Blair Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blair Hill Observatory clear sky chart from 2005-11-06 to 2024-12-24.

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	12% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	101653
	Night	12% 8% 3 7% 5% 4 5% 4 6% 8% 40%	98418
Transparency	Day	42% 8% 12% 21% 14% 3	105316
	Night	39% 8% 12% 22% 15% 4	102881
Seeing	Day	19% 13% 23% 22% 16% 7%	15404
	Night	17% 11% 23% 30% 17% 2	25522

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	14% 8% 4 8% 5% 5% 4 3 5% 7% 37%	2836
	11th to 20th	Day	11% 10% 5% 8% 7% 5% 4 4 6% 6% 34%	2840
	21st to end	Day	15% 12% 4 9% 5% 5% 4 3 4 4 35%	3124
February	1st to 10th	Day	10% 9% 4 9% 6% 5% 5% 4 5% 6% 36%	1710
	11th to 20th	Day	16% 7% 3 9% 6% 5% 5% 5% 4 7% 34%	1742
	21st to end	Day	17% 9% 4 8% 6% 4 4 4 5% 5% 35%	1619
March	1st to 10th	Day	18% 9% 3 8% 4 5% 5% 4 5% 7% 34%	3435
	11th to 20th	Day	10% 6% 3 6% 5% 4 5% 4 5% 6% 47%	3480
	21st to end	Day	15% 10% 3 6% 5% 4 4 4 5% 6% 38%	3815
April	1st to 10th	Day	9% 7% 3 6% 5% 5% 4 4 5% 8% 44%	2133
	11th to 20th	Day	12% 8% 3 6% 4 4 5% 4 6% 7% 42%	2254
	21st to end	Day	14% 7% 3 7% 6% 5% 5% 4 5% 6% 39%	2441
May	1st to 10th	Day	13% 8% 4 10% 6% 6% 6% 5% 7% 8% 27%	4082
	11th to 20th	Day	17% 8% 3 8% 6% 5% 5% 4 6% 9% 29%	4081
	21st to end	Day	7% 5% 2 7% 5% 5% 6% 5% 7% 11% 38%	4476
June	1st to 10th	Day	9% 6% 2 6% 6% 6% 6% 5% 7% 10% 38%	2470
	11th to 20th	Day	7% 6% 2 7% 6% 6% 6% 5% 8% 11% 37%	2470
	21st to end	Day	8% 6% 2 6% 6% 6% 6% 5% 8% 9% 37%	2470
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 7% 6% 9% 11% 36%	4120
	11th to 20th	Day	8% 6% 2 8% 6% 5% 6% 5% 8% 11% 35%	4120
	21st to end	Day	12% 10% 4 10% 8% 8% 7% 5% 7% 8% 23%	4466
August	1st to 10th	Day	12% 7% 2 8% 6% 7% 7% 5% 8% 10% 28%	4104
	11th to 20th	Day	8% 5% 3 7% 6% 6% 7% 7% 9% 10% 33%	4044
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 7% 10% 47%	4254
September	1st to 10th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 7% 43%	2199
	11th to 20th	Day	15% 6% 3 6% 5% 5% 5% 5% 6% 8% 37%	2090
	21st to end	Day	18% 8% 3 7% 5% 5% 4 4 6% 6% 34%	2090
October	1st to 10th	Day	15% 6% 2 5% 3 3 4 2 5% 7% 49%	1936
	11th to 20th	Day	12% 7% 2 7% 3 4 4 4 5% 6% 46%	1900
	21st to end	Day	12% 7% 2 6% 4 4 4 3 5% 6% 47%	2090
November	1st to 10th	Day	11% 6% 2 5% 4 4 4 3 6% 7% 49%	1957
	11th to 20th	Day	10% 7% 2 6% 4 4 4 4 5% 8% 45%	1811
	21st to end	Day	12% 8% 2 6% 5% 5% 3 3 5% 6% 45%	1800
December	1st to 10th	Day	14% 10% 3 8% 5% 5% 4 5% 5% 7% 34%	1800
	11th to 20th	Day	14% 9% 2 7% 3 4 4 3 6% 8% 41%	2243
	21st to end	Day	14% 11% 3 7% 5% 5% 5% 5% 7% 7% 30%	3151
January	1st to 10th	Night	11% 10% 6% 9% 5% 5% 5% 4 7% 6% 31%	4109
	11th to 20th	Night	13% 8% 3 9% 5% 4 4 3 4 6% 40%	4120
	21st to end	Night	17% 14% 3 6% 5% 4 4 4 5% 6% 35%	4532
February	1st to 10th	Night	13% 8% 3 7% 5% 4 5% 4 6% 7% 39%	2470
	11th to 20th	Night	18% 8% 3 7% 6% 4 5% 3 5% 6% 34%	2323
	21st to end	Night	12% 9% 3 7% 6% 5% 5% 4 5% 7% 37%	1833
March	1st to 10th	Night	19% 9% 4 9% 5% 5% 5% 4 6% 5% 31%	3418
	11th to 20th	Night	11% 7% 3 6% 4 4 4 4 6% 7% 44%	3480
	21st to end	Night	14% 8% 2 5% 4 3 4 3 6% 7% 45%	3817
April	1st to 10th	Night	10% 7% 3 7% 5% 4 5% 4 7% 8% 41%	2046
	11th to 20th	Night	12% 5% 2 6% 5% 3 3 4 5% 7% 46%	1875
	21st to end	Night	13% 7% 2 5% 5% 4 4 3 5% 7% 46%	1753
May	1st to 10th	Night	12% 9% 4 8% 5% 5% 5% 4 6% 8% 35%	2813
	11th to 20th	Night	18% 9% 4 8% 5% 4 4 3 5% 7% 33%	2812
	21st to end	Night	8% 5% 2 7% 4 4 5% 5% 7% 11% 41%	3079
June	1st to 10th	Night	7% 5% 2 5% 5% 5% 6% 5% 7% 12% 41%	1710
	11th to 20th	Night	8% 6% 2 5% 4 4 5% 5% 7% 10% 45%	1710
	21st to end	Night	6% 4 2 5% 4 5% 5% 4 8% 11% 45%	1710
July	1st to 10th	Night	5% 5% 2 6% 5% 6% 6% 5% 8% 11% 41%	2840
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 5% 7% 11% 44%	2840
	21st to end	Night	8% 8% 3 9% 7% 6% 6% 5% 7% 11% 30%	3087
August	1st to 10th	Night	13% 7% 2 9% 5% 5% 5% 5% 7% 11% 33%	2825
	11th to 20th	Night	8% 5% 3 6% 6% 5% 6% 5% 7% 10% 39%	2785
	21st to end	Night	5% 3 1 5% 4 5% 5% 4 6% 9% 53%	3369
September	1st to 10th	Night	7% 6% 3 7% 4 4 4 4 6% 7% 46%	1976
	11th to 20th	Night	12% 6% 3 7% 5% 4 4 3 6% 8% 41%	2090
	21st to end	Night	15% 6% 3 7% 6% 5% 5% 4 6% 6% 39%	2090
October	1st to 10th	Night	13% 7% 2 7% 4 3 4 3 4 7% 46%	2242
	11th to 20th	Night	12% 6% 3 6% 4 4 4 3 6% 6% 47%	2280
	21st to end	Night	11% 6% 2 6% 4 3 4 3 5% 7% 49%	2508
November	1st to 10th	Night	12% 7% 3 5% 4 4 4 3 5% 5% 48%	2341
	11th to 20th	Night	10% 7% 2 6% 5% 4 4 4 5% 7% 47%	2585
	21st to end	Night	11% 7% 2 6% 4 4 5% 4 5% 7% 46%	2600
December	1st to 10th	Night	14% 9% 3 8% 5% 4 5% 3 6% 7% 36%	2600
	11th to 20th	Night	14% 9% 3 8% 5% 5% 4 4 6% 7% 35%	3172
	21st to end	Night	14% 11% 4 10% 6% 6% 5% 5% 5% 7% 28%	4578

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	41% 7% 9% 24% 15% 3	2837
	11th to 20th	Day	44% 9% 8% 19% 18% 2	2840
	21st to end	Day	51% 5% 7% 18% 16% 2	3124
February	1st to 10th	Day	46% 6% 10% 19% 16% 1	1710
	11th to 20th	Day	48% 6% 9% 19% 14% 4	1751
	21st to end	Day	50% 5% 11% 14% 16% 4	1619
March	1st to 10th	Day	46% 7% 9% 21% 12% 5%	3435
	11th to 20th	Day	36% 7% 8% 25% 17% 8%	3480
	21st to end	Day	46% 7% 9% 21% 13% 3	3815
April	1st to 10th	Day	37% 7% 11% 23% 17% 5%	2133
	11th to 20th	Day	40% 6% 11% 22% 17% 5%	2254
	21st to end	Day	44% 8% 11% 23% 11% 3	2441
May	1st to 10th	Day	51% 9% 12% 18% 9% 1	4120
	11th to 20th	Day	50% 7% 12% 21% 8% 1	4085
	21st to end	Day	36% 10% 17% 20% 14% 3	4489
June	1st to 10th	Day	39% 10% 17% 22% 11%	2470
	11th to 20th	Day	38% 11% 16% 19% 14% 2	2470
	21st to end	Day	39% 11% 16% 20% 13% 2	2470
July	1st to 10th	Day	38% 11% 20% 19% 8% 4	4055
	11th to 20th	Day	41% 9% 20% 23% 7% 1	4051
	21st to end	Day	57% 9% 11% 16% 7%	4324
August	1st to 10th	Day	48% 9% 18% 20% 5%	4091
	11th to 20th	Day	42% 9% 17% 22% 9% 1	4083
	21st to end	Day	31% 8% 17% 29% 12% 2	4254
September	1st to 10th	Day	39% 8% 12% 20% 18% 4	2199
	11th to 20th	Day	44% 7% 10% 23% 14% 2	2090
	21st to end	Day	49% 6% 10% 18% 15% 1	2090
October	1st to 10th	Day	32% 4 9% 21% 27% 8%	3209
	11th to 20th	Day	28% 4 7% 26% 19% 16%	3140
	21st to end	Day	28% 4 8% 29% 19% 13%	3428
November	1st to 10th	Day	34% 5% 10% 22% 24% 5%	1957
	11th to 20th	Day	37% 5% 9% 21% 23% 5%	1811
	21st to end	Day	39% 4 11% 21% 22% 3	1800
December	1st to 10th	Day	47% 8% 11% 16% 15% 3	1800
	11th to 20th	Day	38% 7% 11% 19% 21% 4	2240
	21st to end	Day	47% 7% 11% 20% 13% 2	3151
January	1st to 10th	Night	44% 9% 11% 18% 15% 3	4109
	11th to 20th	Night	43% 7% 8% 18% 19% 5%	4120
	21st to end	Night	49% 6% 8% 20% 15% 3	4532
February	1st to 10th	Night	42% 6% 12% 23% 16% 1	2470
	11th to 20th	Night	49% 5% 8% 18% 15% 5%	2325
	21st to end	Night	44% 8% 10% 22% 12% 4	1833
March	1st to 10th	Night	50% 9% 9% 19% 11% 2	3418
	11th to 20th	Night	34% 9% 8% 25% 17% 7%	3480
	21st to end	Night	39% 6% 11% 23% 17% 3	3817
April	1st to 10th	Night	39% 8% 13% 20% 17% 4	2046
	11th to 20th	Night	36% 6% 12% 22% 20% 5%	1875
	21st to end	Night	37% 6% 14% 27% 14% 2	1753
May	1st to 10th	Night	46% 9% 15% 21% 9% 1	2840
	11th to 20th	Night	48% 8% 14% 23% 7% 1	2813
	21st to end	Night	33% 11% 20% 24% 9% 4	3087
June	1st to 10th	Night	34% 12% 18% 24% 12% 1	1710
	11th to 20th	Night	33% 10% 19% 24% 12% 1	1710
	21st to end	Night	30% 12% 18% 24% 14% 2	1710
July	1st to 10th	Night	34% 13% 19% 20% 11% 2	2795
	11th to 20th	Night	32% 9% 25% 26% 7% 1	2786
	21st to end	Night	46% 12% 15% 20% 6%	2988
August	1st to 10th	Night	44% 9% 21% 20% 6%	2814
	11th to 20th	Night	37% 9% 18% 27% 9% 1	2812
	21st to end	Night	28% 7% 14% 37% 12% 1	3369
September	1st to 10th	Night	36% 10% 13% 21% 16% 4	1976
	11th to 20th	Night	40% 6% 12% 23% 17% 1	2090
	21st to end	Night	44% 7% 11% 22% 15% 2	2090
October	1st to 10th	Night	36% 3 9% 23% 24% 4	3736
	11th to 20th	Night	27% 4 9% 24% 23% 13%	3779
	21st to end	Night	23% 3 9% 30% 23% 12%	4120
November	1st to 10th	Night	38% 5% 9% 21% 22% 5%	2341
	11th to 20th	Night	38% 4 8% 21% 22% 7%	2585
	21st to end	Night	38% 5% 9% 26% 19% 4	2600
December	1st to 10th	Night	47% 6% 10% 21% 14% 3	2600
	11th to 20th	Night	43% 8% 9% 19% 17% 3	3174
	21st to end	Night	47% 9% 12% 17% 13% 1	4578

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	34% 25% 23% 11% 7%	270
	11th to 20th	Day	36% 22% 30% 11% 1	270
	21st to end	Day	34% 23% 26% 13% 4	297
February	1st to 10th	Day	27% 23% 31% 14% 5%	270
	11th to 20th	Day	40% 29% 22% 7% 1	270
	21st to end	Day	35% 23% 28% 11% 3	274
March	1st to 10th	Day	26% 22% 27% 19% 5% 1	460
	11th to 20th	Day	27% 22% 27% 16% 7% 1	460
	21st to end	Day	26% 19% 30% 20% 5%	506
April	1st to 10th	Day	22% 24% 24% 21% 9% 1	460
	11th to 20th	Day	21% 18% 28% 19% 11% 4	452
	21st to end	Day	24% 14% 29% 18% 12% 4	460
May	1st to 10th	Day	19% 19% 30% 18% 11% 4	463
	11th to 20th	Day	18% 12% 28% 22% 15% 7%	487
	21st to end	Day	13% 7% 23% 27% 19% 11%	529
June	1st to 10th	Day	12% 6% 26% 27% 19% 10%	490
	11th to 20th	Day	9% 7% 22% 29% 22% 11%	490
	21st to end	Day	11% 7% 26% 27% 19% 10%	490
July	1st to 10th	Day	11% 3 16% 27% 26% 17%	485
	11th to 20th	Day	6% 3 17% 32% 27% 15%	484
	21st to end	Day	10% 3 17% 29% 28% 13%	532
August	1st to 10th	Day	16% 3 19% 23% 24% 15%	486
	11th to 20th	Day	10% 4 15% 28% 26% 17%	479
	21st to end	Day	9% 3 12% 26% 27% 22%	539
September	1st to 10th	Day	11% 5% 13% 27% 29% 15%	490
	11th to 20th	Day	15% 4 12% 27% 28% 13%	490
	21st to end	Day	22% 8% 21% 26% 19% 4	490
October	1st to 10th	Day	19% 8% 17% 27% 24% 6%	490
	11th to 20th	Day	16% 15% 21% 24% 22% 3	490
	21st to end	Day	17% 13% 27% 23% 16% 4	539
November	1st to 10th	Day	17% 12% 26% 28% 14% 3	495
	11th to 20th	Day	24% 18% 22% 24% 10% 2	311
	21st to end	Day	25% 21% 24% 23% 7% 1	300
December	1st to 10th	Day	25% 27% 28% 15% 5%	300
	11th to 20th	Day	30% 17% 26% 19% 7% 1	300
	21st to end	Day	27% 17% 27% 15% 11% 1	306
January	1st to 10th	Night	24% 23% 30% 19% 3	760
	11th to 20th	Night	24% 20% 30% 24% 3	760
	21st to end	Night	22% 24% 33% 19% 2	836
February	1st to 10th	Night	21% 18% 33% 24% 3	760
	11th to 20th	Night	30% 22% 30% 17% 1	743
	21st to end	Night	25% 18% 37% 18% 2	628
March	1st to 10th	Night	19% 18% 34% 26% 3	760
	11th to 20th	Night	25% 17% 28% 23% 7%	760
	21st to end	Night	19% 14% 36% 26% 5%	836
April	1st to 10th	Night	20% 18% 29% 26% 6%	760
	11th to 20th	Night	17% 15% 28% 28% 11%	751
	21st to end	Night	17% 11% 28% 34% 9% 1	613
May	1st to 10th	Night	15% 12% 26% 30% 16% 1	570
	11th to 20th	Night	16% 7% 22% 34% 20% 2	561
	21st to end	Night	13% 2 14% 32% 35% 4	617
June	1st to 10th	Night	9% 2 15% 44% 27% 3	570
	11th to 20th	Night	10% 2 15% 34% 35% 4	570
	21st to end	Night	7% 2 12% 41% 34% 4	570
July	1st to 10th	Night	10% 2 8% 36% 39% 5%	555
	11th to 20th	Night	6% 2 9% 33% 45% 6%	552
	21st to end	Night	5% 2 8% 40% 39% 5%	624
August	1st to 10th	Night	10% 3 8% 37% 36% 5%	567
	11th to 20th	Night	10% 2 8% 35% 39% 5%	564
	21st to end	Night	10% 1 9% 32% 40% 8%	768
September	1st to 10th	Night	9% 2 11% 34% 38% 6%	760
	11th to 20th	Night	15% 4 11% 31% 33% 5%	760
	21st to end	Night	19% 6% 19% 32% 24% 1	760
October	1st to 10th	Night	17% 7% 19% 29% 25% 3	760
	11th to 20th	Night	16% 10% 23% 33% 17% 1	760
	21st to end	Night	15% 11% 24% 34% 15% 1	836
November	1st to 10th	Night	17% 9% 22% 35% 15% 2	780
	11th to 20th	Night	17% 15% 25% 34% 10%	800
	21st to end	Night	18% 16% 27% 29% 10% 1	800
December	1st to 10th	Night	22% 17% 32% 23% 5%	800
	11th to 20th	Night	18% 18% 31% 28% 5%	800
	21st to end	Night	23% 16% 28% 27% 5%	851

Page generated at 2024-12-21 04:13:46. Version ac07

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