Hazeldale Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hazeldale Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	132038
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 45%	130446
Transparency	Day	38% 8% 12% 17% 17% 8%	135765
	Night	36% 7% 13% 18% 18% 9%	134781
Seeing	Day	14% 9% 22% 25% 20% 10%	16516
	Night	14% 7% 21% 30% 22% 6%	25957

By Month

Percentage Cloud Cover SE (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	18% 6% 3 6% 4 4 4 3 5% 6% 41%	5856
	11th to 20th	Day	11% 6% 2 5% 4 4 4 3 4 5% 52%	5880
	21st to end	Day	22% 7% 2 6% 4 4 4 4 4 5% 37%	6436
February	1st to 10th	Day	17% 8% 2 7% 4 3 5% 3 5% 5% 41%	2100
	11th to 20th	Day	17% 7% 2 6% 3 4 4 3 6% 7% 41%	2048
	21st to end	Day	16% 9% 3 7% 5% 4 4 4 5% 6% 38%	1730
March	1st to 10th	Day	18% 9% 4 7% 4 4 4 4 5% 6% 36%	4669
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 6% 7% 46%	5090
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 4 5% 5% 40%	5945
April	1st to 10th	Day	12% 7% 3 6% 6% 5% 5% 5% 6% 7% 38%	2520
	11th to 20th	Day	14% 7% 3 6% 4 4 5% 4 7% 6% 41%	2494
	21st to end	Day	9% 6% 2 6% 5% 4 5% 4 7% 7% 45%	2520
May	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 8% 33%	5417
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 7% 10% 43%	5404
	21st to end	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 13% 40%	5972
June	1st to 10th	Day	9% 6% 3 7% 8% 6% 7% 6% 7% 10% 31%	2600
	11th to 20th	Day	5% 5% 2 8% 7% 7% 7% 7% 9% 12% 32%	2657
	21st to end	Day	7% 6% 3 9% 8% 8% 8% 6% 10% 9% 26%	2629
July	1st to 10th	Day	5% 5% 2 9% 7% 8% 8% 6% 9% 12% 28%	5900
	11th to 20th	Day	5% 4 3 8% 6% 7% 9% 7% 9% 12% 31%	5589
	21st to end	Day	5% 5% 2 8% 7% 8% 8% 7% 10% 14% 25%	5922
August	1st to 10th	Day	3 3 2 5% 5% 5% 5% 5% 8% 9% 50%	3905
	11th to 20th	Day	6% 5% 2 6% 6% 6% 7% 6% 8% 11% 37%	3849
	21st to end	Day	7% 7% 3 10% 7% 7% 7% 5% 6% 7% 32%	4312
September	1st to 10th	Day	11% 5% 3 8% 7% 6% 6% 5% 6% 8% 35%	2400
	11th to 20th	Day	9% 8% 2 7% 5% 5% 5% 5% 6% 9% 39%	2309
	21st to end	Day	8% 4 2 7% 5% 4 5% 4 5% 6% 50%	2200
October	1st to 10th	Day	7% 4 2 6% 4 4 4 4 6% 6% 53%	2174
	11th to 20th	Day	6% 4 2 5% 5% 3 4 3 5% 7% 57%	2000
	21st to end	Day	10% 4 1 5% 4 3 3 2 5% 6% 57%	2200
November	1st to 10th	Day	9% 5% 2 5% 3 3 4 4 6% 6% 53%	2058
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 5% 6% 55%	1921
	21st to end	Day	11% 7% 2 7% 4 4 5% 3 6% 7% 45%	1890
December	1st to 10th	Day	18% 7% 2 5% 4 5% 4 3 4 5% 42%	3020
	11th to 20th	Day	11% 6% 2 6% 4 4 4 4 6% 7% 46%	3463
	21st to end	Day	18% 7% 2 6% 4 3 4 3 4 5% 44%	4959
January	1st to 10th	Night	19% 7% 3 5% 3 3 4 3 4 5% 44%	7037
	11th to 20th	Night	17% 8% 2 5% 3 3 3 3 4 6% 48%	7080
	21st to end	Night	22% 8% 3 7% 4 4 4 3 4 6% 36%	7745
February	1st to 10th	Night	17% 7% 2 6% 4 3 4 4 4 6% 43%	2520
	11th to 20th	Night	20% 8% 3 6% 4 3 3 3 5% 6% 40%	2458
	21st to end	Night	17% 8% 3 7% 4 3 3 3 5% 6% 39%	2076
March	1st to 10th	Night	20% 10% 3 7% 3 3 3 3 5% 6% 37%	5446
	11th to 20th	Night	14% 6% 2 4 4 3 4 3 5% 6% 48%	5090
	21st to end	Night	15% 7% 2 6% 4 3 4 4 4 5% 46%	5224
April	1st to 10th	Night	16% 7% 3 6% 4 2 4 4 5% 5% 44%	2100
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 5% 7% 47%	2075
	21st to end	Night	8% 5% 2 5% 4 3 3 4 6% 7% 55%	2100
May	1st to 10th	Night	15% 8% 3 7% 4 4 4 3 5% 8% 38%	4500
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 5% 8% 44%	4489
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 7% 13% 43%	4859
June	1st to 10th	Night	8% 6% 2 5% 4 5% 5% 6% 7% 11% 40%	1800
	11th to 20th	Night	2 3 2 5% 5% 5% 5% 6% 9% 12% 46%	1732
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 5% 8% 10% 42%	1758
July	1st to 10th	Night	3 4 2 6% 5% 6% 7% 6% 9% 14% 38%	4078
	11th to 20th	Night	3 3 2 6% 5% 5% 5% 5% 9% 14% 43%	4341
	21st to end	Night	3 4 2 7% 5% 6% 7% 6% 10% 14% 34%	4929
August	1st to 10th	Night	2 2 1 3 3 4 5% 5% 7% 12% 56%	3244
	11th to 20th	Night	3 3 2 5% 5% 5% 6% 5% 10% 13% 45%	3200
	21st to end	Night	5% 5% 2 7% 5% 7% 6% 6% 8% 10% 39%	3586
September	1st to 10th	Night	8% 6% 2 6% 5% 4 6% 5% 8% 11% 39%	2000
	11th to 20th	Night	8% 4 2 6% 5% 4 5% 4 6% 9% 46%	2070
	21st to end	Night	6% 6% 2 5% 5% 4 4 4 5% 7% 52%	2200
October	1st to 10th	Night	7% 5% 1 5% 5% 3 3 3 5% 7% 57%	2220
	11th to 20th	Night	7% 4 2 4 3 2 3 3 5% 7% 62%	2400
	21st to end	Night	11% 5% 2 5% 3 3 3 3 4 6% 57%	2640
November	1st to 10th	Night	10% 5% 2 5% 4 3 3 2 4 5% 57%	2460
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 4 5% 56%	2694
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 6% 49%	2730
December	1st to 10th	Night	15% 7% 2 7% 4 4 4 3 5% 6% 44%	4380
	11th to 20th	Night	12% 7% 2 5% 4 4 4 3 5% 6% 49%	4952
	21st to end	Night	20% 9% 2 6% 3 3 3 2 3 4 43%	6233

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% 5% 8% 16% 22% 6%	5857
	11th to 20th	Day	34% 4 5% 17% 29% 12%	5880
	21st to end	Day	49% 4 6% 12% 16% 13%	6436
February	1st to 10th	Day	45% 5% 7% 12% 21% 10%	2100
	11th to 20th	Day	41% 5% 8% 18% 18% 10%	2058
	21st to end	Day	46% 5% 7% 15% 17% 9%	1730
March	1st to 10th	Day	47% 5% 8% 16% 15% 10%	4669
	11th to 20th	Day	36% 6% 7% 15% 19% 16%	5090
	21st to end	Day	43% 6% 11% 15% 16% 9%	5947
April	1st to 10th	Day	42% 7% 10% 15% 16% 9%	2520
	11th to 20th	Day	40% 6% 9% 17% 20% 8%	2494
	21st to end	Day	34% 6% 12% 21% 20% 7%	2520
May	1st to 10th	Day	43% 9% 14% 21% 11% 3	5452
	11th to 20th	Day	33% 7% 17% 24% 18% 1	5408
	21st to end	Day	31% 9% 17% 28% 12% 2	5986
June	1st to 10th	Day	44% 11% 14% 21% 9% 1	2600
	11th to 20th	Day	39% 11% 17% 20% 11% 2	2657
	21st to end	Day	46% 13% 16% 16% 7% 1	2629
July	1st to 10th	Day	42% 15% 24% 16% 3	5836
	11th to 20th	Day	38% 17% 33% 10% 1	5522
	21st to end	Day	42% 16% 21% 19% 2	5752
August	1st to 10th	Day	27% 10% 15% 26% 23% 1	3893
	11th to 20th	Day	35% 11% 19% 24% 10% 1	3885
	21st to end	Day	48% 10% 13% 14% 13% 2	4311
September	1st to 10th	Day	44% 10% 15% 16% 12% 4	2400
	11th to 20th	Day	40% 7% 14% 17% 15% 7%	2309
	21st to end	Day	33% 7% 12% 20% 20% 9%	2200
October	1st to 10th	Day	28% 4 7% 11% 21% 29%	3538
	11th to 20th	Day	17% 3 5% 16% 28% 30%	3240
	21st to end	Day	20% 4 5% 12% 37% 21%	3538
November	1st to 10th	Day	30% 5% 9% 18% 26% 13%	2058
	11th to 20th	Day	30% 4 7% 17% 28% 13%	1921
	21st to end	Day	38% 5% 8% 18% 21% 11%	1890
December	1st to 10th	Day	44% 4 7% 14% 24% 7%	3020
	11th to 20th	Day	35% 6% 10% 16% 22% 10%	3460
	21st to end	Day	42% 5% 6% 16% 25% 5%	4959
January	1st to 10th	Night	41% 5% 10% 14% 22% 8%	7038
	11th to 20th	Night	39% 3 5% 16% 26% 12%	7080
	21st to end	Night	50% 3 6% 13% 19% 9%	7745
February	1st to 10th	Night	41% 4 8% 16% 21% 9%	2520
	11th to 20th	Night	46% 4 7% 13% 19% 11%	2459
	21st to end	Night	44% 6% 10% 14% 15% 12%	2076
March	1st to 10th	Night	47% 5% 9% 12% 18% 9%	5446
	11th to 20th	Night	36% 6% 9% 15% 20% 15%	5090
	21st to end	Night	38% 6% 10% 20% 17% 8%	5224
April	1st to 10th	Night	41% 6% 9% 16% 17% 11%	2100
	11th to 20th	Night	36% 6% 8% 22% 18% 9%	2075
	21st to end	Night	28% 6% 13% 23% 23% 8%	2100
May	1st to 10th	Night	43% 8% 14% 18% 14% 3	4530
	11th to 20th	Night	36% 7% 18% 23% 15% 1	4490
	21st to end	Night	29% 9% 23% 28% 10% 1	4867
June	1st to 10th	Night	33% 12% 19% 24% 11% 1	1800
	11th to 20th	Night	25% 13% 23% 28% 10% 1	1732
	21st to end	Night	34% 12% 24% 21% 9% 1	1758
July	1st to 10th	Night	31% 16% 30% 18% 4	4035
	11th to 20th	Night	27% 19% 42% 11% 1	4288
	21st to end	Night	35% 18% 21% 24% 2	4785
August	1st to 10th	Night	17% 11% 22% 28% 22% 1	3232
	11th to 20th	Night	25% 14% 27% 26% 8%	3230
	21st to end	Night	36% 14% 18% 18% 10% 4	3586
September	1st to 10th	Night	36% 11% 19% 17% 12% 5%	2000
	11th to 20th	Night	32% 10% 17% 17% 16% 8%	2070
	21st to end	Night	30% 7% 14% 23% 16% 10%	2200
October	1st to 10th	Night	30% 5% 8% 13% 21% 23%	3623
	11th to 20th	Night	15% 4 6% 18% 27% 30%	3899
	21st to end	Night	20% 3 5% 11% 37% 24%	4252
November	1st to 10th	Night	30% 4 8% 18% 25% 15%	2460
	11th to 20th	Night	30% 4 7% 17% 27% 15%	2694
	21st to end	Night	35% 5% 8% 19% 20% 13%	2730
December	1st to 10th	Night	42% 5% 7% 15% 24% 8%	4380
	11th to 20th	Night	36% 4 11% 18% 20% 11%	4954
	21st to end	Night	44% 5% 7% 16% 21% 7%	6233

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	25% 17% 38% 15% 5% 1	330
	11th to 20th	Day	23% 19% 37% 16% 5%	330
	21st to end	Day	28% 20% 36% 13% 3	363
February	1st to 10th	Day	27% 18% 36% 14% 4 1	330
	11th to 20th	Day	29% 22% 30% 15% 4	330
	21st to end	Day	19% 30% 38% 10% 3	273
March	1st to 10th	Day	18% 22% 40% 17% 2	330
	11th to 20th	Day	26% 21% 34% 14% 5% 1	330
	21st to end	Day	17% 19% 29% 25% 9% 1	518
April	1st to 10th	Day	18% 17% 33% 23% 9% 1	540
	11th to 20th	Day	20% 15% 29% 23% 11% 2	532
	21st to end	Day	11% 12% 31% 28% 14% 4	540
May	1st to 10th	Day	15% 10% 29% 25% 19% 3	507
	11th to 20th	Day	12% 7% 25% 30% 19% 7%	527
	21st to end	Day	7% 2 21% 36% 23% 11%	573
June	1st to 10th	Day	11% 5% 14% 33% 24% 14%	530
	11th to 20th	Day	6% 3 14% 33% 29% 15%	587
	21st to end	Day	9% 3 14% 31% 25% 19%	730
July	1st to 10th	Day	8% 2 7% 30% 35% 18%	721
	11th to 20th	Day	7% 2 4 21% 38% 28%	591
	21st to end	Day	7% 1 6% 20% 40% 24%	576
August	1st to 10th	Day	4 2 6% 24% 36% 29%	526
	11th to 20th	Day	8% 2 5% 18% 38% 29%	519
	21st to end	Day	13% 4 7% 23% 31% 22%	583
September	1st to 10th	Day	10% 2 9% 26% 35% 17%	530
	11th to 20th	Day	11% 4 10% 26% 34% 15%	530
	21st to end	Day	9% 5% 13% 31% 33% 8%	530
October	1st to 10th	Day	7% 7% 20% 38% 23% 5%	504
	11th to 20th	Day	5% 8% 28% 32% 22% 4	330
	21st to end	Day	18% 9% 25% 34% 11% 3	363
November	1st to 10th	Day	16% 8% 27% 33% 15% 1	330
	11th to 20th	Day	16% 11% 34% 25% 11% 2	330
	21st to end	Day	15% 18% 39% 19% 7% 2	330
December	1st to 10th	Day	22% 15% 33% 23% 6% 1	330
	11th to 20th	Day	21% 15% 38% 21% 6%	330
	21st to end	Day	26% 14% 31% 19% 9% 1	363
January	1st to 10th	Night	22% 12% 32% 28% 6%	840
	11th to 20th	Night	20% 13% 30% 30% 6%	840
	21st to end	Night	24% 14% 30% 27% 4	924
February	1st to 10th	Night	20% 11% 35% 28% 5%	840
	11th to 20th	Night	25% 15% 28% 25% 6%	823
	21st to end	Night	22% 17% 35% 23% 3	692
March	1st to 10th	Night	19% 15% 32% 28% 6%	819
	11th to 20th	Night	17% 9% 30% 32% 12%	630
	21st to end	Night	15% 10% 32% 34% 10%	693
April	1st to 10th	Night	20% 11% 28% 31% 11%	630
	11th to 20th	Night	13% 9% 33% 29% 15% 1	623
	21st to end	Night	9% 7% 28% 38% 18% 1	630
May	1st to 10th	Night	12% 4 22% 42% 19% 1	603
	11th to 20th	Night	8% 5% 19% 40% 24% 4	591
	21st to end	Night	6% 2 15% 37% 34% 6%	650
June	1st to 10th	Night	9% 3 9% 35% 36% 8%	600
	11th to 20th	Night	3 2 8% 31% 44% 12%	600
	21st to end	Night	8% 1 7% 26% 42% 16%	600
July	1st to 10th	Night	5% 2 4 19% 50% 20%	585
	11th to 20th	Night	4 1 2 14% 50% 29%	582
	21st to end	Night	5% 1 3 19% 50% 22%	657
August	1st to 10th	Night	2 2 1 17% 50% 28%	597
	11th to 20th	Night	5% 1 3 14% 48% 29%	594
	21st to end	Night	7% 2 5% 19% 43% 23%	660
September	1st to 10th	Night	9% 3 6% 23% 44% 17%	600
	11th to 20th	Night	10% 4 6% 28% 43% 9%	670
	21st to end	Night	8% 4 10% 35% 37% 7%	800
October	1st to 10th	Night	8% 2 19% 38% 29% 4	800
	11th to 20th	Night	8% 5% 21% 37% 26% 3	800
	21st to end	Night	13% 6% 20% 37% 22% 2	880
November	1st to 10th	Night	13% 7% 19% 41% 19% 2	820
	11th to 20th	Night	15% 8% 27% 34% 15% 2	840
	21st to end	Night	15% 8% 30% 32% 14% 1	840
December	1st to 10th	Night	18% 10% 30% 28% 13% 1	840
	11th to 20th	Night	20% 8% 30% 32% 9%	840
	21st to end	Night	25% 11% 29% 27% 7%	924

Page generated at 2026-04-29 01:21:43. Version ac07

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