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 2025-05-26.

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 6% 4 4 5% 4 6% 8% 43%	114341
	Night	12% 6% 2 6% 4 4 4 4 5% 7% 45%	114969
Transparency	Day	38% 6% 11% 20% 19% 6%	118174
	Night	37% 5% 10% 20% 21% 8%	119276
Seeing	Day	14% 11% 23% 24% 19% 9%	15277
	Night	14% 9% 23% 30% 21% 4	25312

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	15% 9% 4 5% 3 3 3 2 4 4 46%	4056
	11th to 20th	Day	10% 7% 5% 5% 3 3 3 3 5% 6% 50%	4216
	21st to end	Day	21% 9% 2 6% 4 3 3 3 4 5% 38%	4972
February	1st to 10th	Day	16% 9% 3 7% 4 5% 4 3 4 6% 39%	2000
	11th to 20th	Day	21% 9% 2 6% 4 4 4 3 4 5% 40%	1948
	21st to end	Day	17% 8% 3 7% 5% 3 5% 3 4 6% 39%	1650
March	1st to 10th	Day	19% 8% 2 7% 4 4 4 4 6% 6% 36%	4480
	11th to 20th	Day	11% 7% 2 6% 4 4 4 3 6% 7% 47%	4832
	21st to end	Day	15% 7% 3 7% 5% 4 4 3 5% 6% 41%	5642
April	1st to 10th	Day	10% 7% 2 6% 5% 4 4 4 5% 6% 46%	2400
	11th to 20th	Day	11% 6% 2 6% 4 4 4 3 6% 7% 47%	2374
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 9% 49%	2400
May	1st to 10th	Day	8% 4 2 7% 5% 5% 5% 5% 7% 10% 43%	4177
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 6% 9% 38%	4211
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	4541
June	1st to 10th	Day	9% 6% 3 7% 5% 6% 6% 5% 8% 10% 34%	2470
	11th to 20th	Day	5% 4 2 5% 5% 4 6% 6% 8% 11% 43%	2470
	21st to end	Day	6% 5% 2 7% 5% 5% 6% 5% 8% 12% 39%	2470
July	1st to 10th	Day	8% 6% 2 8% 6% 6% 7% 7% 9% 12% 29%	4120
	11th to 20th	Day	7% 6% 2 7% 5% 7% 7% 6% 8% 11% 34%	4120
	21st to end	Day	8% 8% 3 9% 8% 8% 8% 6% 9% 11% 22%	4405
August	1st to 10th	Day	3 3 1 5% 4 5% 6% 5% 8% 11% 48%	3785
	11th to 20th	Day	7% 4 2 5% 5% 4 6% 5% 7% 9% 48%	3729
	21st to end	Day	6% 4 2 5% 4 5% 5% 4 7% 8% 50%	4180
September	1st to 10th	Day	9% 6% 3 7% 6% 5% 5% 4 6% 7% 41%	2211
	11th to 20th	Day	12% 8% 3 6% 4 4 5% 4 5% 6% 42%	2090
	21st to end	Day	9% 6% 2 6% 4 4 5% 3 5% 7% 49%	2090
October	1st to 10th	Day	10% 4 1 6% 4 4 3 4 5% 6% 54%	2090
	11th to 20th	Day	9% 5% 2 4 3 3 4 2 5% 7% 57%	2090
	21st to end	Day	11% 5% 2 5% 3 3 3 2 5% 5% 55%	1991
November	1st to 10th	Day	12% 7% 2 6% 4 2 3 3 4 5% 52%	1762
	11th to 20th	Day	13% 5% 2 5% 4 3 3 3 4 5% 53%	1800
	21st to end	Day	12% 8% 3 5% 4 3 4 3 5% 7% 46%	1800
December	1st to 10th	Day	18% 6% 2 6% 4 3 4 3 5% 6% 44%	2930
	11th to 20th	Day	14% 9% 2 7% 4 4 4 3 4 4 44%	3373
	21st to end	Day	18% 8% 4 7% 3 3 4 2 4 5% 40%	4466
January	1st to 10th	Night	15% 9% 4 6% 3 3 4 2 4 5% 45%	5889
	11th to 20th	Night	14% 8% 3 6% 4 4 3 3 4 6% 45%	5739
	21st to end	Night	17% 11% 2 5% 4 4 3 3 4 4 43%	5984
February	1st to 10th	Night	15% 7% 2 7% 5% 3 4 3 5% 6% 43%	2400
	11th to 20th	Night	21% 8% 2 7% 4 3 4 3 4 5% 42%	2338
	21st to end	Night	16% 5% 3 6% 5% 4 4 3 6% 7% 42%	1980
March	1st to 10th	Night	20% 9% 3 7% 4 5% 4 3 6% 6% 34%	5373
	11th to 20th	Night	10% 6% 2 5% 3 3 4 3 5% 6% 52%	5127
	21st to end	Night	13% 6% 3 6% 5% 5% 5% 4 5% 6% 45%	5315
April	1st to 10th	Night	10% 5% 2 5% 4 4 4 4 6% 8% 47%	2000
	11th to 20th	Night	11% 5% 2 6% 3 3 3 3 5% 7% 51%	1975
	21st to end	Night	8% 5% 2 5% 5% 4 4 4 5% 6% 53%	2000
May	1st to 10th	Night	8% 5% 3 7% 5% 5% 5% 4 7% 10% 42%	2936
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	2902
	21st to end	Night	5% 2 2 4 4 5% 5% 5% 8% 12% 48%	3130
June	1st to 10th	Night	5% 5% 3 7% 7% 6% 7% 6% 7% 10% 36%	1710
	11th to 20th	Night	3 3 2 4 4 4 4 5% 8% 14% 48%	1710
	21st to end	Night	4 4 2 7% 4 5% 5% 5% 9% 12% 42%	1710
July	1st to 10th	Night	6% 6% 2 8% 7% 7% 6% 5% 9% 12% 31%	2840
	11th to 20th	Night	7% 5% 2 7% 5% 5% 6% 6% 10% 14% 33%	2840
	21st to end	Night	9% 8% 4 10% 6% 6% 7% 5% 8% 10% 27%	3142
August	1st to 10th	Night	3 3 1 4 4 4 4 4 8% 13% 52%	3144
	11th to 20th	Night	5% 3 2 5% 4 5% 4 3 6% 10% 52%	3100
	21st to end	Night	6% 5% 2 5% 4 4 5% 4 6% 8% 52%	3476
September	1st to 10th	Night	7% 6% 2 6% 6% 5% 6% 6% 7% 8% 42%	1948
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 6% 7% 47%	2090
	21st to end	Night	8% 4 2 6% 4 4 4 4 6% 7% 52%	2090
October	1st to 10th	Night	8% 4 2 4 3 3 3 3 4 6% 59%	2090
	11th to 20th	Night	9% 6% 1 5% 3 3 3 3 4 6% 57%	2090
	21st to end	Night	12% 7% 2 5% 4 3 3 3 3 4 56%	2603
November	1st to 10th	Night	12% 4 1 4 3 3 3 4 4 6% 56%	2536
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 6% 54%	2600
	21st to end	Night	12% 5% 2 6% 4 4 4 4 5% 6% 49%	2600
December	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 4 6% 49%	4250
	11th to 20th	Night	15% 7% 3 6% 4 3 3 3 4 5% 46%	4822
	21st to end	Night	18% 8% 3 6% 4 4 4 3 5% 6% 41%	6490

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% 3 4 25% 22% 4	4057
	11th to 20th	Day	36% 3 5% 23% 28% 5%	4216
	21st to end	Day	49% 2 6% 14% 19% 9%	4972
February	1st to 10th	Day	47% 4 7% 18% 19% 5%	2000
	11th to 20th	Day	48% 3 4 18% 20% 7%	1958
	21st to end	Day	47% 5% 6% 14% 20% 8%	1650
March	1st to 10th	Day	48% 4 8% 14% 21% 6%	4480
	11th to 20th	Day	37% 4 6% 16% 20% 17%	4832
	21st to end	Day	44% 4 8% 17% 20% 6%	5644
April	1st to 10th	Day	37% 6% 10% 21% 19% 8%	2400
	11th to 20th	Day	35% 5% 7% 20% 25% 8%	2374
	21st to end	Day	31% 6% 12% 23% 25% 3	2400
May	1st to 10th	Day	33% 9% 13% 26% 17% 3	4215
	11th to 20th	Day	42% 7% 12% 27% 11% 1	4215
	21st to end	Day	34% 8% 13% 26% 16% 2	4554
June	1st to 10th	Day	41% 10% 15% 19% 13% 1	2470
	11th to 20th	Day	31% 11% 18% 25% 14% 2	2470
	21st to end	Day	34% 12% 18% 22% 10% 4	2470
July	1st to 10th	Day	42% 11% 25% 16% 5% 1	4055
	11th to 20th	Day	40% 10% 21% 20% 7% 1	4051
	21st to end	Day	52% 13% 15% 15% 5%	4264
August	1st to 10th	Day	25% 10% 13% 35% 17% 1	3773
	11th to 20th	Day	32% 6% 17% 27% 15% 2	3765
	21st to end	Day	30% 7% 17% 22% 20% 3	4179
September	1st to 10th	Day	41% 9% 13% 15% 17% 6%	2211
	11th to 20th	Day	42% 7% 11% 18% 19% 4	2090
	21st to end	Day	35% 5% 11% 21% 21% 7%	2090
October	1st to 10th	Day	24% 5% 6% 12% 31% 21%	3480
	11th to 20th	Day	18% 2 4 16% 30% 30%	3458
	21st to end	Day	22% 2 7% 19% 38% 11%	3253
November	1st to 10th	Day	35% 3 7% 16% 27% 11%	1762
	11th to 20th	Day	34% 3 6% 17% 27% 12%	1800
	21st to end	Day	38% 5% 8% 17% 25% 8%	1800
December	1st to 10th	Day	42% 3 5% 19% 27% 4	2930
	11th to 20th	Day	45% 3 8% 20% 19% 6%	3370
	21st to end	Day	46% 3 6% 23% 20% 2	4466
January	1st to 10th	Night	43% 3 5% 24% 23% 4	5889
	11th to 20th	Night	42% 3 4 19% 24% 9%	5739
	21st to end	Night	46% 2 4 15% 27% 6%	5984
February	1st to 10th	Night	43% 4 6% 18% 20% 9%	2400
	11th to 20th	Night	47% 4 4 17% 19% 10%	2339
	21st to end	Night	42% 6% 7% 16% 19% 10%	1980
March	1st to 10th	Night	51% 4 6% 14% 17% 8%	5373
	11th to 20th	Night	33% 4 8% 15% 22% 18%	5127
	21st to end	Night	41% 3 7% 22% 20% 8%	5315
April	1st to 10th	Night	36% 6% 11% 19% 19% 9%	2000
	11th to 20th	Night	32% 5% 8% 20% 24% 11%	1975
	21st to end	Night	31% 6% 12% 24% 21% 7%	2000
May	1st to 10th	Night	34% 10% 13% 20% 20% 3	2963
	11th to 20th	Night	42% 8% 13% 23% 13% 1	2903
	21st to end	Night	24% 10% 16% 29% 18% 3	3138
June	1st to 10th	Night	36% 14% 15% 17% 16% 1	1710
	11th to 20th	Night	24% 10% 21% 27% 15% 3	1710
	21st to end	Night	30% 11% 17% 21% 16% 4	1710
July	1st to 10th	Night	42% 11% 23% 16% 6% 1	2795
	11th to 20th	Night	36% 12% 25% 21% 6% 1	2786
	21st to end	Night	50% 11% 18% 14% 6% 1	3042
August	1st to 10th	Night	22% 9% 14% 38% 16% 1	3132
	11th to 20th	Night	28% 8% 17% 30% 14% 3	3130
	21st to end	Night	29% 8% 14% 21% 24% 4	3476
September	1st to 10th	Night	35% 11% 14% 17% 16% 7%	1948
	11th to 20th	Night	35% 9% 10% 20% 20% 5%	2090
	21st to end	Night	30% 7% 10% 23% 23% 8%	2090
October	1st to 10th	Night	25% 3 7% 14% 29% 23%	3480
	11th to 20th	Night	18% 3 4 16% 29% 30%	3461
	21st to end	Night	22% 2 4 21% 33% 17%	4291
November	1st to 10th	Night	30% 4 10% 17% 26% 14%	2536
	11th to 20th	Night	33% 3 6% 17% 27% 14%	2600
	21st to end	Night	36% 5% 7% 18% 24% 10%	2600
December	1st to 10th	Night	38% 3 6% 21% 27% 5%	4250
	11th to 20th	Night	42% 2 10% 16% 25% 5%	4824
	21st to end	Night	44% 3 7% 24% 20% 2	6490

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% 25% 35% 14% 4	300
	11th to 20th	Day	20% 26% 32% 16% 5% 1	300
	21st to end	Day	23% 25% 35% 12% 4 1	330
February	1st to 10th	Day	23% 21% 35% 16% 4	300
	11th to 20th	Day	35% 23% 29% 11% 1	300
	21st to end	Day	22% 26% 42% 8% 2	249
March	1st to 10th	Day	24% 23% 34% 14% 4 1	300
	11th to 20th	Day	20% 20% 35% 17% 7% 1	430
	21st to end	Day	21% 18% 33% 20% 8%	550
April	1st to 10th	Day	21% 14% 33% 20% 10% 1	500
	11th to 20th	Day	15% 14% 33% 24% 12% 2	492
	21st to end	Day	10% 13% 30% 32% 13% 2	500
May	1st to 10th	Day	11% 11% 27% 31% 15% 5%	503
	11th to 20th	Day	11% 5% 26% 30% 18% 9%	527
	21st to end	Day	10% 6% 23% 30% 19% 13%	549
June	1st to 10th	Day	11% 3 17% 31% 25% 13%	490
	11th to 20th	Day	7% 2 17% 24% 30% 19%	490
	21st to end	Day	8% 5% 12% 24% 34% 17%	490
July	1st to 10th	Day	7% 2 7% 25% 33% 25%	485
	11th to 20th	Day	5% 2 6% 23% 34% 31%	484
	21st to end	Day	5% 2 8% 22% 39% 24%	532
August	1st to 10th	Day	7% 2 9% 24% 32% 26%	486
	11th to 20th	Day	8% 2 10% 27% 36% 18%	479
	21st to end	Day	13% 2 7% 26% 32% 19%	539
September	1st to 10th	Day	8% 5% 11% 30% 32% 14%	490
	11th to 20th	Day	13% 5% 13% 26% 33% 10%	490
	21st to end	Day	10% 5% 18% 26% 32% 10%	490
October	1st to 10th	Day	12% 5% 23% 30% 23% 7%	490
	11th to 20th	Day	11% 8% 22% 35% 22% 2	490
	21st to end	Day	15% 11% 23% 31% 16% 4	392
November	1st to 10th	Day	19% 8% 31% 28% 9% 5%	300
	11th to 20th	Day	16% 17% 34% 24% 10%	300
	21st to end	Day	12% 20% 34% 22% 10% 2	300
December	1st to 10th	Day	24% 18% 35% 17% 5%	300
	11th to 20th	Day	25% 19% 31% 18% 6% 1	300
	21st to end	Day	25% 22% 27% 20% 5% 1	330
January	1st to 10th	Night	22% 17% 32% 25% 5%	800
	11th to 20th	Night	21% 18% 29% 25% 7%	800
	21st to end	Night	21% 14% 37% 25% 3	880
February	1st to 10th	Night	20% 14% 32% 30% 5%	800
	11th to 20th	Night	27% 14% 31% 24% 4	783
	21st to end	Night	19% 17% 36% 25% 3	660
March	1st to 10th	Night	19% 16% 30% 31% 5%	800
	11th to 20th	Night	17% 14% 35% 24% 9% 1	800
	21st to end	Night	16% 13% 33% 28% 9%	818
April	1st to 10th	Night	12% 13% 34% 28% 13% 1	600
	11th to 20th	Night	13% 11% 31% 30% 13% 1	593
	21st to end	Night	10% 10% 27% 35% 17% 2	600
May	1st to 10th	Night	9% 7% 27% 32% 23% 2	600
	11th to 20th	Night	9% 4 23% 36% 24% 5%	591
	21st to end	Night	8% 1 14% 37% 32% 8%	634
June	1st to 10th	Night	7% 2 10% 41% 35% 5%	570
	11th to 20th	Night	4 1 10% 30% 40% 15%	570
	21st to end	Night	6% 2 7% 30% 45% 11%	570
July	1st to 10th	Night	7% 1 6% 27% 44% 15%	555
	11th to 20th	Night	5% 1 7% 22% 44% 23%	552
	21st to end	Night	6% 1 7% 26% 45% 16%	624
August	1st to 10th	Night	5% 1 5% 24% 50% 16%	567
	11th to 20th	Night	8% 1 8% 30% 44% 9%	564
	21st to end	Night	12% 2 7% 27% 40% 13%	627
September	1st to 10th	Night	9% 2 10% 28% 42% 10%	618
	11th to 20th	Night	14% 4 11% 29% 36% 5%	760
	21st to end	Night	10% 2 15% 33% 36% 4	760
October	1st to 10th	Night	10% 3 20% 36% 26% 5%	760
	11th to 20th	Night	12% 5% 24% 35% 23% 2	760
	21st to end	Night	15% 8% 21% 35% 19% 2	836
November	1st to 10th	Night	13% 7% 21% 36% 21% 2	780
	11th to 20th	Night	14% 11% 26% 34% 14% 1	800
	21st to end	Night	15% 13% 29% 31% 12% 1	800
December	1st to 10th	Night	20% 12% 30% 30% 8% 1	800
	11th to 20th	Night	21% 12% 30% 27% 10%	800
	21st to end	Night	24% 13% 27% 29% 6%	880

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