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 2024-11-02.

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% 5% 4 5% 4 6% 8% 42%	100820
	Night	11% 7% 2 6% 4 4 4 4 6% 8% 44%	98109
Transparency	Day	39% 7% 12% 20% 18% 5%	100631
	Night	37% 6% 11% 20% 20% 6%	97967
Seeing	Day	14% 10% 22% 24% 20% 10%	14589
	Night	14% 8% 22% 30% 21% 4	24549

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	16% 11% 3 6% 4 4 4 3 6% 5% 37%	2836
	11th to 20th	Day	11% 7% 3 6% 4 3 4 3 4 5% 50%	2940
	21st to end	Day	22% 10% 2 5% 3 2 3 3 4 5% 40%	3476
February	1st to 10th	Day	17% 9% 3 7% 4 4 4 3 4 6% 39%	1900
	11th to 20th	Day	20% 9% 2 6% 4 4 3 2 4 5% 41%	1848
	21st to end	Day	18% 9% 3 7% 5% 3 4 3 5% 6% 38%	1570
March	1st to 10th	Day	21% 9% 2 7% 4 4 4 3 6% 6% 34%	3120
	11th to 20th	Day	13% 8% 2 6% 5% 4 4 3 6% 7% 42%	3374
	21st to end	Day	16% 7% 2 7% 5% 4 4 4 5% 7% 39%	3938
April	1st to 10th	Day	10% 7% 2 6% 5% 5% 5% 4 5% 6% 46%	2280
	11th to 20th	Day	12% 6% 2 6% 4 4 4 3 6% 7% 46%	2254
	21st to end	Day	7% 5% 1 6% 4 4 5% 3 6% 9% 50%	2280
May	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 5% 7% 9% 44%	4048
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 6% 9% 39%	4081
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	4476
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	10% 5% 2 5% 3 3 4 2 5% 5% 55%	1991
November	1st to 10th	Day	11% 7% 2 6% 4 2 3 2 4 5% 54%	1681
	11th to 20th	Day	13% 5% 2 5% 4 3 3 3 4 5% 53%	1710
	21st to end	Day	12% 8% 3 5% 4 3 4 3 5% 7% 45%	1710
December	1st to 10th	Day	18% 7% 2 6% 5% 4 5% 3 5% 6% 39%	1710
	11th to 20th	Day	16% 9% 2 8% 5% 3 4 3 4 4 43%	2153
	21st to end	Day	18% 8% 4 8% 4 4 4 3 5% 6% 36%	3124
January	1st to 10th	Night	17% 11% 3 6% 4 4 4 3 5% 5% 39%	4109
	11th to 20th	Night	13% 9% 3 6% 4 4 3 3 4 6% 46%	4015
	21st to end	Night	19% 14% 3 5% 4 3 3 3 3 4 40%	4180
February	1st to 10th	Night	15% 7% 2 7% 5% 4 4 3 5% 6% 42%	2280
	11th to 20th	Night	20% 8% 2 6% 3 3 4 2 4 5% 43%	2218
	21st to end	Night	17% 6% 3 6% 5% 4 4 3 6% 7% 40%	1884
March	1st to 10th	Night	22% 10% 3 7% 4 4 4 3 5% 5% 35%	3733
	11th to 20th	Night	10% 6% 2 5% 4 3 5% 4 6% 7% 48%	3585
	21st to end	Night	14% 7% 3 6% 5% 5% 5% 4 5% 6% 41%	3707
April	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 7% 46%	1900
	11th to 20th	Night	12% 5% 2 6% 4 4 3 3 5% 7% 50%	1875
	21st to end	Night	8% 5% 2 5% 5% 4 4 4 5% 6% 54%	1900
May	1st to 10th	Night	7% 5% 3 7% 5% 4 5% 4 7% 10% 42%	2845
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	2812
	21st to end	Night	5% 2 2 4 4 5% 5% 5% 8% 12% 48%	3079
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	11% 4 1 3 3 3 3 4 4 6% 57%	2420
	11th to 20th	Night	11% 6% 2 4 3 3 3 3 4 6% 54%	2470
	21st to end	Night	12% 5% 2 6% 4 4 4 4 5% 6% 48%	2470
December	1st to 10th	Night	16% 8% 3 5% 3 3 4 4 5% 7% 43%	2470
	11th to 20th	Night	15% 9% 3 5% 4 3 3 3 3 5% 47%	3042
	21st to end	Night	17% 10% 3 6% 4 4 4 4 6% 7% 35%	4532

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	48% 4 5% 15% 23% 5%	2837
	11th to 20th	Day	37% 4 6% 26% 21% 6%	2940
	21st to end	Day	47% 3 6% 16% 20% 7%	3476
February	1st to 10th	Day	47% 4 7% 18% 19% 5%	1900
	11th to 20th	Day	47% 4 3 17% 21% 7%	1858
	21st to end	Day	48% 5% 6% 13% 20% 8%	1570
March	1st to 10th	Day	49% 5% 10% 13% 17% 7%	3120
	11th to 20th	Day	41% 5% 8% 17% 19% 9%	3374
	21st to end	Day	44% 5% 10% 15% 19% 6%	3938
April	1st to 10th	Day	37% 6% 10% 20% 19% 9%	2280
	11th to 20th	Day	36% 6% 7% 20% 23% 8%	2254
	21st to end	Day	30% 6% 11% 23% 27% 3	2280
May	1st to 10th	Day	32% 9% 13% 26% 17% 3	4086
	11th to 20th	Day	41% 7% 12% 27% 11% 1	4085
	21st to end	Day	34% 8% 14% 26% 16% 2	4489
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	31% 6% 9% 17% 25% 11%	2090
	11th to 20th	Day	28% 4 7% 22% 28% 11%	2090
	21st to end	Day	32% 4 6% 17% 31% 11%	1991
November	1st to 10th	Day	34% 3 6% 16% 29% 11%	1681
	11th to 20th	Day	35% 3 6% 18% 27% 11%	1710
	21st to end	Day	38% 5% 8% 17% 24% 8%	1710
December	1st to 10th	Day	45% 5% 8% 16% 20% 6%	1710
	11th to 20th	Day	46% 3 5% 17% 22% 6%	2150
	21st to end	Day	49% 4 7% 17% 20% 3	3124
January	1st to 10th	Night	47% 3 6% 15% 25% 5%	4109
	11th to 20th	Night	41% 4 5% 21% 20% 9%	4015
	21st to end	Night	50% 3 3 14% 25% 5%	4180
February	1st to 10th	Night	44% 4 6% 18% 20% 8%	2280
	11th to 20th	Night	46% 4 4 16% 20% 10%	2219
	21st to end	Night	43% 6% 8% 15% 18% 9%	1884
March	1st to 10th	Night	51% 5% 8% 14% 15% 8%	3733
	11th to 20th	Night	34% 6% 10% 17% 22% 12%	3585
	21st to end	Night	42% 4 8% 19% 21% 6%	3707
April	1st to 10th	Night	37% 6% 10% 19% 19% 9%	1900
	11th to 20th	Night	33% 5% 8% 19% 24% 11%	1875
	21st to end	Night	30% 6% 10% 25% 22% 7%	1900
May	1st to 10th	Night	33% 10% 13% 20% 20% 3	2872
	11th to 20th	Night	42% 8% 13% 23% 13% 1	2813
	21st to end	Night	24% 10% 16% 29% 18% 3	3087
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	26% 4 10% 20% 26% 14%	2090
	11th to 20th	Night	28% 4 7% 20% 26% 14%	2090
	21st to end	Night	33% 4 5% 15% 30% 13%	2603
November	1st to 10th	Night	29% 4 9% 18% 26% 14%	2420
	11th to 20th	Night	32% 4 6% 18% 26% 14%	2470
	21st to end	Night	37% 5% 7% 17% 24% 10%	2470
December	1st to 10th	Night	42% 5% 9% 18% 21% 5%	2470
	11th to 20th	Night	42% 3 6% 14% 30% 6%	3044
	21st to end	Night	47% 4 8% 19% 19% 3	4532

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	20% 25% 36% 15% 4	270
	11th to 20th	Day	20% 24% 32% 17% 6% 1	270
	21st to end	Day	24% 26% 35% 12% 3 1	297
February	1st to 10th	Day	24% 23% 33% 16% 3	270
	11th to 20th	Day	34% 22% 30% 12% 1	270
	21st to end	Day	24% 26% 42% 8%	225
March	1st to 10th	Day	24% 21% 35% 15% 4 1	270
	11th to 20th	Day	19% 20% 35% 18% 8% 1	393
	21st to end	Day	21% 18% 32% 20% 8%	506
April	1st to 10th	Day	22% 13% 33% 21% 11% 1	460
	11th to 20th	Day	16% 15% 31% 25% 12% 2	452
	21st to end	Day	11% 13% 30% 31% 12% 2	460
May	1st to 10th	Day	10% 11% 26% 31% 16% 5%	463
	11th to 20th	Day	10% 5% 25% 31% 20% 10%	487
	21st to end	Day	10% 5% 21% 30% 19% 13%	529
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% 22% 32% 16% 4	392
November	1st to 10th	Day	18% 9% 29% 29% 10% 6%	273
	11th to 20th	Day	15% 17% 34% 24% 9%	270
	21st to end	Day	13% 19% 34% 23% 10% 1	270
December	1st to 10th	Day	24% 19% 33% 19% 5%	270
	11th to 20th	Day	26% 20% 29% 18% 6% 1	270
	21st to end	Day	24% 23% 29% 20% 5%	297
January	1st to 10th	Night	22% 16% 32% 25% 5%	760
	11th to 20th	Night	20% 18% 29% 25% 7%	760
	21st to end	Night	21% 14% 37% 24% 4	836
February	1st to 10th	Night	20% 14% 32% 30% 4	760
	11th to 20th	Night	26% 13% 32% 25% 4	743
	21st to end	Night	20% 17% 36% 24% 3	628
March	1st to 10th	Night	19% 15% 31% 31% 5%	760
	11th to 20th	Night	17% 14% 35% 24% 9% 1	760
	21st to end	Night	16% 13% 33% 27% 10%	777
April	1st to 10th	Night	12% 12% 34% 28% 13% 1	570
	11th to 20th	Night	14% 12% 30% 29% 14% 1	563
	21st to end	Night	9% 10% 28% 35% 16% 2	570
May	1st to 10th	Night	8% 7% 28% 31% 24% 2	570
	11th to 20th	Night	8% 4 22% 36% 24% 5%	561
	21st to end	Night	8% 1 13% 37% 33% 8%	617
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% 8% 20% 36% 22% 3	745
	11th to 20th	Night	13% 11% 26% 34% 14% 1	760
	21st to end	Night	15% 12% 29% 32% 12% 1	760
December	1st to 10th	Night	20% 11% 30% 29% 9% 1	760
	11th to 20th	Night	21% 12% 30% 26% 10%	760
	21st to end	Night	24% 13% 28% 29% 6%	836

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