Bush Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bush Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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% 8% 3 7% 4 4 4 4 5% 7% 43%	109000
	Night	11% 8% 3 7% 4 4 4 3 5% 6% 46%	110206
Transparency	Day	38% 6% 13% 24% 16% 2	112646
	Night	37% 6% 12% 25% 18% 3	114700
Seeing	Day	14% 10% 29% 28% 15% 4	16137
	Night	14% 9% 32% 32% 12% 1	26185

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	13% 10% 4 10% 5% 5% 5% 4 5% 7% 34%	3962
	11th to 20th	Day	8% 9% 4 9% 5% 6% 7% 3 3 6% 40%	4080
	21st to end	Day	11% 13% 6% 12% 5% 5% 5% 3 4 5% 30%	4488
February	1st to 10th	Day	12% 12% 4 9% 6% 5% 5% 3 5% 8% 32%	1800
	11th to 20th	Day	12% 11% 4 7% 5% 3 3 4 5% 7% 38%	1754
	21st to end	Day	12% 9% 3 7% 4 4 4 3 5% 6% 44%	1653
March	1st to 10th	Day	16% 9% 3 6% 3 4 3 3 4 5% 45%	3546
	11th to 20th	Day	11% 8% 2 6% 4 3 4 3 4 6% 47%	3491
	21st to end	Day	20% 12% 6% 7% 5% 4 5% 3 4 7% 27%	3814
April	1st to 10th	Day	16% 8% 3 7% 4 3 3 3 5% 6% 42%	2223
	11th to 20th	Day	14% 10% 3 7% 4 4 4 4 4 7% 39%	2254
	21st to end	Day	22% 7% 2 6% 5% 4 5% 4 5% 6% 35%	2423
May	1st to 10th	Day	16% 7% 3 7% 5% 4 4 4 6% 8% 37%	4082
	11th to 20th	Day	16% 7% 3 7% 5% 4 4 3 6% 7% 39%	4082
	21st to end	Day	13% 6% 2 6% 4 3 4 3 5% 8% 45%	4824
June	1st to 10th	Day	7% 5% 2 5% 6% 4 5% 4 7% 8% 47%	2660
	11th to 20th	Day	7% 5% 2 5% 5% 4 4 4 7% 9% 46%	2660
	21st to end	Day	7% 5% 2 6% 5% 4 5% 5% 6% 8% 47%	2660
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 4 7% 9% 38%	4410
	11th to 20th	Day	7% 4 1 4 3 4 5% 4 6% 8% 54%	4120
	21st to end	Day	5% 3 1 4 3 4 4 5% 8% 11% 51%	4465
August	1st to 10th	Day	6% 4 1 5% 4 4 5% 5% 7% 11% 48%	4104
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 6% 8% 44%	4041
	21st to end	Day	6% 4 2 6% 4 4 5% 4 6% 9% 51%	4279
September	1st to 10th	Day	11% 4 2 4 3 3 3 3 5% 6% 53%	2280
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 5% 9% 50%	2150
	21st to end	Day	11% 3 2 5% 3 4 3 3 5% 6% 55%	2055
October	1st to 10th	Day	13% 7% 2 5% 4 3 3 3 4 6% 50%	1900
	11th to 20th	Day	13% 7% 3 4 2 3 2 3 4 5% 55%	1900
	21st to end	Day	13% 9% 3 5% 3 3 3 3 5% 6% 45%	2090
November	1st to 10th	Day	9% 7% 2 5% 3 3 3 3 4 7% 52%	1935
	11th to 20th	Day	14% 8% 2 5% 4 4 3 2 5% 6% 48%	1615
	21st to end	Day	16% 10% 3 6% 3 3 4 4 5% 6% 41%	1600
December	1st to 10th	Day	6% 6% 2 7% 4 4 4 3 5% 6% 55%	2610
	11th to 20th	Day	11% 10% 3 9% 5% 4 4 3 4 6% 42%	3008
	21st to end	Day	19% 15% 7% 12% 5% 4 4 2 4 6% 22%	3982
January	1st to 10th	Night	10% 9% 5% 17% 7% 5% 5% 4 5% 6% 28%	5998
	11th to 20th	Night	8% 9% 5% 9% 5% 6% 6% 4 6% 6% 37%	5900
	21st to end	Night	12% 12% 4 11% 5% 4 3 2 3 4 37%	6490
February	1st to 10th	Night	11% 12% 3 8% 5% 4 4 4 5% 6% 37%	2600
	11th to 20th	Night	12% 9% 3 7% 5% 4 4 3 5% 6% 41%	2533
	21st to end	Night	10% 10% 3 6% 4 4 4 2 4 5% 48%	1994
March	1st to 10th	Night	13% 7% 2 6% 4 3 3 3 5% 5% 48%	3529
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 4 5% 55%	3497
	21st to end	Night	19% 11% 5% 9% 4 4 4 3 5% 6% 29%	3817
April	1st to 10th	Night	12% 7% 2 5% 3 4 4 3 5% 7% 47%	1956
	11th to 20th	Night	14% 7% 2 6% 4 3 4 3 5% 5% 46%	1875
	21st to end	Night	17% 8% 3 6% 4 4 4 3 4 6% 43%	1753
May	1st to 10th	Night	14% 7% 2 6% 5% 4 4 4 5% 7% 42%	2823
	11th to 20th	Night	14% 7% 2 6% 4 4 4 2 4 6% 46%	2806
	21st to end	Night	12% 6% 2 6% 4 3 4 3 4 7% 49%	2743
June	1st to 10th	Night	5% 4 1 3 4 3 5% 4 7% 10% 53%	1520
	11th to 20th	Night	5% 3 2 4 4 4 4 4 8% 11% 52%	1520
	21st to end	Night	5% 4 1 4 3 4 4 4 5% 9% 56%	1520
July	1st to 10th	Night	11% 6% 2 6% 4 3 3 4 6% 8% 47%	2555
	11th to 20th	Night	4 3 1 3 3 3 4 3 5% 8% 63%	2850
	21st to end	Night	6% 3 2 4 3 4 4 4 7% 11% 50%	3098
August	1st to 10th	Night	3 3 1 4 3 3 5% 4 6% 9% 61%	2836
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 7% 53%	2798
	21st to end	Night	5% 4 2 5% 4 3 4 3 5% 7% 59%	3378
September	1st to 10th	Night	9% 4 2 4 3 3 3 2 5% 7% 58%	1900
	11th to 20th	Night	10% 4 2 5% 3 3 4 4 5% 8% 54%	2026
	21st to end	Night	8% 4 2 4 4 2 4 4 4 7% 57%	2104
October	1st to 10th	Night	12% 6% 2 4 3 3 3 3 5% 6% 54%	2280
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 3 5% 55%	2280
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 5% 50%	2508
November	1st to 10th	Night	9% 4 2 3 3 3 3 3 5% 7% 58%	2360
	11th to 20th	Night	11% 7% 2 5% 5% 3 4 3 4 6% 50%	2784
	21st to end	Night	15% 8% 3 6% 3 3 3 3 4 6% 47%	2800
December	1st to 10th	Night	7% 7% 2 5% 4 4 4 3 5% 6% 52%	4580
	11th to 20th	Night	12% 8% 3 8% 4 3 4 3 5% 6% 44%	5199
	21st to end	Night	20% 16% 6% 11% 4 3 3 3 4 4 27%	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	49% 6% 13% 21% 10% 1	3963
	11th to 20th	Day	47% 4 11% 19% 15% 4	4080
	21st to end	Day	56% 5% 11% 18% 9% 1	4488
February	1st to 10th	Day	50% 6% 14% 18% 10% 1	1800
	11th to 20th	Day	45% 6% 13% 21% 13% 2	1763
	21st to end	Day	41% 5% 12% 24% 15% 2	1653
March	1st to 10th	Day	42% 6% 10% 21% 20% 2	3546
	11th to 20th	Day	36% 6% 12% 26% 18% 2	3491
	21st to end	Day	56% 8% 17% 14% 5% 1	3814
April	1st to 10th	Day	42% 7% 12% 26% 12% 2	2223
	11th to 20th	Day	44% 6% 12% 23% 14% 1	2254
	21st to end	Day	48% 7% 11% 24% 10% 1	2423
May	1st to 10th	Day	42% 8% 14% 22% 13% 1	4120
	11th to 20th	Day	43% 9% 11% 25% 12%	4086
	21st to end	Day	37% 8% 12% 26% 16% 2	4837
June	1st to 10th	Day	32% 9% 16% 27% 13% 3	2660
	11th to 20th	Day	32% 8% 15% 25% 19% 1	2660
	21st to end	Day	32% 8% 17% 25% 15% 3	2660
July	1st to 10th	Day	39% 7% 16% 25% 11% 2	4341
	11th to 20th	Day	26% 7% 12% 32% 19% 2	4051
	21st to end	Day	23% 10% 19% 31% 14% 2	4325
August	1st to 10th	Day	27% 8% 16% 31% 17% 1	4090
	11th to 20th	Day	36% 7% 16% 25% 15% 1	4080
	21st to end	Day	30% 8% 16% 29% 14% 5%	4278
September	1st to 10th	Day	30% 6% 12% 27% 20% 4	2280
	11th to 20th	Day	31% 6% 13% 28% 21% 1	2150
	21st to end	Day	30% 5% 10% 31% 22% 2	2055
October	1st to 10th	Day	22% 3 6% 20% 46% 2	3160
	11th to 20th	Day	21% 3 7% 27% 33% 8%	3140
	21st to end	Day	34% 5% 10% 27% 22% 3	3428
November	1st to 10th	Day	32% 5% 15% 25% 19% 4	1935
	11th to 20th	Day	39% 3 12% 27% 17% 2	1615
	21st to end	Day	42% 6% 13% 25% 14%	1600
December	1st to 10th	Day	30% 5% 19% 30% 15% 1	2610
	11th to 20th	Day	44% 4 15% 25% 10% 1	3005
	21st to end	Day	64% 5% 13% 13% 5% 1	3982
January	1st to 10th	Night	55% 8% 10% 18% 8% 1	5998
	11th to 20th	Night	45% 6% 12% 20% 14% 3	5900
	21st to end	Night	51% 4 11% 16% 15% 3	6490
February	1st to 10th	Night	47% 6% 12% 19% 14% 2	2600
	11th to 20th	Night	43% 5% 12% 22% 16% 2	2534
	21st to end	Night	39% 5% 10% 22% 19% 5%	1994
March	1st to 10th	Night	38% 5% 11% 23% 20% 2	3529
	11th to 20th	Night	32% 5% 9% 30% 18% 5%	3497
	21st to end	Night	53% 8% 17% 17% 5% 1	3817
April	1st to 10th	Night	35% 7% 12% 26% 17% 3	1956
	11th to 20th	Night	39% 6% 11% 26% 14% 3	1875
	21st to end	Night	43% 7% 10% 24% 16% 2	1753
May	1st to 10th	Night	39% 8% 13% 21% 18% 1	2850
	11th to 20th	Night	38% 7% 11% 26% 17% 1	2807
	21st to end	Night	34% 8% 11% 24% 20% 2	2751
June	1st to 10th	Night	24% 9% 20% 29% 15% 4	1520
	11th to 20th	Night	25% 8% 14% 29% 20% 3	1520
	21st to end	Night	24% 7% 17% 30% 18% 4	1520
July	1st to 10th	Night	34% 6% 13% 25% 17% 5%	2515
	11th to 20th	Night	19% 6% 17% 34% 22% 2	2796
	21st to end	Night	24% 9% 20% 29% 17% 1	2999
August	1st to 10th	Night	19% 7% 16% 32% 24% 1	2826
	11th to 20th	Night	33% 5% 12% 28% 21% 1	2825
	21st to end	Night	25% 6% 16% 30% 18% 5%	3378
September	1st to 10th	Night	26% 6% 11% 26% 24% 7%	1900
	11th to 20th	Night	29% 5% 11% 30% 22% 3	2026
	21st to end	Night	26% 5% 10% 29% 26% 3	2104
October	1st to 10th	Night	19% 3 7% 18% 49% 3	3800
	11th to 20th	Night	23% 3 7% 25% 33% 9%	3779
	21st to end	Night	36% 4 8% 26% 24% 2	4120
November	1st to 10th	Night	26% 5% 16% 26% 22% 5%	2360
	11th to 20th	Night	35% 4 9% 30% 18% 3	2784
	21st to end	Night	39% 5% 10% 28% 16% 2	2800
December	1st to 10th	Night	33% 4 12% 37% 13% 1	4580
	11th to 20th	Night	40% 5% 15% 28% 12% 1	5201
	21st to end	Night	62% 4 11% 17% 5% 1	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	22% 15% 31% 24% 7%	300
	11th to 20th	Day	20% 16% 42% 18% 4	300
	21st to end	Day	16% 12% 42% 27% 4	330
February	1st to 10th	Day	16% 20% 41% 19% 3 1	300
	11th to 20th	Day	19% 19% 38% 18% 6% 1	300
	21st to end	Day	15% 20% 44% 18% 2 1	249
March	1st to 10th	Day	19% 14% 43% 21% 3	300
	11th to 20th	Day	26% 13% 36% 22% 3 1	273
	21st to end	Day	35% 21% 28% 13% 3	297
April	1st to 10th	Day	22% 16% 38% 17% 7% 1	403
	11th to 20th	Day	18% 14% 36% 25% 6%	452
	21st to end	Day	23% 16% 33% 18% 9% 1	603
May	1st to 10th	Day	20% 13% 31% 27% 7% 1	653
	11th to 20th	Day	19% 11% 30% 27% 12% 2	675
	21st to end	Day	17% 10% 28% 27% 16% 2	734
June	1st to 10th	Day	7% 5% 23% 34% 24% 7%	680
	11th to 20th	Day	8% 4 27% 39% 18% 4	680
	21st to end	Day	7% 8% 35% 33% 15% 2	680
July	1st to 10th	Day	10% 4 14% 32% 29% 11%	671
	11th to 20th	Day	7% 3 20% 39% 24% 7%	668
	21st to end	Day	7% 2 16% 40% 29% 6%	739
August	1st to 10th	Day	8% 2 19% 38% 26% 6%	676
	11th to 20th	Day	10% 5% 18% 35% 27% 6%	665
	21st to end	Day	9% 3 17% 33% 28% 11%	599
September	1st to 10th	Day	13% 2 21% 29% 30% 6%	490
	11th to 20th	Day	8% 2 19% 38% 28% 6%	360
	21st to end	Day	13% 8% 27% 30% 18% 5%	300
October	1st to 10th	Day	16% 7% 31% 26% 12% 8%	300
	11th to 20th	Day	13% 19% 39% 21% 7% 1	300
	21st to end	Day	17% 16% 36% 23% 6% 2	330
November	1st to 10th	Day	16% 11% 41% 21% 10% 2	300
	11th to 20th	Day	19% 15% 37% 20% 7% 1	300
	21st to end	Day	22% 12% 35% 22% 9%	300
December	1st to 10th	Day	10% 17% 37% 26% 8% 2	300
	11th to 20th	Day	17% 18% 34% 24% 5% 1	300
	21st to end	Day	23% 18% 38% 18% 3	330
January	1st to 10th	Night	13% 13% 39% 29% 7%	1000
	11th to 20th	Night	15% 13% 39% 26% 6% 1	1000
	21st to end	Night	14% 12% 42% 28% 5%	1100
February	1st to 10th	Night	18% 12% 41% 26% 4	1000
	11th to 20th	Night	16% 13% 39% 27% 5%	979
	21st to end	Night	16% 16% 42% 21% 4 1	674
March	1st to 10th	Night	15% 14% 38% 27% 6%	600
	11th to 20th	Night	20% 9% 36% 29% 6%	575
	21st to end	Night	26% 11% 39% 19% 5%	627
April	1st to 10th	Night	15% 13% 41% 26% 4	570
	11th to 20th	Night	17% 12% 37% 29% 5%	563
	21st to end	Night	21% 9% 35% 28% 6% 1	570
May	1st to 10th	Night	14% 12% 35% 33% 6% 1	570
	11th to 20th	Night	16% 7% 30% 35% 11% 1	561
	21st to end	Night	13% 7% 28% 35% 17%	617
June	1st to 10th	Night	6% 4 24% 43% 22% 2	570
	11th to 20th	Night	7% 3 26% 44% 18% 1	570
	21st to end	Night	6% 3 30% 46% 15%	570
July	1st to 10th	Night	7% 3 15% 40% 33% 3	555
	11th to 20th	Night	5% 2 15% 47% 28% 1	552
	21st to end	Night	5% 2 15% 48% 28% 2	624
August	1st to 10th	Night	5% 1 19% 47% 28% 1	567
	11th to 20th	Night	8% 2 15% 43% 29% 2	564
	21st to end	Night	7% 2 15% 44% 29% 3	627
September	1st to 10th	Night	11% 2 19% 39% 26% 3	570
	11th to 20th	Night	12% 3 18% 42% 22% 2	570
	21st to end	Night	10% 4 25% 41% 19% 1	584
October	1st to 10th	Night	15% 7% 28% 31% 18% 2	760
	11th to 20th	Night	17% 8% 36% 30% 9%	760
	21st to end	Night	19% 10% 35% 28% 8%	836
November	1st to 10th	Night	11% 10% 34% 35% 10%	800
	11th to 20th	Night	14% 13% 39% 27% 7%	1000
	21st to end	Night	19% 11% 34% 28% 8%	1000
December	1st to 10th	Night	16% 11% 38% 28% 7% 1	1000
	11th to 20th	Night	19% 9% 35% 30% 6%	1000
	21st to end	Night	21% 14% 35% 27% 3	1100

Page generated at 2025-03-09 07:08:43. Version ac07

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