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 2024-11-05.

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 6% 5% 5% 5% 4 6% 8% 41%	101447
	Night	12% 7% 2 6% 4 4 4 4 5% 7% 45%	97545
Transparency	Day	38% 8% 11% 20% 18% 5%	105211
	Night	36% 7% 11% 21% 20% 6%	101894
Seeing	Day	15% 10% 22% 24% 19% 10%	14910
	Night	15% 8% 24% 29% 21% 4	24427

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% 12% 3 8% 4 3 4 4 6% 7% 35%	2836
	11th to 20th	Day	17% 9% 3 8% 4 4 3 3 4 4 41%	2950
	21st to end	Day	21% 11% 3 5% 3 3 3 3 4 6% 39%	3476
February	1st to 10th	Day	16% 10% 3 6% 5% 4 4 3 5% 6% 39%	1900
	11th to 20th	Day	17% 9% 4 8% 5% 4 3 3 4 5% 38%	1848
	21st to end	Day	18% 11% 3 6% 4 4 4 4 5% 5% 38%	1570
March	1st to 10th	Day	21% 10% 3 7% 5% 5% 4 3 5% 5% 31%	3120
	11th to 20th	Day	12% 8% 2 6% 4 4 3 4 5% 7% 45%	3288
	21st to end	Day	12% 7% 3 7% 6% 5% 5% 4 6% 7% 38%	4089
April	1st to 10th	Day	12% 6% 2 5% 4 3 4 4 6% 7% 47%	2280
	11th to 20th	Day	12% 7% 2 6% 4 4 4 4 6% 7% 45%	2254
	21st to end	Day	9% 6% 2 5% 5% 4 5% 4 6% 8% 45%	2280
May	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 7% 9% 41%	4014
	11th to 20th	Day	14% 7% 3 7% 5% 4 5% 4 7% 9% 37%	4081
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 7% 9% 41%	4476
June	1st to 10th	Day	9% 5% 3 8% 6% 6% 7% 5% 7% 9% 37%	2470
	11th to 20th	Day	4 4 2 5% 5% 5% 6% 5% 9% 10% 45%	2529
	21st to end	Day	6% 5% 2 6% 5% 6% 6% 4 8% 9% 43%	2660
July	1st to 10th	Day	6% 5% 2 8% 7% 7% 7% 7% 8% 11% 31%	4440
	11th to 20th	Day	8% 5% 2 6% 6% 7% 7% 6% 10% 12% 31%	4162
	21st to end	Day	13% 9% 4 10% 8% 6% 7% 5% 7% 9% 23%	4377
August	1st to 10th	Day	3 3 1 4 4 5% 5% 4 6% 11% 53%	3785
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 5% 8% 9% 46%	3729
	21st to end	Day	6% 3 2 6% 5% 5% 5% 4 6% 7% 51%	4180
September	1st to 10th	Day	11% 5% 2 6% 4 5% 6% 5% 7% 8% 42%	2263
	11th to 20th	Day	12% 5% 3 7% 5% 5% 6% 4 6% 8% 41%	2090
	21st to end	Day	10% 5% 2 5% 4 3 5% 4 5% 7% 51%	2090
October	1st to 10th	Day	9% 4 2 6% 4 3 4 3 5% 5% 55%	2090
	11th to 20th	Day	9% 5% 1 4 4 4 3 3 5% 6% 56%	2049
	21st to end	Day	13% 5% 2 6% 3 3 4 3 4 5% 52%	1956
November	1st to 10th	Day	11% 5% 2 4 3 3 4 3 3 6% 54%	1708
	11th to 20th	Day	11% 6% 2 6% 4 4 3 3 4 6% 52%	1710
	21st to end	Day	14% 6% 2 6% 5% 3 4 4 5% 6% 44%	1710
December	1st to 10th	Day	18% 8% 3 7% 6% 4 5% 5% 6% 4 36%	1710
	11th to 20th	Day	14% 8% 3 7% 4 5% 5% 3 4 6% 41%	2153
	21st to end	Day	13% 10% 4 8% 5% 4 4 3 6% 7% 35%	3124
January	1st to 10th	Night	17% 11% 2 6% 4 4 4 3 4 4 41%	4109
	11th to 20th	Night	17% 8% 3 7% 3 3 4 2 4 5% 44%	3994
	21st to end	Night	18% 14% 3 6% 4 3 3 2 3 3 39%	4180
February	1st to 10th	Night	17% 10% 3 6% 4 3 3 3 4 5% 41%	2280
	11th to 20th	Night	19% 9% 3 6% 4 4 4 3 5% 4 41%	2218
	21st to end	Night	20% 8% 4 6% 5% 4 4 3 5% 5% 37%	1884
March	1st to 10th	Night	22% 9% 3 7% 4 4 4 3 4 5% 35%	3733
	11th to 20th	Night	11% 7% 2 5% 4 3 4 4 6% 7% 48%	3657
	21st to end	Night	15% 8% 3 7% 5% 5% 3 4 5% 4 41%	3570
April	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 46%	1900
	11th to 20th	Night	13% 5% 2 4 5% 3 4 3 5% 7% 49%	1875
	21st to end	Night	11% 5% 2 5% 3 4 3 4 5% 7% 51%	1900
May	1st to 10th	Night	10% 7% 2 7% 5% 4 4 3 6% 8% 44%	2878
	11th to 20th	Night	12% 7% 3 7% 4 5% 4 4 6% 8% 40%	2812
	21st to end	Night	7% 5% 2 6% 4 5% 5% 5% 7% 11% 44%	3079
June	1st to 10th	Night	6% 5% 3 8% 6% 4 5% 4 7% 10% 44%	1710
	11th to 20th	Night	3 3 1 3 4 3 5% 5% 7% 12% 52%	1642
	21st to end	Night	4 4 2 5% 4 4 4 5% 7% 10% 50%	1520
July	1st to 10th	Night	8% 4 2 8% 5% 7% 6% 5% 7% 10% 38%	2530
	11th to 20th	Night	7% 6% 2 5% 5% 5% 5% 5% 9% 12% 39%	2795
	21st to end	Night	9% 8% 4 11% 7% 7% 6% 5% 8% 9% 26%	3171
August	1st to 10th	Night	3 2 1 4 4 4 5% 4 6% 10% 57%	3144
	11th to 20th	Night	5% 3 1 5% 4 3 5% 5% 7% 11% 52%	3100
	21st to end	Night	6% 3 1 4 4 4 5% 3 6% 7% 58%	3476
September	1st to 10th	Night	8% 4 1 6% 4 4 4 4 7% 8% 49%	1900
	11th to 20th	Night	10% 6% 2 7% 5% 4 4 3 6% 8% 46%	2086
	21st to end	Night	10% 4 2 4 4 4 4 3 5% 7% 54%	2090
October	1st to 10th	Night	10% 4 1 4 2 3 3 2 3 6% 61%	2090
	11th to 20th	Night	12% 5% 2 6% 3 3 3 3 4 5% 54%	2127
	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% 4 55%	2470
	11th to 20th	Night	11% 7% 2 5% 4 3 3 3 4 5% 53%	2470
	21st to end	Night	16% 5% 2 5% 5% 3 4 3 5% 5% 47%	2470
December	1st to 10th	Night	17% 9% 2 6% 4 4 4 4 5% 6% 39%	2470
	11th to 20th	Night	14% 7% 2 6% 3 4 4 3 5% 6% 46%	3042
	21st to end	Night	14% 9% 4 8% 5% 5% 6% 4 6% 6% 32%	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	46% 6% 8% 17% 19% 3	2837
	11th to 20th	Day	45% 5% 8% 18% 21% 2	2950
	21st to end	Day	47% 4 8% 20% 18% 2	3476
February	1st to 10th	Day	47% 4 7% 19% 19% 3	1900
	11th to 20th	Day	50% 5% 5% 18% 17% 5%	1858
	21st to end	Day	48% 5% 9% 13% 19% 6%	1570
March	1st to 10th	Day	52% 7% 8% 10% 13% 9%	3120
	11th to 20th	Day	38% 6% 9% 23% 18% 6%	3288
	21st to end	Day	43% 7% 12% 19% 17% 3	4089
April	1st to 10th	Day	35% 7% 10% 23% 18% 6%	2280
	11th to 20th	Day	37% 6% 9% 20% 22% 6%	2254
	21st to end	Day	34% 8% 11% 27% 19% 2	2280
May	1st to 10th	Day	35% 12% 12% 24% 14% 2	4051
	11th to 20th	Day	41% 10% 14% 24% 11%	4085
	21st to end	Day	36% 10% 13% 24% 16% 1	4489
June	1st to 10th	Day	39% 10% 14% 19% 16% 2	2470
	11th to 20th	Day	29% 11% 17% 25% 16% 2	2529
	21st to end	Day	34% 11% 16% 20% 14% 4	2660
July	1st to 10th	Day	40% 13% 21% 20% 6% 1	4371
	11th to 20th	Day	39% 13% 20% 21% 5% 1	4091
	21st to end	Day	55% 11% 12% 16% 6%	4237
August	1st to 10th	Day	23% 9% 12% 36% 18% 1	3773
	11th to 20th	Day	31% 10% 15% 26% 15% 3	3765
	21st to end	Day	30% 9% 12% 19% 25% 4	4179
September	1st to 10th	Day	36% 10% 15% 18% 17% 5%	2263
	11th to 20th	Day	39% 9% 11% 16% 21% 4	2090
	21st to end	Day	33% 6% 12% 23% 20% 7%	2090
October	1st to 10th	Day	21% 5% 6% 15% 31% 21%	3480
	11th to 20th	Day	19% 4 6% 18% 31% 22%	3380
	21st to end	Day	26% 4 5% 21% 33% 10%	3194
November	1st to 10th	Day	31% 4 9% 20% 27% 9%	1708
	11th to 20th	Day	35% 4 7% 20% 26% 8%	1710
	21st to end	Day	41% 5% 6% 19% 23% 6%	1710
December	1st to 10th	Day	49% 6% 8% 15% 18% 4	1710
	11th to 20th	Day	45% 5% 7% 20% 19% 4	2150
	21st to end	Day	45% 8% 11% 19% 13% 3	3124
January	1st to 10th	Night	46% 5% 7% 17% 24% 2	4109
	11th to 20th	Night	43% 5% 7% 20% 17% 6%	3994
	21st to end	Night	50% 4 5% 19% 18% 4	4180
February	1st to 10th	Night	46% 4 8% 18% 19% 5%	2280
	11th to 20th	Night	47% 5% 7% 17% 18% 7%	2219
	21st to end	Night	50% 5% 6% 15% 18% 7%	1884
March	1st to 10th	Night	49% 6% 10% 13% 15% 7%	3733
	11th to 20th	Night	34% 7% 10% 20% 20% 9%	3657
	21st to end	Night	45% 6% 9% 18% 21% 2	3570
April	1st to 10th	Night	34% 6% 12% 24% 19% 4	1900
	11th to 20th	Night	33% 6% 9% 19% 22% 11%	1875
	21st to end	Night	32% 6% 12% 25% 21% 4	1900
May	1st to 10th	Night	35% 10% 13% 23% 17% 2	2906
	11th to 20th	Night	39% 10% 17% 21% 11% 1	2813
	21st to end	Night	29% 12% 15% 26% 16% 2	3087
June	1st to 10th	Night	33% 10% 15% 21% 18% 2	1710
	11th to 20th	Night	22% 9% 18% 28% 20% 3	1642
	21st to end	Night	27% 10% 16% 26% 17% 4	1520
July	1st to 10th	Night	38% 9% 24% 20% 6% 2	2490
	11th to 20th	Night	33% 11% 25% 23% 6% 1	2743
	21st to end	Night	52% 12% 14% 15% 7%	3070
August	1st to 10th	Night	18% 10% 14% 36% 21% 1	3132
	11th to 20th	Night	23% 10% 21% 29% 16% 2	3130
	21st to end	Night	24% 7% 15% 22% 26% 5%	3476
September	1st to 10th	Night	30% 9% 18% 20% 17% 5%	1900
	11th to 20th	Night	35% 8% 14% 17% 23% 4	2086
	21st to end	Night	30% 6% 9% 23% 23% 8%	2090
October	1st to 10th	Night	19% 4 6% 17% 34% 20%	3480
	11th to 20th	Night	20% 3 5% 16% 31% 25%	3522
	21st to end	Night	23% 3 5% 25% 28% 16%	4340
November	1st to 10th	Night	31% 5% 9% 20% 23% 12%	2470
	11th to 20th	Night	33% 4 7% 16% 28% 11%	2470
	21st to end	Night	38% 5% 7% 20% 24% 6%	2470
December	1st to 10th	Night	45% 6% 8% 18% 19% 3	2470
	11th to 20th	Night	39% 5% 7% 19% 25% 5%	3044
	21st to end	Night	47% 9% 10% 18% 14% 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	23% 27% 34% 11% 4	270
	11th to 20th	Day	26% 25% 33% 10% 4 2	270
	21st to end	Day	22% 27% 36% 12% 3 1	297
February	1st to 10th	Day	23% 26% 31% 17% 3	270
	11th to 20th	Day	33% 19% 34% 11% 1 1	270
	21st to end	Day	34% 24% 32% 8% 1	225
March	1st to 10th	Day	26% 21% 30% 18% 3 2	270
	11th to 20th	Day	22% 19% 35% 18% 6% 1	346
	21st to end	Day	21% 18% 36% 18% 8%	506
April	1st to 10th	Day	19% 17% 32% 22% 9% 2	460
	11th to 20th	Day	19% 17% 29% 22% 11% 1	452
	21st to end	Day	15% 13% 30% 29% 12% 2	460
May	1st to 10th	Day	17% 8% 29% 27% 12% 6%	463
	11th to 20th	Day	11% 8% 24% 28% 20% 9%	487
	21st to end	Day	11% 6% 19% 30% 20% 14%	529
June	1st to 10th	Day	12% 4 16% 30% 22% 15%	490
	11th to 20th	Day	5% 3 15% 29% 26% 21%	549
	21st to end	Day	8% 3 17% 28% 30% 14%	680
July	1st to 10th	Day	9% 2 10% 30% 30% 19%	671
	11th to 20th	Day	7% 2 9% 23% 30% 29%	511
	21st to end	Day	7% 2 11% 23% 34% 24%	532
August	1st to 10th	Day	7% 2 10% 24% 33% 23%	486
	11th to 20th	Day	9% 1 9% 30% 34% 16%	479
	21st to end	Day	13% 1 9% 20% 34% 22%	539
September	1st to 10th	Day	10% 3 10% 26% 36% 14%	490
	11th to 20th	Day	11% 3 14% 22% 32% 18%	490
	21st to end	Day	14% 4 14% 27% 31% 9%	490
October	1st to 10th	Day	10% 4 20% 31% 26% 10%	490
	11th to 20th	Day	10% 11% 27% 34% 16% 2	449
	21st to end	Day	19% 13% 22% 28% 15% 2	330
November	1st to 10th	Day	20% 9% 28% 26% 15% 2	282
	11th to 20th	Day	15% 17% 37% 25% 6% 1	270
	21st to end	Day	16% 19% 32% 24% 7% 2	270
December	1st to 10th	Day	27% 20% 28% 19% 5%	270
	11th to 20th	Day	17% 25% 33% 15% 10%	270
	21st to end	Day	24% 20% 39% 12% 5%	297
January	1st to 10th	Night	21% 18% 37% 20% 5%	760
	11th to 20th	Night	22% 18% 32% 21% 7%	760
	21st to end	Night	20% 16% 37% 24% 2	836
February	1st to 10th	Night	22% 13% 35% 26% 4	760
	11th to 20th	Night	26% 16% 32% 23% 3	743
	21st to end	Night	26% 17% 33% 22% 2	628
March	1st to 10th	Night	20% 15% 33% 27% 6%	760
	11th to 20th	Night	18% 14% 37% 24% 7% 1	760
	21st to end	Night	16% 12% 35% 25% 11%	692
April	1st to 10th	Night	14% 12% 35% 26% 12%	570
	11th to 20th	Night	14% 9% 34% 30% 11% 1	563
	21st to end	Night	13% 9% 32% 28% 16% 1	570
May	1st to 10th	Night	9% 7% 28% 33% 21% 3	570
	11th to 20th	Night	11% 6% 21% 34% 24% 4	561
	21st to end	Night	7% 2 18% 35% 32% 6%	617
June	1st to 10th	Night	7% 3 15% 36% 33% 7%	570
	11th to 20th	Night	5% 1 11% 34% 39% 10%	570
	21st to end	Night	5% 2 12% 33% 41% 8%	570
July	1st to 10th	Night	8% 3 7% 28% 43% 12%	555
	11th to 20th	Night	7% 1 8% 27% 42% 16%	552
	21st to end	Night	7% 1 10% 29% 41% 12%	624
August	1st to 10th	Night	5% 1 3 28% 53% 11%	567
	11th to 20th	Night	7% 1 7% 33% 42% 10%	564
	21st to end	Night	11% 1 8% 27% 44% 9%	627
September	1st to 10th	Night	9% 1 10% 31% 42% 7%	570
	11th to 20th	Night	13% 2 13% 29% 36% 6%	756
	21st to end	Night	11% 2 15% 33% 36% 4	760
October	1st to 10th	Night	11% 3 18% 31% 29% 7%	760
	11th to 20th	Night	14% 6% 24% 34% 19% 2	760
	21st to end	Night	16% 8% 22% 32% 20% 2	836
November	1st to 10th	Night	14% 8% 21% 34% 19% 3	760
	11th to 20th	Night	14% 10% 32% 31% 12% 1	760
	21st to end	Night	18% 12% 28% 29% 11% 1	760
December	1st to 10th	Night	22% 14% 29% 27% 7%	760
	11th to 20th	Night	20% 12% 32% 26% 9%	760
	21st to end	Night	23% 14% 33% 25% 5%	836

Page generated at 2024-11-03 01:11:16. Version ac07

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