White House Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the White House Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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% 7% 3 7% 5% 5% 5% 4 6% 8% 41%	127076
	Night	12% 7% 2 6% 4 4 4 4 5% 7% 44%	122651
Transparency	Day	38% 8% 12% 21% 17% 5%	130800
	Night	37% 7% 11% 21% 19% 6%	126974
Seeing	Day	15% 10% 22% 24% 19% 10%	16194
	Night	15% 8% 24% 29% 20% 4	25645

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% 9% 4 10% 4 3 3 3 5% 5% 40%	4056
	11th to 20th	Day	13% 7% 3 10% 3 3 3 2 4 4 47%	4226
	21st to end	Day	18% 12% 4 6% 4 3 3 3 4 6% 38%	4972
February	1st to 10th	Day	16% 10% 3 6% 5% 4 4 3 5% 6% 39%	2000
	11th to 20th	Day	18% 9% 4 8% 5% 4 3 3 4 4 37%	1948
	21st to end	Day	17% 11% 3 6% 4 4 4 4 5% 5% 39%	1650
March	1st to 10th	Day	17% 9% 3 8% 5% 5% 5% 4 5% 4 34%	4480
	11th to 20th	Day	10% 7% 2 6% 4 4 4 5% 5% 6% 46%	4704
	21st to end	Day	11% 7% 3 7% 5% 5% 4 4 5% 7% 42%	5863
April	1st to 10th	Day	11% 6% 2 6% 4 3 4 4 6% 7% 46%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 6% 7% 47%	2374
	21st to end	Day	10% 6% 2 6% 5% 4 5% 4 6% 8% 44%	2400
May	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 5% 7% 9% 33%	5766
	11th to 20th	Day	15% 7% 3 8% 6% 6% 6% 4 7% 9% 29%	5861
	21st to end	Day	14% 7% 3 7% 5% 5% 4 4 6% 8% 38%	6433
June	1st to 10th	Day	9% 5% 3 8% 6% 5% 7% 5% 7% 10% 37%	2600
	11th to 20th	Day	4 4 2 5% 5% 5% 6% 5% 9% 11% 44%	2662
	21st to end	Day	5% 4 2 6% 5% 6% 6% 4 8% 10% 42%	2800
July	1st to 10th	Day	5% 5% 2 7% 6% 7% 7% 6% 8% 11% 36%	6360
	11th to 20th	Day	7% 4 2 6% 5% 7% 7% 6% 10% 14% 31%	5956
	21st to end	Day	9% 6% 3 8% 6% 6% 7% 6% 9% 12% 28%	6293
August	1st to 10th	Day	4 3 1 4 4 5% 5% 4 7% 11% 52%	3905
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 9% 45%	3849
	21st to end	Day	6% 3 2 6% 4 5% 5% 4 6% 7% 51%	4312
September	1st to 10th	Day	10% 5% 2 6% 4 5% 6% 4 7% 8% 42%	2382
	11th to 20th	Day	11% 5% 2 7% 4 5% 6% 4 6% 8% 41%	2200
	21st to end	Day	10% 5% 2 5% 4 3 5% 4 5% 7% 50%	2200
October	1st to 10th	Day	9% 4 2 6% 4 3 4 3 5% 5% 55%	2200
	11th to 20th	Day	9% 4 1 4 4 4 3 3 5% 6% 57%	2137
	21st to end	Day	13% 5% 2 6% 3 3 4 3 4 5% 52%	1956
November	1st to 10th	Day	11% 5% 2 5% 4 3 4 3 4 6% 53%	1762
	11th to 20th	Day	11% 6% 2 6% 4 4 3 3 4 6% 52%	1800
	21st to end	Day	13% 7% 2 6% 5% 3 4 4 5% 6% 45%	1800
December	1st to 10th	Day	13% 7% 3 7% 5% 4 4 4 5% 4 44%	2930
	11th to 20th	Day	15% 9% 4 8% 5% 4 4 3 4 5% 38%	3373
	21st to end	Day	14% 9% 4 7% 5% 4 4 3 6% 7% 37%	4466
January	1st to 10th	Night	15% 10% 3 8% 4 4 4 3 4 4 41%	5889
	11th to 20th	Night	16% 8% 3 6% 3 2 3 2 3 4 48%	5718
	21st to end	Night	17% 11% 3 6% 4 3 3 3 4 4 42%	5984
February	1st to 10th	Night	17% 10% 3 6% 4 3 3 3 4 5% 41%	2400
	11th to 20th	Night	20% 9% 3 6% 4 4 4 3 4 4 39%	2338
	21st to end	Night	19% 7% 3 6% 5% 4 4 3 5% 5% 39%	1980
March	1st to 10th	Night	20% 9% 3 7% 5% 5% 4 3 4 6% 35%	5373
	11th to 20th	Night	10% 6% 2 5% 3 3 4 4 6% 7% 50%	5241
	21st to end	Night	14% 7% 3 7% 5% 4 3 3 4 4 44%	5108
April	1st to 10th	Night	10% 5% 2 6% 4 4 4 5% 7% 8% 46%	2000
	11th to 20th	Night	13% 5% 2 5% 4 3 4 4 5% 7% 49%	1975
	21st to end	Night	11% 5% 2 5% 4 4 3 4 5% 7% 50%	2000
May	1st to 10th	Night	11% 9% 3 8% 5% 5% 5% 4 6% 8% 37%	4126
	11th to 20th	Night	16% 8% 4 10% 5% 5% 5% 4 5% 7% 31%	4032
	21st to end	Night	11% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	4421
June	1st to 10th	Night	6% 5% 3 7% 6% 4 4 4 7% 10% 44%	1800
	11th to 20th	Night	3 3 1 4 4 4 5% 5% 7% 12% 51%	1728
	21st to end	Night	4 4 2 5% 4 4 4 5% 7% 10% 51%	1600
July	1st to 10th	Night	6% 4 2 7% 5% 6% 6% 5% 8% 11% 40%	3618
	11th to 20th	Night	6% 5% 2 6% 5% 5% 6% 5% 9% 14% 36%	3986
	21st to end	Night	7% 6% 3 9% 6% 6% 6% 6% 10% 13% 29%	4553
August	1st to 10th	Night	3 2 1 4 4 4 5% 5% 6% 10% 57%	3244
	11th to 20th	Night	5% 3 1 5% 4 4 5% 5% 7% 11% 52%	3200
	21st to end	Night	6% 3 1 4 4 4 5% 3 6% 7% 57%	3586
September	1st to 10th	Night	8% 5% 1 6% 4 4 4 4 7% 8% 49%	2000
	11th to 20th	Night	10% 6% 2 7% 5% 4 4 3 6% 8% 46%	2196
	21st to end	Night	10% 4 2 4 4 4 4 3 5% 7% 53%	2200
October	1st to 10th	Night	9% 4 1 4 3 3 3 2 3 6% 60%	2200
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 4 5% 56%	2216
	21st to end	Night	14% 6% 2 4 3 3 3 3 3 5% 54%	2641
November	1st to 10th	Night	12% 5% 2 4 3 3 3 3 5% 5% 54%	2536
	11th to 20th	Night	11% 7% 2 5% 4 3 3 3 4 6% 53%	2600
	21st to end	Night	15% 5% 2 5% 5% 3 4 3 5% 5% 48%	2600
December	1st to 10th	Night	16% 8% 2 5% 4 3 4 3 4 5% 45%	4250
	11th to 20th	Night	18% 9% 2 6% 4 3 4 3 4 5% 42%	4822
	21st to end	Night	15% 9% 4 8% 5% 4 5% 4 5% 6% 36%	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	45% 5% 8% 20% 20% 2	4057
	11th to 20th	Day	40% 4 7% 21% 25% 2	4226
	21st to end	Day	47% 4 7% 19% 19% 4	4972
February	1st to 10th	Day	47% 4 7% 19% 19% 3	2000
	11th to 20th	Day	51% 5% 6% 18% 16% 5%	1958
	21st to end	Day	47% 5% 8% 14% 20% 6%	1650
March	1st to 10th	Day	51% 6% 6% 12% 16% 8%	4480
	11th to 20th	Day	37% 5% 7% 19% 19% 12%	4704
	21st to end	Day	41% 6% 9% 24% 18% 2	5865
April	1st to 10th	Day	35% 7% 11% 24% 17% 6%	2400
	11th to 20th	Day	36% 6% 9% 21% 21% 6%	2374
	21st to end	Day	35% 8% 11% 26% 18% 2	2400
May	1st to 10th	Day	42% 11% 14% 20% 10% 2	5802
	11th to 20th	Day	48% 9% 15% 18% 9%	5865
	21st to end	Day	42% 9% 11% 21% 16% 2	6447
June	1st to 10th	Day	39% 10% 14% 19% 17% 2	2600
	11th to 20th	Day	30% 11% 17% 24% 16% 2	2662
	21st to end	Day	34% 11% 17% 20% 14% 4	2800
July	1st to 10th	Day	36% 12% 18% 24% 9% 1	6291
	11th to 20th	Day	36% 14% 25% 21% 4 1	5885
	21st to end	Day	45% 12% 23% 16% 4	6111
August	1st to 10th	Day	24% 9% 13% 36% 18% 1	3893
	11th to 20th	Day	31% 10% 15% 26% 15% 3	3885
	21st to end	Day	30% 9% 12% 19% 24% 4	4311
September	1st to 10th	Day	35% 10% 15% 19% 17% 4	2382
	11th to 20th	Day	37% 9% 12% 18% 21% 3	2200
	21st to end	Day	33% 6% 12% 22% 20% 7%	2200
October	1st to 10th	Day	22% 5% 6% 15% 31% 21%	3590
	11th to 20th	Day	19% 3 6% 18% 31% 22%	3468
	21st to end	Day	26% 4 5% 21% 33% 10%	3194
November	1st to 10th	Day	33% 4 9% 19% 26% 8%	1762
	11th to 20th	Day	35% 4 7% 19% 27% 8%	1800
	21st to end	Day	40% 5% 7% 20% 23% 6%	1800
December	1st to 10th	Day	42% 4 6% 20% 23% 4	2930
	11th to 20th	Day	49% 5% 6% 21% 14% 5%	3370
	21st to end	Day	45% 6% 10% 22% 14% 2	4466
January	1st to 10th	Night	46% 4 7% 20% 21% 3	5889
	11th to 20th	Night	41% 4 6% 18% 25% 6%	5718
	21st to end	Night	45% 4 5% 18% 21% 7%	5984
February	1st to 10th	Night	46% 4 8% 19% 19% 4	2400
	11th to 20th	Night	48% 5% 7% 17% 17% 6%	2339
	21st to end	Night	48% 5% 6% 16% 19% 7%	1980
March	1st to 10th	Night	49% 5% 7% 15% 17% 6%	5373
	11th to 20th	Night	32% 6% 8% 17% 22% 15%	5241
	21st to end	Night	44% 5% 8% 20% 19% 4	5108
April	1st to 10th	Night	34% 6% 12% 24% 19% 5%	2000
	11th to 20th	Night	33% 6% 9% 20% 22% 10%	1975
	21st to end	Night	32% 7% 13% 25% 20% 4	2000
May	1st to 10th	Night	42% 9% 14% 19% 13% 1	4154
	11th to 20th	Night	49% 10% 15% 17% 8% 1	4033
	21st to end	Night	38% 10% 13% 23% 15% 2	4429
June	1st to 10th	Night	33% 10% 15% 21% 19% 2	1800
	11th to 20th	Night	23% 9% 18% 27% 20% 3	1728
	21st to end	Night	27% 10% 16% 26% 17% 4	1600
July	1st to 10th	Night	35% 10% 22% 23% 8% 2	3580
	11th to 20th	Night	33% 13% 27% 22% 4 1	3937
	21st to end	Night	42% 13% 24% 16% 5%	4421
August	1st to 10th	Night	19% 10% 14% 36% 20% 1	3232
	11th to 20th	Night	23% 10% 21% 28% 15% 2	3230
	21st to end	Night	25% 7% 15% 22% 26% 5%	3586
September	1st to 10th	Night	30% 9% 18% 20% 18% 5%	2000
	11th to 20th	Night	34% 8% 14% 18% 22% 3	2196
	21st to end	Night	31% 6% 10% 23% 23% 8%	2200
October	1st to 10th	Night	20% 4 6% 17% 34% 20%	3590
	11th to 20th	Night	20% 3 5% 17% 31% 25%	3611
	21st to end	Night	23% 3 5% 25% 28% 16%	4340
November	1st to 10th	Night	32% 5% 9% 20% 23% 12%	2536
	11th to 20th	Night	33% 4 7% 17% 28% 11%	2600
	21st to end	Night	38% 4 7% 21% 24% 6%	2600
December	1st to 10th	Night	41% 4 6% 22% 24% 3	4250
	11th to 20th	Night	46% 5% 5% 18% 22% 4	4824
	21st to end	Night	46% 7% 9% 24% 12% 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	23% 27% 33% 13% 4	300
	11th to 20th	Day	24% 25% 35% 11% 4 2	300
	21st to end	Day	21% 25% 37% 13% 3 1	330
February	1st to 10th	Day	23% 25% 31% 18% 3	300
	11th to 20th	Day	33% 20% 34% 10% 1 1	300
	21st to end	Day	31% 23% 34% 10% 3	249
March	1st to 10th	Day	26% 22% 31% 17% 3 2	300
	11th to 20th	Day	22% 19% 36% 17% 6% 1	380
	21st to end	Day	20% 19% 36% 18% 7%	550
April	1st to 10th	Day	19% 17% 32% 21% 9% 2	500
	11th to 20th	Day	17% 17% 32% 21% 12% 1	492
	21st to end	Day	16% 13% 29% 29% 12% 2	500
May	1st to 10th	Day	18% 9% 29% 27% 12% 6%	503
	11th to 20th	Day	11% 9% 25% 28% 18% 9%	527
	21st to end	Day	11% 8% 20% 30% 19% 13%	573
June	1st to 10th	Day	12% 4 16% 30% 22% 15%	530
	11th to 20th	Day	6% 3 16% 29% 26% 20%	592
	21st to end	Day	8% 3 16% 27% 30% 17%	730
July	1st to 10th	Day	9% 2 9% 29% 31% 19%	721
	11th to 20th	Day	7% 2 9% 21% 30% 30%	552
	21st to end	Day	7% 2 10% 22% 35% 25%	576
August	1st to 10th	Day	7% 3 10% 24% 34% 23%	526
	11th to 20th	Day	9% 1 9% 28% 34% 19%	519
	21st to end	Day	13% 2 10% 22% 33% 21%	583
September	1st to 10th	Day	10% 3 10% 28% 36% 14%	530
	11th to 20th	Day	10% 2 13% 23% 33% 18%	530
	21st to end	Day	14% 4 14% 28% 31% 9%	530
October	1st to 10th	Day	10% 4 19% 32% 26% 9%	530
	11th to 20th	Day	10% 10% 25% 35% 17% 2	481
	21st to end	Day	19% 13% 22% 28% 15% 2	330
November	1st to 10th	Day	21% 9% 28% 25% 15% 1	300
	11th to 20th	Day	16% 17% 36% 25% 5% 2	300
	21st to end	Day	15% 19% 33% 23% 8% 2	300
December	1st to 10th	Day	24% 20% 31% 19% 5%	300
	11th to 20th	Day	17% 24% 34% 16% 9%	300
	21st to end	Day	24% 20% 37% 13% 6%	330
January	1st to 10th	Night	20% 18% 37% 21% 4	800
	11th to 20th	Night	22% 18% 32% 22% 6%	800
	21st to end	Night	20% 16% 38% 25% 2	880
February	1st to 10th	Night	22% 13% 35% 26% 5%	800
	11th to 20th	Night	26% 16% 32% 23% 3	783
	21st to end	Night	25% 16% 33% 24% 2	660
March	1st to 10th	Night	20% 15% 32% 27% 6%	800
	11th to 20th	Night	18% 14% 37% 24% 7% 1	800
	21st to end	Night	16% 12% 35% 26% 11%	728
April	1st to 10th	Night	14% 13% 36% 26% 12%	600
	11th to 20th	Night	14% 9% 35% 30% 11% 1	593
	21st to end	Night	13% 8% 31% 29% 17% 1	600
May	1st to 10th	Night	10% 8% 28% 33% 20% 3	600
	11th to 20th	Night	13% 5% 21% 34% 23% 4	591
	21st to end	Night	8% 2 18% 36% 30% 6%	650
June	1st to 10th	Night	7% 3 16% 36% 33% 6%	600
	11th to 20th	Night	5% 1 11% 35% 39% 10%	600
	21st to end	Night	5% 2 11% 32% 41% 10%	600
July	1st to 10th	Night	8% 3 7% 27% 44% 12%	585
	11th to 20th	Night	7% 1 7% 26% 43% 16%	582
	21st to end	Night	6% 1 9% 28% 42% 13%	657
August	1st to 10th	Night	5% 1 3 28% 53% 11%	597
	11th to 20th	Night	7% 1 6% 31% 43% 11%	594
	21st to end	Night	10% 2 8% 28% 43% 9%	660
September	1st to 10th	Night	9% 2 10% 31% 42% 7%	600
	11th to 20th	Night	12% 2 12% 30% 37% 6%	796
	21st to end	Night	11% 2 15% 33% 36% 3	800
October	1st to 10th	Night	11% 3 18% 32% 29% 7%	800
	11th to 20th	Night	14% 6% 23% 35% 21% 2	793
	21st to end	Night	16% 8% 22% 32% 20% 2	836
November	1st to 10th	Night	14% 9% 21% 35% 18% 3	780
	11th to 20th	Night	15% 10% 32% 31% 13% 1	800
	21st to end	Night	18% 12% 29% 29% 11% 1	800
December	1st to 10th	Night	22% 14% 30% 27% 7%	800
	11th to 20th	Night	21% 12% 31% 27% 8%	800
	21st to end	Night	23% 14% 32% 25% 6%	880

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