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 2025-01-07.

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% 39%	105078
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 45%	103884
Transparency	Day	39% 8% 12% 17% 17% 7%	108841
	Night	35% 7% 13% 18% 18% 9%	108247
Seeing	Day	14% 9% 21% 25% 21% 10%	14813
	Night	14% 7% 21% 30% 23% 6%	24282

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	17% 7% 2 5% 4 4 4 3 5% 7% 42%	3216
	11th to 20th	Day	13% 5% 2 4 3 3 3 3 4 5% 55%	3160
	21st to end	Day	26% 7% 2 5% 3 3 3 3 4 5% 38%	3476
February	1st to 10th	Day	18% 8% 2 7% 4 3 5% 3 5% 5% 39%	1900
	11th to 20th	Day	18% 7% 3 5% 3 3 3 3 6% 7% 42%	1848
	21st to end	Day	16% 9% 3 7% 5% 4 4 4 6% 6% 37%	1570
March	1st to 10th	Day	23% 9% 4 7% 3 4 4 4 6% 7% 29%	3179
	11th to 20th	Day	14% 8% 3 8% 5% 5% 5% 4 7% 8% 35%	3480
	21st to end	Day	11% 6% 3 7% 5% 6% 6% 5% 6% 6% 40%	4057
April	1st to 10th	Day	12% 6% 2 7% 6% 5% 5% 5% 6% 7% 39%	2280
	11th to 20th	Day	15% 7% 3 7% 4 4 5% 4 7% 6% 39%	2254
	21st to end	Day	9% 6% 1 6% 6% 4 5% 4 7% 6% 47%	2280
May	1st to 10th	Day	11% 5% 2 8% 6% 6% 6% 5% 6% 9% 36%	3765
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 4 6% 8% 38%	3764
	21st to end	Day	5% 4 2 6% 5% 6% 6% 5% 7% 12% 42%	4155
June	1st to 10th	Day	10% 6% 3 7% 8% 6% 7% 6% 7% 10% 30%	2470
	11th to 20th	Day	5% 5% 2 8% 6% 7% 7% 7% 8% 12% 32%	2524
	21st to end	Day	7% 7% 3 9% 8% 8% 8% 6% 10% 9% 26%	2498
July	1st to 10th	Day	7% 6% 3 10% 8% 8% 9% 6% 9% 11% 22%	4120
	11th to 20th	Day	6% 5% 3 8% 7% 8% 9% 7% 9% 10% 26%	3907
	21st to end	Day	7% 7% 3 10% 8% 8% 8% 7% 9% 12% 22%	4118
August	1st to 10th	Day	3 3 2 5% 5% 5% 5% 5% 7% 9% 51%	3785
	11th to 20th	Day	7% 5% 2 7% 6% 6% 7% 6% 8% 11% 36%	3729
	21st to end	Day	8% 7% 3 10% 7% 7% 7% 5% 6% 7% 32%	4180
September	1st to 10th	Day	11% 5% 3 8% 7% 6% 6% 5% 6% 8% 33%	2280
	11th to 20th	Day	10% 8% 2 8% 5% 6% 5% 5% 6% 8% 38%	2194
	21st to end	Day	8% 5% 2 7% 5% 4 5% 4 5% 6% 49%	2090
October	1st to 10th	Day	7% 4 2 6% 4 4 4 4 5% 6% 54%	2066
	11th to 20th	Day	7% 4 2 5% 5% 3 4 4 5% 7% 55%	1900
	21st to end	Day	10% 4 1 5% 4 3 3 2 5% 6% 56%	2090
November	1st to 10th	Day	9% 5% 2 5% 4 3 4 4 6% 6% 52%	1958
	11th to 20th	Day	11% 5% 2 4 4 3 3 3 5% 6% 55%	1830
	21st to end	Day	12% 7% 2 7% 5% 4 5% 3 6% 6% 45%	1800
December	1st to 10th	Day	18% 7% 2 5% 4 5% 4 3 4 5% 42%	2930
	11th to 20th	Day	11% 7% 2 6% 4 4 4 4 6% 7% 45%	3373
	21st to end	Day	18% 7% 2 6% 4 3 4 3 4 5% 43%	4852
January	1st to 10th	Night	17% 7% 2 4 3 3 4 3 4 5% 48%	3862
	11th to 20th	Night	16% 7% 2 3 2 2 3 3 4 5% 54%	3800
	21st to end	Night	26% 8% 2 5% 2 2 3 3 4 5% 40%	4180
February	1st to 10th	Night	18% 7% 2 6% 4 3 4 4 4 6% 42%	2280
	11th to 20th	Night	19% 8% 2 6% 3 3 3 3 5% 6% 42%	2218
	21st to end	Night	18% 9% 3 7% 4 3 4 3 5% 6% 37%	1884
March	1st to 10th	Night	24% 11% 4 7% 4 3 3 3 4 5% 31%	3701
	11th to 20th	Night	16% 7% 2 6% 5% 4 5% 4 6% 7% 38%	3480
	21st to end	Night	14% 7% 2 6% 4 3 4 4 5% 6% 45%	3572
April	1st to 10th	Night	15% 6% 3 5% 4 3 4 3 5% 6% 45%	1900
	11th to 20th	Night	13% 6% 2 7% 5% 4 4 4 5% 7% 44%	1875
	21st to end	Night	7% 4 2 5% 4 3 3 4 6% 6% 57%	1900
May	1st to 10th	Night	12% 7% 2 7% 4 3 4 3 5% 8% 45%	3130
	11th to 20th	Night	14% 6% 3 6% 4 5% 4 4 6% 7% 42%	3129
	21st to end	Night	6% 3 1 4 4 4 4 5% 8% 14% 45%	3377
June	1st to 10th	Night	8% 6% 2 6% 4 5% 6% 5% 7% 11% 40%	1710
	11th to 20th	Night	2 3 2 5% 5% 4 5% 6% 9% 12% 47%	1646
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 5% 8% 10% 42%	1670
July	1st to 10th	Night	4 5% 2 6% 5% 6% 7% 6% 9% 15% 35%	2850
	11th to 20th	Night	3 3 2 6% 5% 6% 6% 5% 10% 13% 42%	3036
	21st to end	Night	5% 5% 3 8% 5% 6% 7% 6% 10% 15% 29%	3435
August	1st to 10th	Night	2 2 1 3 3 4 5% 5% 7% 12% 56%	3144
	11th to 20th	Night	3 3 2 5% 4 5% 6% 5% 10% 13% 45%	3100
	21st to end	Night	5% 5% 2 7% 5% 7% 6% 6% 8% 10% 39%	3476
September	1st to 10th	Night	8% 7% 2 7% 5% 5% 6% 5% 8% 11% 38%	1900
	11th to 20th	Night	9% 4 2 7% 5% 4 5% 4 6% 9% 45%	1966
	21st to end	Night	6% 6% 2 5% 5% 4 4 4 5% 7% 52%	2090
October	1st to 10th	Night	6% 4 1 5% 5% 3 3 3 5% 7% 58%	2109
	11th to 20th	Night	7% 4 2 4 3 2 3 3 5% 7% 61%	2280
	21st to end	Night	11% 5% 2 4 3 3 3 3 4 5% 58%	2508
November	1st to 10th	Night	10% 6% 2 5% 4 3 3 2 4 5% 56%	2340
	11th to 20th	Night	12% 5% 2 4 3 3 3 3 5% 5% 55%	2565
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 6% 50%	2600
December	1st to 10th	Night	15% 7% 2 7% 4 4 4 3 5% 6% 44%	4250
	11th to 20th	Night	13% 7% 2 5% 4 4 4 3 5% 6% 48%	4822
	21st to end	Night	20% 9% 2 6% 3 3 3 2 4 4 43%	6099

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	40% 5% 7% 15% 23% 11%	3217
	11th to 20th	Day	32% 5% 6% 17% 31% 10%	3160
	21st to end	Day	50% 4 4 14% 20% 9%	3476
February	1st to 10th	Day	47% 5% 8% 11% 19% 10%	1900
	11th to 20th	Day	41% 4 8% 17% 18% 11%	1858
	21st to end	Day	47% 5% 8% 15% 16% 9%	1570
March	1st to 10th	Day	53% 6% 9% 16% 12% 4	3179
	11th to 20th	Day	44% 7% 9% 18% 17% 5%	3480
	21st to end	Day	40% 7% 12% 13% 19% 9%	4057
April	1st to 10th	Day	41% 7% 10% 15% 17% 9%	2280
	11th to 20th	Day	42% 6% 9% 17% 18% 8%	2254
	21st to end	Day	34% 6% 10% 21% 21% 7%	2280
May	1st to 10th	Day	38% 10% 15% 20% 13% 4	3800
	11th to 20th	Day	41% 7% 12% 18% 21% 2	3768
	21st to end	Day	30% 9% 17% 31% 11% 2	4168
June	1st to 10th	Day	45% 11% 14% 20% 9% 2	2470
	11th to 20th	Day	39% 10% 17% 20% 11% 2	2524
	21st to end	Day	47% 13% 15% 16% 8% 1	2498
July	1st to 10th	Day	49% 15% 20% 14% 2	4056
	11th to 20th	Day	43% 17% 29% 8% 2 1	3840
	21st to end	Day	49% 15% 19% 14% 3	3987
August	1st to 10th	Day	26% 9% 14% 26% 23% 1	3773
	11th to 20th	Day	36% 11% 18% 24% 10% 1	3765
	21st to end	Day	48% 10% 12% 14% 13% 2	4179
September	1st to 10th	Day	46% 10% 14% 15% 11% 4	2280
	11th to 20th	Day	42% 7% 15% 15% 14% 7%	2194
	21st to end	Day	34% 7% 11% 20% 19% 9%	2090
October	1st to 10th	Day	27% 4 7% 11% 21% 30%	3430
	11th to 20th	Day	17% 3 5% 16% 28% 30%	3140
	21st to end	Day	20% 4 5% 12% 37% 22%	3428
November	1st to 10th	Day	30% 5% 10% 18% 26% 12%	1958
	11th to 20th	Day	31% 4 6% 16% 28% 13%	1830
	21st to end	Day	38% 4 7% 18% 21% 12%	1800
December	1st to 10th	Day	44% 4 7% 14% 24% 7%	2930
	11th to 20th	Day	35% 6% 11% 16% 22% 10%	3370
	21st to end	Day	42% 5% 6% 16% 25% 5%	4852
January	1st to 10th	Night	38% 4 7% 14% 26% 11%	3862
	11th to 20th	Night	35% 3 5% 18% 28% 11%	3800
	21st to end	Night	47% 3 6% 14% 22% 8%	4180
February	1st to 10th	Night	42% 4 9% 17% 20% 9%	2280
	11th to 20th	Night	43% 4 6% 14% 20% 12%	2219
	21st to end	Night	46% 6% 10% 13% 13% 12%	1884
March	1st to 10th	Night	54% 6% 9% 12% 13% 6%	3701
	11th to 20th	Night	43% 7% 11% 16% 17% 6%	3480
	21st to end	Night	38% 7% 10% 20% 18% 7%	3572
April	1st to 10th	Night	40% 6% 10% 16% 17% 11%	1900
	11th to 20th	Night	38% 6% 9% 20% 17% 9%	1875
	21st to end	Night	26% 6% 12% 23% 25% 9%	1900
May	1st to 10th	Night	37% 8% 14% 20% 17% 4	3160
	11th to 20th	Night	39% 7% 14% 21% 17% 2	3130
	21st to end	Night	25% 10% 23% 27% 12% 2	3385
June	1st to 10th	Night	34% 12% 19% 24% 10% 1	1710
	11th to 20th	Night	25% 13% 22% 29% 10% 1	1646
	21st to end	Night	34% 12% 23% 21% 10% 1	1670
July	1st to 10th	Night	33% 17% 31% 15% 4	2805
	11th to 20th	Night	27% 19% 41% 10% 2	2980
	21st to end	Night	39% 19% 21% 18% 3	3325
August	1st to 10th	Night	17% 11% 22% 27% 23% 1	3132
	11th to 20th	Night	25% 14% 26% 26% 8% 1	3130
	21st to end	Night	36% 14% 18% 18% 10% 4	3476
September	1st to 10th	Night	37% 11% 19% 16% 11% 6%	1900
	11th to 20th	Night	33% 11% 17% 16% 14% 8%	1966
	21st to end	Night	30% 7% 13% 23% 16% 10%	2090
October	1st to 10th	Night	29% 5% 8% 13% 21% 24%	3512
	11th to 20th	Night	15% 4 6% 19% 27% 30%	3779
	21st to end	Night	19% 3 5% 11% 37% 25%	4120
November	1st to 10th	Night	31% 4 8% 19% 24% 15%	2340
	11th to 20th	Night	31% 4 7% 16% 27% 15%	2565
	21st to end	Night	36% 4 7% 19% 21% 13%	2600
December	1st to 10th	Night	41% 5% 7% 15% 24% 8%	4250
	11th to 20th	Night	36% 4 11% 18% 20% 11%	4824
	21st to end	Night	44% 5% 7% 16% 21% 7%	6099

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	26% 16% 37% 15% 6% 1	288
	11th to 20th	Day	24% 17% 36% 17% 6%	270
	21st to end	Day	27% 21% 38% 11% 3	297
February	1st to 10th	Day	30% 20% 32% 14% 4 1	270
	11th to 20th	Day	30% 20% 29% 16% 4	270
	21st to end	Day	20% 31% 37% 11% 1	225
March	1st to 10th	Day	18% 23% 41% 16% 2	270
	11th to 20th	Day	26% 20% 34% 14% 4	270
	21st to end	Day	18% 21% 31% 21% 9%	437
April	1st to 10th	Day	16% 18% 33% 23% 9% 1	460
	11th to 20th	Day	21% 17% 27% 22% 11% 2	452
	21st to end	Day	12% 11% 30% 29% 15% 3	460
May	1st to 10th	Day	13% 10% 29% 25% 19% 3	463
	11th to 20th	Day	13% 6% 22% 31% 20% 8%	487
	21st to end	Day	7% 2 19% 36% 24% 12%	529
June	1st to 10th	Day	11% 5% 13% 32% 23% 16%	490
	11th to 20th	Day	7% 3 14% 33% 29% 14%	544
	21st to end	Day	9% 3 15% 30% 25% 18%	680
July	1st to 10th	Day	8% 2 7% 28% 35% 19%	671
	11th to 20th	Day	7% 2 4 21% 39% 27%	548
	21st to end	Day	8% 1 6% 20% 40% 24%	532
August	1st to 10th	Day	5% 2 6% 23% 35% 30%	486
	11th to 20th	Day	9% 3 5% 18% 38% 27%	479
	21st to end	Day	13% 4 7% 23% 31% 22%	539
September	1st to 10th	Day	11% 2 8% 25% 36% 18%	490
	11th to 20th	Day	12% 5% 10% 27% 33% 13%	490
	21st to end	Day	10% 5% 13% 29% 34% 9%	490
October	1st to 10th	Day	7% 6% 19% 38% 24% 5%	466
	11th to 20th	Day	6% 8% 29% 32% 21% 4	300
	21st to end	Day	17% 9% 23% 35% 12% 4	330
November	1st to 10th	Day	17% 7% 28% 34% 14% 1	300
	11th to 20th	Day	18% 12% 33% 25% 11% 1	300
	21st to end	Day	17% 16% 39% 19% 7% 2	300
December	1st to 10th	Day	20% 14% 34% 24% 7% 1	300
	11th to 20th	Day	21% 16% 37% 20% 6%	300
	21st to end	Day	28% 14% 32% 19% 7% 1	330
January	1st to 10th	Night	22% 12% 31% 28% 6%	785
	11th to 20th	Night	20% 12% 30% 31% 7%	760
	21st to end	Night	24% 14% 30% 28% 5%	836
February	1st to 10th	Night	21% 11% 35% 27% 6%	760
	11th to 20th	Night	24% 14% 29% 27% 6%	743
	21st to end	Night	22% 16% 36% 24% 2	628
March	1st to 10th	Night	18% 14% 33% 28% 7%	741
	11th to 20th	Night	17% 8% 31% 33% 11%	570
	21st to end	Night	14% 11% 32% 33% 10%	627
April	1st to 10th	Night	18% 11% 28% 30% 12%	570
	11th to 20th	Night	14% 9% 33% 28% 15% 1	563
	21st to end	Night	9% 6% 28% 39% 18% 1	570
May	1st to 10th	Night	11% 4 22% 42% 20% 1	570
	11th to 20th	Night	8% 4 17% 41% 25% 4	561
	21st to end	Night	6% 2 15% 37% 34% 6%	617
June	1st to 10th	Night	9% 2 9% 34% 37% 9%	570
	11th to 20th	Night	4 2 8% 30% 44% 13%	570
	21st to end	Night	8% 1 8% 26% 41% 16%	570
July	1st to 10th	Night	5% 2 4 19% 50% 20%	555
	11th to 20th	Night	4 1 2 14% 51% 28%	552
	21st to end	Night	5% 1 3 19% 50% 22%	624
August	1st to 10th	Night	2 1 1 18% 50% 28%	567
	11th to 20th	Night	5% 3 14% 49% 29%	564
	21st to end	Night	7% 2 6% 19% 44% 22%	627
September	1st to 10th	Night	9% 3 5% 21% 44% 17%	570
	11th to 20th	Night	11% 4 7% 28% 42% 9%	636
	21st to end	Night	9% 3 9% 34% 37% 7%	760
October	1st to 10th	Night	8% 2 18% 38% 29% 4	760
	11th to 20th	Night	8% 5% 22% 37% 25% 3	760
	21st to end	Night	13% 6% 20% 36% 22% 2	836
November	1st to 10th	Night	13% 7% 19% 40% 19% 2	780
	11th to 20th	Night	16% 8% 27% 33% 15% 2	800
	21st to end	Night	15% 8% 28% 33% 15% 1	800
December	1st to 10th	Night	17% 10% 30% 29% 14% 1	800
	11th to 20th	Night	21% 8% 30% 32% 9%	800
	21st to end	Night	26% 11% 29% 27% 7%	880

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