night	40% 13% 15% 27% 5%	5121
	21st to end	Night	50% 16% 16% 15% 3	5595
April	1st to 10th	Night	47% 12% 18% 19% 4	2100
	11th to 20th	Night	46% 11% 16% 19% 7% 1	1887
	21st to end	Night	44% 12% 14% 21% 8%	1890
May	1st to 10th	Night	31% 7% 13% 37% 12%	5319
	11th to 20th	Night	31% 11% 17% 25% 15% 1	5058
	21st to end	Night	36% 8% 17% 32% 7%	4590
June	1st to 10th	Night	41% 11% 19% 21% 7% 1	1470
	11th to 20th	Night	39% 9% 14% 23% 13% 3	1470
	21st to end	Night	22% 9% 16% 33% 18% 2	1470
July	1st to 10th	Night	10% 4 12% 29% 31% 13%	4105
	11th to 20th	Night	12% 4 8% 25% 27% 25%	4684
	21st to end	Night	11% 4 13% 28% 24% 20%	5040
August	1st to 10th	Night	11% 4 9% 24% 33% 19%	4005
	11th to 20th	Night	15% 6% 19% 31% 23% 5%	4168
	21st to end	Night	17% 6% 13% 38% 23% 3	4578
September	1st to 10th	Night	22% 6% 12% 27% 25% 8%	2150
	11th to 20th	Night	28% 8% 14% 28% 17% 5%	2310
	21st to end	Night	28% 7% 15% 27% 20% 3	2288
October	1st to 10th	Night	25% 7% 11% 35% 21% 1	3660
	11th to 20th	Night	29% 9% 20% 30% 12% 1	4014
	21st to end	Night	34% 10% 17% 28% 11% 1	4600
November	1st to 10th	Night	43% 12% 17% 21% 5% 2	2665
	11th to 20th	Night	48% 13% 17% 17% 3 1	2821
	21st to end	Night	40% 12% 18% 20% 9% 1	2982
December	1st to 10th	Night	31% 11% 29% 21% 7% 1	5040
	11th to 20th	Night	42% 16% 23% 17% 2	5697
	21st to end	Night	44% 12% 18% 18% 7% 2	7623

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	8% 24% 25% 29% 12% 3	220
	11th to 20th	Day	7% 16% 38% 23% 10% 5%	220
	21st to end	Day	10% 20% 31% 28% 8% 3	242
February	1st to 10th	Day	12% 31% 34% 16% 5% 1	297
	11th to 20th	Day	8% 25% 33% 27% 7%	405
	21st to end	Day	14% 25% 36% 20% 4	446
March	1st to 10th	Day	12% 25% 35% 20% 7% 1	540
	11th to 20th	Day	15% 23% 27% 23% 13%	540
	21st to end	Day	15% 27% 30% 22% 5% 1	594
April	1st to 10th	Day	15% 26% 28% 22% 7% 1	540
	11th to 20th	Day	18% 26% 28% 21% 7% 1	532
	21st to end	Day	21% 22% 26% 19% 9% 2	540
May	1st to 10th	Day	15% 16% 30% 26% 12% 1	543
	11th to 20th	Day	23% 16% 27% 21% 11% 1	568
	21st to end	Day	19% 15% 20% 26% 16% 4	814
June	1st to 10th	Day	14% 15% 25% 28% 15% 3	780
	11th to 20th	Day	19% 19% 27% 22% 11% 1	780
	21st to end	Day	8% 14% 26% 31% 19% 3	780
July	1st to 10th	Day	3 12% 29% 30% 22% 4	771
	11th to 20th	Day	6% 15% 36% 28% 14% 2	567
	21st to end	Day	4 10% 32% 34% 16% 4	619
August	1st to 10th	Day	5% 10% 29% 34% 18% 5%	567
	11th to 20th	Day	5% 10% 25% 36% 18% 5%	557
	21st to end	Day	5% 12% 30% 31% 15% 7%	627
September	1st to 10th	Day	5% 12% 30% 28% 18% 8%	570
	11th to 20th	Day	8% 14% 30% 27% 17% 5%	570
	21st to end	Day	12% 16% 24% 25% 18% 6%	563
October	1st to 10th	Day	14% 18% 24% 25% 15% 3	493
	11th to 20th	Day	13% 20% 26% 22% 13% 7%	330
	21st to end	Day	14% 20% 32% 23% 9% 2	363
November	1st to 10th	Day	17% 20% 31% 22% 9% 1	330
	11th to 20th	Day	11% 21% 35% 21% 12% 1	330
	21st to end	Day	10% 17% 33% 33% 7% 1	307
December	1st to 10th	Day	12% 19% 29% 24% 12% 5%	220
	11th to 20th	Day	14% 27% 31% 23% 5%	220
	21st to end	Day	9% 22% 34% 26% 8% 2	242
January	1st to 10th	Night	8% 18% 32% 31% 10% 1	934
	11th to 20th	Night	7% 19% 35% 27% 10% 2	840
	21st to end	Night	7% 15% 35% 32% 10% 1	924
February	1st to 10th	Night	11% 17% 35% 28% 7% 1	840
	11th to 20th	Night	7% 19% 33% 30% 9% 2	824
	21st to end	Night	9% 17% 36% 28% 10% 1	692
March	1st to 10th	Night	7% 15% 35% 34% 9% 1	840
	11th to 20th	Night	10% 17% 31% 29% 13% 1	840
	21st to end	Night	10% 15% 31% 34% 9% 1	924
April	1st to 10th	Night	13% 13% 26% 33% 14% 2	840
	11th to 20th	Night	11% 17% 30% 31% 11%	639
	21st to end	Night	13% 15% 23% 36% 12% 1	630
May	1st to 10th	Night	12% 10% 22% 36% 19% 2	630
	11th to 20th	Night	13% 9% 20% 34% 22% 2	540
	21st to end	Night	18% 9% 14% 29% 28% 2	455
June	1st to 10th	Night	15% 7% 18% 37% 21% 1	420
	11th to 20th	Night	17% 8% 22% 40% 13%	420
	21st to end	Night	5% 5% 15% 51% 24% 1	420
July	1st to 10th	Night	2 1 11% 52% 31% 1	410
	11th to 20th	Night	4 1 13% 58% 23%	408
	21st to end	Night	2 2 10% 53% 33% 2	460
August	1st to 10th	Night	3 3 13% 48% 31% 2	553
	11th to 20th	Night	4 2 13% 47% 32% 3	624
	21st to end	Night	5% 4 17% 45% 27% 1	693
September	1st to 10th	Night	5% 4 15% 45% 27% 4	838
	11th to 20th	Night	7% 4 15% 41% 28% 5%	840
	21st to end	Night	9% 6% 17% 39% 25% 4	832
October	1st to 10th	Night	13% 9% 20% 38% 20% 2	800
	11th to 20th	Night	10% 10% 22% 37% 17% 3	800
	21st to end	Night	11% 14% 27% 32% 12% 3	880
November	1st to 10th	Night	12% 15% 29% 33% 10%	820
	11th to 20th	Night	14% 18% 31% 29% 8%	931
	21st to end	Night	8% 16% 32% 32% 10% 1	1050
December	1st to 10th	Night	10% 18% 30% 29% 11% 1	1050
	11th to 20th	Night	8% 17% 39% 28% 8%	1050
	21st to end	Night	8% 20% 38% 26% 7%	1155

Page generated at 2026-09-26 02:18:41. Version ac07

Copyright © 2026 Attilla Danko.