Crawford Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Crawford Observatory 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	11% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	148540
	Night	12% 6% 2 6% 4 4 4 4 6% 8% 43%	142897
Transparency	Day	40% 6% 13% 20% 17% 5%	152330
	Night	38% 5% 12% 20% 19% 7%	147181
Seeing	Day	14% 10% 23% 24% 20% 9%	17099
	Night	14% 8% 22% 30% 21% 4	27007

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% 10% 4 6% 4 3 3 2 4 4 42%	5258
	11th to 20th	Day	9% 6% 4 5% 3 2 3 2 4 6% 56%	5478
	21st to end	Day	24% 9% 3 7% 5% 3 3 3 4 5% 35%	6436
February	1st to 10th	Day	16% 9% 3 7% 4 5% 4 3 4 6% 40%	2100
	11th to 20th	Day	20% 9% 2 6% 4 4 4 3 4 5% 40%	2048
	21st to end	Day	17% 8% 3 7% 5% 3 5% 3 4 6% 39%	1730
March	1st to 10th	Day	18% 8% 2 7% 4 5% 4 4 6% 6% 36%	4580
	11th to 20th	Day	11% 7% 2 6% 4 4 4 3 6% 7% 47%	4938
	21st to end	Day	15% 6% 3 7% 5% 4 4 4 5% 6% 42%	5767
April	1st to 10th	Day	10% 7% 2 6% 4 4 5% 4 5% 7% 46%	2520
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 6% 7% 49%	2494
	21st to end	Day	7% 5% 2 7% 4 4 5% 3 6% 9% 49%	2520
May	1st to 10th	Day	14% 7% 3 8% 5% 5% 5% 5% 7% 8% 34%	7554
	11th to 20th	Day	12% 6% 2 7% 5% 5% 6% 5% 7% 9% 36%	7626
	21st to end	Day	13% 8% 3 9% 6% 5% 5% 5% 6% 8% 32%	8391
June	1st to 10th	Day	10% 6% 3 7% 5% 6% 6% 5% 8% 10% 35%	2730
	11th to 20th	Day	6% 5% 2 5% 5% 5% 6% 6% 9% 11% 41%	2730
	21st to end	Day	5% 5% 2 7% 5% 5% 6% 5% 9% 12% 39%	2730
July	1st to 10th	Day	6% 5% 2 7% 5% 6% 8% 6% 10% 12% 33%	7680
	11th to 20th	Day	6% 6% 2 7% 5% 7% 7% 6% 9% 13% 32%	7680
	21st to end	Day	6% 6% 3 8% 7% 7% 8% 6% 9% 14% 26%	8223
August	1st to 10th	Day	4 4 2 7% 5% 6% 7% 6% 10% 12% 37%	5545
	11th to 20th	Day	5% 3 2 4 4 4 6% 5% 7% 12% 49%	5486
	21st to end	Day	5% 4 2 6% 4 5% 5% 5% 8% 10% 47%	6080
September	1st to 10th	Day	9% 5% 3 6% 6% 5% 5% 5% 6% 8% 43%	2443
	11th to 20th	Day	11% 7% 3 6% 4 4 5% 4 5% 6% 45%	2310
	21st to end	Day	9% 5% 2 6% 4 4 5% 4 5% 7% 49%	2288
October	1st to 10th	Day	10% 4 2 6% 4 4 4 4 5% 7% 53%	2200
	11th to 20th	Day	8% 5% 2 4 3 3 4 2 5% 7% 59%	2200
	21st to end	Day	11% 5% 2 5% 3 3 3 2 4 5% 55%	2095
November	1st to 10th	Day	11% 7% 2 5% 4 2 3 3 4 6% 53%	1852
	11th to 20th	Day	12% 5% 2 5% 4 3 3 3 4 6% 54%	1890
	21st to end	Day	12% 7% 3 5% 4 3 4 3 6% 7% 46%	1890
December	1st to 10th	Day	18% 7% 2 6% 4 3 4 3 5% 6% 43%	3020
	11th to 20th	Day	14% 9% 2 7% 4 4 4 3 4 4 45%	3463
	21st to end	Day	18% 8% 4 7% 4 3 4 2 4 5% 41%	4565
January	1st to 10th	Night	16% 9% 3 6% 3 4 4 3 5% 5% 42%	7635
	11th to 20th	Night	14% 8% 3 6% 3 4 3 3 4 6% 47%	7463
	21st to end	Night	19% 10% 3 7% 4 3 3 2 3 4 41%	7745
February	1st to 10th	Night	15% 7% 3 7% 5% 3 4 3 5% 6% 43%	2520
	11th to 20th	Night	20% 8% 2 7% 3 3 4 2 4 5% 41%	2458
	21st to end	Night	16% 6% 3 6% 5% 4 4 3 5% 7% 42%	2076
March	1st to 10th	Night	20% 9% 3 7% 4 5% 4 3 6% 6% 34%	5493
	11th to 20th	Night	10% 6% 2 5% 3 3 4 3 5% 6% 52%	5240
	21st to end	Night	13% 6% 2 6% 5% 5% 4 4 5% 6% 45%	5433
April	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 46%	2100
	11th to 20th	Night	11% 5% 2 5% 3 4 3 3 5% 7% 52%	2075
	21st to end	Night	9% 5% 2 6% 5% 4 4 3 4 6% 52%	2100
May	1st to 10th	Night	15% 7% 3 7% 5% 5% 5% 4 6% 8% 36%	5297
	11th to 20th	Night	12% 6% 3 8% 6% 5% 6% 5% 6% 8% 36%	5234
	21st to end	Night	11% 6% 3 8% 5% 5% 6% 5% 7% 10% 34%	5761
June	1st to 10th	Night	5% 5% 3 7% 6% 6% 7% 6% 7% 11% 38%	1890
	11th to 20th	Night	5% 4 1 5% 4 5% 5% 5% 8% 13% 46%	1890
	21st to end	Night	5% 4 2 6% 4 5% 6% 5% 9% 12% 42%	1890
July	1st to 10th	Night	5% 5% 2 7% 5% 6% 6% 6% 9% 13% 36%	5278
	11th to 20th	Night	7% 5% 2 7% 5% 5% 6% 6% 10% 14% 32%	5277
	21st to end	Night	6% 6% 3 9% 6% 6% 7% 5% 9% 13% 32%	5847
August	1st to 10th	Night	4 4 2 6% 5% 5% 5% 5% 9% 13% 42%	4604
	11th to 20th	Night	4 3 2 5% 4 5% 5% 4 8% 13% 47%	4560
	21st to end	Night	5% 4 2 5% 4 5% 5% 4 7% 9% 49%	5046
September	1st to 10th	Night	7% 6% 2 6% 5% 5% 6% 5% 7% 8% 43%	2154
	11th to 20th	Night	11% 6% 2 5% 4 3 4 3 6% 7% 48%	2310
	21st to end	Night	7% 4 2 6% 4 4 4 4 6% 7% 51%	2289
October	1st to 10th	Night	8% 4 2 5% 4 4 3 3 5% 6% 58%	2200
	11th to 20th	Night	9% 6% 1 5% 3 3 3 3 4 5% 59%	2200
	21st to end	Night	12% 7% 2 5% 4 3 3 3 3 4 55%	2741
November	1st to 10th	Night	11% 4 2 4 3 3 3 3 4 6% 56%	2666
	11th to 20th	Night	11% 6% 2 4 3 3 3 3 4 6% 55%	2730
	21st to end	Night	12% 5% 2 6% 4 4 4 4 5% 6% 49%	2730
December	1st to 10th	Night	17% 7% 2 5% 3 3 3 3 4 6% 49%	4380
	11th to 20th	Night	15% 7% 3 6% 4 3 3 3 4 5% 47%	4952
	21st to end	Night	18% 8% 3 6% 4 4 4 3 4 6% 41%	6633

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	47% 2 6% 25% 17% 3	5259
	11th to 20th	Day	31% 3 5% 23% 30% 8%	5478
	21st to end	Day	53% 2 5% 12% 19% 9%	6436
February	1st to 10th	Day	46% 4 7% 18% 20% 5%	2100
	11th to 20th	Day	48% 3 4 18% 20% 7%	2058
	21st to end	Day	46% 5% 6% 14% 20% 9%	1730
March	1st to 10th	Day	47% 4 8% 14% 20% 6%	4580
	11th to 20th	Day	37% 4 7% 16% 21% 16%	4938
	21st to end	Day	43% 4 9% 18% 20% 6%	5769
April	1st to 10th	Day	37% 6% 10% 21% 18% 8%	2520
	11th to 20th	Day	34% 5% 7% 20% 26% 8%	2494
	21st to end	Day	32% 6% 12% 22% 25% 3	2520
May	1st to 10th	Day	46% 5% 12% 20% 14% 2	7590
	11th to 20th	Day	43% 6% 16% 22% 12% 1	7630
	21st to end	Day	49% 7% 12% 18% 13% 1	8405
June	1st to 10th	Day	41% 9% 15% 19% 14% 2	2730
	11th to 20th	Day	33% 10% 19% 23% 13% 2	2730
	21st to end	Day	33% 12% 19% 23% 10% 3	2730
July	1st to 10th	Day	39% 8% 29% 18% 6%	7615
	11th to 20th	Day	42% 9% 31% 14% 4	7611
	21st to end	Day	45% 9% 23% 19% 3	8040
August	1st to 10th	Day	34% 9% 18% 26% 12%	5533
	11th to 20th	Day	28% 5% 24% 30% 11% 2	5522
	21st to end	Day	30% 6% 17% 27% 17% 3	6079
September	1st to 10th	Day	38% 8% 13% 18% 17% 5%	2443
	11th to 20th	Day	39% 7% 11% 21% 19% 3	2310
	21st to end	Day	34% 5% 12% 21% 21% 7%	2288
October	1st to 10th	Day	25% 5% 6% 13% 31% 20%	3590
	11th to 20th	Day	18% 2 4 16% 31% 29%	3568
	21st to end	Day	23% 2 7% 19% 38% 11%	3357
November	1st to 10th	Day	34% 3 7% 17% 28% 11%	1852
	11th to 20th	Day	34% 3 6% 18% 27% 11%	1890
	21st to end	Day	38% 5% 8% 18% 25% 7%	1890
December	1st to 10th	Day	43% 3 5% 18% 26% 4	3020
	11th to 20th	Day	44% 3 8% 21% 19% 6%	3460
	21st to end	Day	46% 3 6% 23% 20% 2	4565
January	1st to 10th	Night	45% 2 7% 25% 18% 3	7636
	11th to 20th	Night	40% 3 5% 17% 25% 9%	7463
	21st to end	Night	49% 2 4 15% 24% 6%	7745
February	1st to 10th	Night	43% 4 6% 18% 21% 8%	2520
	11th to 20th	Night	47% 3 5% 17% 18% 9%	2459
	21st to end	Night	42% 6% 7% 16% 19% 10%	2076
March	1st to 10th	Night	51% 4 6% 15% 17% 8%	5493
	11th to 20th	Night	33% 4 8% 15% 22% 18%	5240
	21st to end	Night	40% 3 7% 22% 20% 8%	5433
April	1st to 10th	Night	36% 6% 11% 19% 19% 9%	2100
	11th to 20th	Night	32% 5% 7% 21% 25% 10%	2075
	21st to end	Night	32% 6% 11% 24% 21% 6%	2100
May	1st to 10th	Night	44% 8% 13% 16% 16% 3	5324
	11th to 20th	Night	44% 6% 15% 22% 12% 1	5235
	21st to end	Night	44% 8% 14% 19% 13% 1	5771
June	1st to 10th	Night	36% 13% 16% 17% 17% 2	1890
	11th to 20th	Night	27% 10% 21% 25% 14% 3	1890
	21st to end	Night	31% 10% 17% 23% 15% 4	1890
July	1st to 10th	Night	37% 9% 29% 19% 6% 1	5235
	11th to 20th	Night	39% 11% 33% 14% 3	5226
	21st to end	Night	42% 9% 27% 17% 5%	5716
August	1st to 10th	Night	32% 8% 19% 28% 12% 1	4592
	11th to 20th	Night	28% 6% 24% 30% 11% 2	4590
	21st to end	Night	30% 6% 15% 27% 19% 3	5046
September	1st to 10th	Night	35% 10% 14% 19% 16% 6%	2154
	11th to 20th	Night	34% 9% 10% 21% 21% 5%	2310
	21st to end	Night	31% 6% 11% 22% 23% 7%	2289
October	1st to 10th	Night	25% 3 7% 14% 29% 22%	3590
	11th to 20th	Night	17% 3 4 16% 29% 30%	3571
	21st to end	Night	23% 2 4 21% 33% 16%	4429
November	1st to 10th	Night	30% 4 9% 17% 27% 14%	2666
	11th to 20th	Night	32% 3 6% 18% 28% 13%	2730
	21st to end	Night	36% 5% 7% 18% 24% 10%	2730
December	1st to 10th	Night	39% 3 6% 21% 27% 5%	4380
	11th to 20th	Night	42% 2 10% 16% 25% 5%	4954
	21st to end	Night	44% 3 7% 24% 20% 2	6633

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	24% 23% 35% 14% 4	330
	11th to 20th	Day	20% 26% 32% 15% 5% 1	330
	21st to end	Day	26% 24% 35% 12% 3 1	363
February	1st to 10th	Day	22% 23% 36% 15% 4	330
	11th to 20th	Day	34% 22% 32% 11% 1	330
	21st to end	Day	23% 25% 40% 9% 3	273
March	1st to 10th	Day	22% 22% 35% 16% 5% 1	330
	11th to 20th	Day	21% 19% 35% 17% 7% 1	466
	21st to end	Day	19% 17% 33% 23% 8%	594
April	1st to 10th	Day	21% 14% 32% 22% 10% 1	540
	11th to 20th	Day	14% 14% 33% 24% 12% 2	532
	21st to end	Day	11% 13% 31% 31% 12% 2	540
May	1st to 10th	Day	12% 12% 27% 29% 14% 5%	543
	11th to 20th	Day	11% 5% 27% 30% 18% 8%	567
	21st to end	Day	12% 6% 23% 30% 18% 12%	617
June	1st to 10th	Day	12% 3 18% 31% 24% 12%	570
	11th to 20th	Day	8% 2 17% 25% 30% 17%	570
	21st to end	Day	7% 4 12% 24% 34% 19%	570
July	1st to 10th	Day	6% 2 7% 26% 34% 25%	565
	11th to 20th	Day	5% 2 6% 23% 33% 29%	564
	21st to end	Day	5% 2 8% 24% 38% 23%	620
August	1st to 10th	Day	8% 2 10% 23% 31% 25%	566
	11th to 20th	Day	7% 2 9% 25% 37% 21%	559
	21st to end	Day	11% 3 9% 28% 32% 18%	627
September	1st to 10th	Day	7% 4 11% 32% 32% 15%	570
	11th to 20th	Day	12% 5% 12% 26% 34% 12%	570
	21st to end	Day	9% 4 18% 28% 32% 9%	562
October	1st to 10th	Day	13% 5% 24% 30% 22% 6%	530
	11th to 20th	Day	11% 8% 22% 35% 22% 2	530
	21st to end	Day	16% 11% 23% 30% 15% 4	428
November	1st to 10th	Day	18% 8% 32% 27% 10% 5%	330
	11th to 20th	Day	15% 15% 36% 25% 10%	330
	21st to end	Day	13% 21% 35% 20% 9% 2	330
December	1st to 10th	Day	27% 18% 34% 17% 4	330
	11th to 20th	Day	23% 19% 31% 19% 7% 1	330
	21st to end	Day	24% 21% 28% 21% 5% 1	363
January	1st to 10th	Night	22% 16% 32% 25% 5%	840
	11th to 20th	Night	20% 18% 30% 25% 7%	840
	21st to end	Night	21% 14% 37% 25% 3	924
February	1st to 10th	Night	20% 14% 32% 30% 4	840
	11th to 20th	Night	27% 14% 31% 24% 4	823
	21st to end	Night	20% 17% 35% 25% 3	692
March	1st to 10th	Night	19% 15% 30% 31% 4	840
	11th to 20th	Night	17% 14% 35% 25% 9% 1	840
	21st to end	Night	15% 13% 34% 28% 9%	859
April	1st to 10th	Night	12% 12% 34% 29% 12% 1	630
	11th to 20th	Night	13% 11% 30% 30% 14% 1	623
	21st to end	Night	10% 10% 27% 35% 17% 1	630
May	1st to 10th	Night	10% 7% 27% 31% 23% 2	630
	11th to 20th	Night	9% 4 23% 36% 24% 5%	621
	21st to end	Night	10% 2 13% 37% 31% 8%	683
June	1st to 10th	Night	7% 2 10% 41% 34% 5%	630
	11th to 20th	Night	6% 1 10% 30% 39% 13%	630
	21st to end	Night	6% 2 7% 29% 45% 11%	630
July	1st to 10th	Night	6% 1 6% 26% 46% 15%	615
	11th to 20th	Night	5% 1 6% 22% 43% 23%	612
	21st to end	Night	6% 1 6% 27% 45% 15%	690
August	1st to 10th	Night	6% 1 5% 23% 50% 15%	627
	11th to 20th	Night	8% 1 7% 28% 45% 11%	624
	21st to end	Night	12% 2 7% 28% 40% 12%	693
September	1st to 10th	Night	8% 2 9% 29% 42% 10%	684
	11th to 20th	Night	13% 4 11% 29% 37% 6%	840
	21st to end	Night	9% 2 14% 34% 36% 3	833
October	1st to 10th	Night	10% 3 21% 36% 25% 5%	800
	11th to 20th	Night	11% 5% 23% 36% 23% 2	800
	21st to end	Night	16% 8% 21% 35% 18% 2	880
November	1st to 10th	Night	14% 7% 20% 36% 21% 2	820
	11th to 20th	Night	13% 11% 26% 34% 14% 1	840
	21st to end	Night	15% 12% 29% 31% 11% 1	840
December	1st to 10th	Night	21% 11% 30% 29% 8% 1	840
	11th to 20th	Night	21% 12% 29% 27% 10%	840
	21st to end	Night	23% 14% 27% 30% 6%	924

Page generated at 2026-09-26 03:18:38. Version ac07

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