Waltman Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Waltman Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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% 43%	100560
	Night	12% 6% 2 5% 4 4 4 3 5% 7% 46%	98972
Transparency	Day	37% 7% 12% 19% 19% 6%	104236
	Night	34% 6% 11% 19% 20% 9%	103436
Seeing	Day	13% 7% 20% 25% 23% 11%	15133
	Night	13% 6% 23% 32% 22% 4	24871

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% 12% 2 5% 3 2 3 3 5% 6% 42%	2847
	11th to 20th	Day	17% 7% 2 6% 3 3 3 3 4 4 47%	2850
	21st to end	Day	22% 9% 3 6% 4 3 3 2 4 5% 41%	3149
February	1st to 10th	Day	18% 9% 3 6% 4 3 5% 3 5% 6% 40%	1900
	11th to 20th	Day	19% 10% 2 6% 5% 3 3 3 3 6% 40%	1848
	21st to end	Day	19% 9% 3 7% 4 5% 4 3 5% 5% 38%	1570
March	1st to 10th	Day	21% 10% 3 8% 5% 5% 4 4 5% 6% 29%	3276
	11th to 20th	Day	15% 8% 3 7% 4 4 6% 4 7% 8% 35%	3480
	21st to end	Day	11% 10% 3 9% 6% 5% 4 3 4 5% 40%	3991
April	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 6% 8% 43%	2280
	11th to 20th	Day	16% 6% 3 6% 4 4 4 3 5% 6% 43%	2254
	21st to end	Day	8% 5% 2 5% 5% 4 4 4 6% 8% 48%	2280
May	1st to 10th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	3765
	11th to 20th	Day	12% 6% 3 6% 5% 5% 5% 3 6% 8% 43%	3764
	21st to end	Day	8% 5% 2 6% 4 5% 5% 5% 8% 11% 41%	4302
June	1st to 10th	Day	9% 5% 2 6% 6% 6% 7% 5% 7% 10% 38%	2660
	11th to 20th	Day	4 5% 2 5% 6% 6% 6% 5% 7% 9% 44%	2660
	21st to end	Day	4 5% 2 7% 6% 6% 7% 5% 7% 10% 41%	2608
July	1st to 10th	Day	5% 5% 3 8% 7% 7% 8% 6% 9% 11% 31%	4120
	11th to 20th	Day	8% 5% 2 7% 5% 7% 8% 6% 9% 11% 31%	4120
	21st to end	Day	13% 9% 3 9% 7% 6% 6% 4 7% 9% 28%	4354
August	1st to 10th	Day	3 2 1 3 3 4 4 3 5% 8% 64%	3785
	11th to 20th	Day	5% 3 2 5% 4 5% 6% 5% 7% 10% 47%	3729
	21st to end	Day	9% 5% 2 6% 5% 4 5% 4 6% 8% 46%	4180
September	1st to 10th	Day	7% 5% 2 6% 6% 6% 5% 5% 6% 8% 42%	2280
	11th to 20th	Day	12% 7% 3 6% 4 5% 5% 3 5% 7% 43%	2243
	21st to end	Day	11% 5% 1 4 3 3 4 3 4 8% 54%	2090
October	1st to 10th	Day	6% 3 2 4 4 4 4 3 5% 7% 59%	1948
	11th to 20th	Day	8% 5% 2 5% 4 4 3 3 5% 6% 56%	1900
	21st to end	Day	13% 5% 2 4 3 3 3 3 3 5% 56%	2090
November	1st to 10th	Day	11% 6% 2 4 3 3 4 3 4 6% 54%	1958
	11th to 20th	Day	12% 6% 2 5% 4 3 3 2 4 6% 53%	1818
	21st to end	Day	15% 6% 2 6% 4 4 3 3 5% 6% 47%	1773
December	1st to 10th	Day	17% 8% 3 6% 3 4 3 3 5% 6% 40%	1710
	11th to 20th	Day	14% 11% 4 7% 3 3 4 3 4 6% 41%	2029
	21st to end	Day	14% 6% 3 6% 4 3 3 3 6% 7% 44%	2949
January	1st to 10th	Night	18% 9% 2 5% 3 3 3 3 4 5% 44%	4108
	11th to 20th	Night	15% 7% 3 7% 4 3 3 3 5% 5% 47%	4120
	21st to end	Night	24% 9% 2 5% 3 2 2 3 3 4 43%	4502
February	1st to 10th	Night	17% 8% 3 6% 4 3 4 3 5% 6% 41%	2280
	11th to 20th	Night	19% 8% 2 5% 3 3 3 3 3 5% 45%	2218
	21st to end	Night	21% 8% 2 5% 4 3 3 3 4 5% 41%	1884
March	1st to 10th	Night	19% 10% 3 7% 4 4 3 3 5% 6% 35%	3605
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 6% 9% 40%	3480
	21st to end	Night	16% 8% 2 6% 5% 4 5% 3 5% 6% 40%	3640
April	1st to 10th	Night	11% 5% 2 5% 4 4 4 4 6% 7% 48%	1900
	11th to 20th	Night	13% 5% 2 5% 3 4 4 4 5% 7% 48%	1875
	21st to end	Night	8% 4 2 5% 4 4 4 3 6% 7% 53%	1900
May	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 5% 9% 47%	3130
	11th to 20th	Night	15% 6% 2 5% 4 3 3 4 6% 9% 42%	3129
	21st to end	Night	7% 5% 2 7% 5% 5% 6% 5% 7% 9% 43%	3243
June	1st to 10th	Night	6% 5% 1 6% 6% 5% 5% 3 8% 10% 44%	1520
	11th to 20th	Night	4 3 2 3 4 4 4 5% 8% 11% 52%	1520
	21st to end	Night	4 4 2 6% 5% 4 4 4 8% 13% 48%	1564
July	1st to 10th	Night	5% 5% 3 7% 5% 5% 6% 4 7% 12% 42%	2850
	11th to 20th	Night	7% 5% 2 6% 5% 5% 7% 6% 10% 12% 36%	2850
	21st to end	Night	9% 8% 4 9% 5% 5% 5% 4 7% 10% 35%	3189
August	1st to 10th	Night	2 2 1 3 3 3 4 3 6% 10% 64%	3144
	11th to 20th	Night	4 2 1 4 4 4 5% 6% 9% 12% 49%	3100
	21st to end	Night	6% 4 2 6% 5% 5% 4 4 6% 8% 49%	3476
September	1st to 10th	Night	7% 5% 2 5% 4 5% 5% 5% 6% 9% 47%	1900
	11th to 20th	Night	9% 6% 2 5% 4 4 5% 4 6% 8% 47%	1915
	21st to end	Night	9% 4 2 5% 3 4 3 3 5% 6% 57%	2090
October	1st to 10th	Night	6% 3 1 3 3 3 4 3 5% 6% 64%	2228
	11th to 20th	Night	11% 4 2 4 3 3 3 3 5% 5% 56%	2280
	21st to end	Night	13% 5% 2 5% 3 3 3 2 3 3 58%	2508
November	1st to 10th	Night	13% 5% 1 4 3 3 3 2 4 5% 57%	2340
	11th to 20th	Night	13% 5% 1 4 2 3 3 3 5% 6% 55%	2579
	21st to end	Night	13% 5% 2 5% 4 4 4 3 5% 6% 50%	2562
December	1st to 10th	Night	16% 6% 3 5% 5% 3 4 3 5% 6% 44%	2470
	11th to 20th	Night	15% 7% 2 7% 4 4 3 3 4 5% 46%	3162
	21st to end	Night	16% 10% 3 6% 4 3 3 3 4 6% 41%	4711

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	43% 4 6% 17% 26% 5%	2848
	11th to 20th	Day	39% 5% 10% 20% 21% 5%	2850
	21st to end	Day	48% 3 6% 15% 21% 7%	3149
February	1st to 10th	Day	45% 5% 8% 17% 16% 9%	1900
	11th to 20th	Day	46% 4 6% 17% 19% 8%	1858
	21st to end	Day	48% 5% 6% 14% 18% 9%	1570
March	1st to 10th	Day	54% 6% 8% 11% 15% 6%	3276
	11th to 20th	Day	43% 9% 9% 19% 15% 5%	3480
	21st to end	Day	45% 6% 7% 17% 20% 5%	3991
April	1st to 10th	Day	37% 7% 10% 21% 17% 8%	2280
	11th to 20th	Day	41% 5% 8% 16% 23% 6%	2254
	21st to end	Day	32% 7% 11% 24% 23% 3	2280
May	1st to 10th	Day	34% 11% 15% 24% 13% 3	3800
	11th to 20th	Day	36% 7% 14% 21% 20% 2	3768
	21st to end	Day	32% 11% 20% 26% 11% 1	4315
June	1st to 10th	Day	38% 10% 15% 23% 13% 2	2660
	11th to 20th	Day	34% 9% 16% 23% 15% 3	2660
	21st to end	Day	35% 10% 16% 22% 14% 2	2608
July	1st to 10th	Day	41% 13% 19% 18% 7% 1	4056
	11th to 20th	Day	40% 11% 24% 20% 3 1	4051
	21st to end	Day	51% 9% 13% 21% 6% 1	4216
August	1st to 10th	Day	19% 7% 12% 31% 29% 2	3773
	11th to 20th	Day	29% 9% 18% 27% 14% 1	3765
	21st to end	Day	35% 8% 14% 21% 20% 3	4179
September	1st to 10th	Day	37% 9% 14% 16% 17% 7%	2280
	11th to 20th	Day	39% 7% 10% 17% 20% 7%	2243
	21st to end	Day	28% 6% 12% 23% 20% 11%	2090
October	1st to 10th	Day	17% 3 6% 14% 27% 33%	3234
	11th to 20th	Day	17% 3 5% 13% 34% 29%	3140
	21st to end	Day	23% 3 4 17% 37% 16%	3428
November	1st to 10th	Day	32% 4 9% 16% 29% 10%	1958
	11th to 20th	Day	34% 4 6% 15% 28% 14%	1818
	21st to end	Day	38% 4 7% 18% 22% 10%	1773
December	1st to 10th	Day	43% 5% 10% 16% 16% 8%	1710
	11th to 20th	Day	44% 5% 6% 15% 24% 6%	2026
	21st to end	Day	37% 6% 9% 19% 26% 3	2949
January	1st to 10th	Night	41% 4 7% 14% 27% 7%	4108
	11th to 20th	Night	38% 6% 7% 19% 19% 10%	4120
	21st to end	Night	47% 3 6% 16% 22% 6%	4502
February	1st to 10th	Night	44% 5% 7% 16% 21% 8%	2280
	11th to 20th	Night	42% 5% 6% 17% 20% 10%	2219
	21st to end	Night	46% 5% 7% 13% 18% 11%	1884
March	1st to 10th	Night	47% 7% 9% 14% 15% 7%	3605
	11th to 20th	Night	39% 8% 10% 19% 18% 7%	3480
	21st to end	Night	42% 7% 9% 16% 22% 4	3640
April	1st to 10th	Night	35% 7% 11% 18% 19% 9%	1900
	11th to 20th	Night	35% 5% 9% 19% 21% 10%	1875
	21st to end	Night	29% 6% 11% 24% 26% 5%	1900
May	1st to 10th	Night	31% 9% 13% 26% 18% 4	3160
	11th to 20th	Night	37% 7% 13% 26% 16% 1	3130
	21st to end	Night	32% 12% 16% 23% 15% 2	3251
June	1st to 10th	Night	32% 10% 15% 25% 16% 2	1520
	11th to 20th	Night	22% 8% 20% 27% 17% 5%	1520
	21st to end	Night	27% 8% 20% 24% 17% 3	1564
July	1st to 10th	Night	34% 11% 24% 22% 8% 2	2805
	11th to 20th	Night	34% 12% 26% 23% 5% 1	2796
	21st to end	Night	43% 9% 18% 22% 8% 1	3087
August	1st to 10th	Night	15% 7% 18% 30% 29% 2	3132
	11th to 20th	Night	23% 10% 21% 32% 12% 2	3130
	21st to end	Night	32% 8% 13% 20% 22% 5%	3476
September	1st to 10th	Night	31% 10% 15% 20% 17% 7%	1900
	11th to 20th	Night	33% 8% 14% 15% 21% 9%	1915
	21st to end	Night	27% 6% 10% 26% 20% 11%	2090
October	1st to 10th	Night	14% 3 6% 16% 24% 36%	3722
	11th to 20th	Night	19% 3 5% 14% 27% 33%	3779
	21st to end	Night	21% 3 6% 14% 35% 22%	4120
November	1st to 10th	Night	31% 4 7% 18% 28% 13%	2340
	11th to 20th	Night	30% 4 8% 16% 27% 15%	2579
	21st to end	Night	35% 4 8% 20% 21% 12%	2562
December	1st to 10th	Night	40% 6% 10% 18% 19% 7%	2470
	11th to 20th	Night	41% 5% 7% 12% 27% 8%	3164
	21st to end	Night	43% 5% 7% 17% 24% 4	4711

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% 17% 39% 15% 5% 1	270
	11th to 20th	Day	27% 17% 30% 18% 7% 1	270
	21st to end	Day	24% 21% 33% 18% 4	297
February	1st to 10th	Day	24% 17% 35% 16% 6% 1	270
	11th to 20th	Day	32% 11% 39% 14% 3	270
	21st to end	Day	27% 23% 35% 13% 2	225
March	1st to 10th	Day	24% 15% 36% 20% 5%	270
	11th to 20th	Day	22% 16% 36% 23% 4	270
	21st to end	Day	18% 16% 32% 24% 9%	397
April	1st to 10th	Day	17% 14% 33% 24% 11% 1	460
	11th to 20th	Day	22% 12% 28% 25% 11% 2	452
	21st to end	Day	13% 9% 27% 32% 18% 1	460
May	1st to 10th	Day	13% 8% 27% 32% 16% 4	463
	11th to 20th	Day	12% 4 24% 30% 22% 9%	487
	21st to end	Day	7% 4 20% 31% 25% 13%	536
June	1st to 10th	Day	10% 4 11% 32% 29% 14%	680
	11th to 20th	Day	6% 2 12% 30% 35% 14%	680
	21st to end	Day	7% 1 10% 29% 36% 17%	680
July	1st to 10th	Day	6% 2 9% 25% 37% 21%	671
	11th to 20th	Day	7% 1 4 20% 40% 28%	668
	21st to end	Day	8% 1 7% 22% 39% 23%	677
August	1st to 10th	Day	5% 1 6% 21% 37% 30%	486
	11th to 20th	Day	6% 1 5% 19% 41% 28%	479
	21st to end	Day	13% 1 7% 18% 33% 28%	539
September	1st to 10th	Day	7% 2 8% 27% 37% 19%	490
	11th to 20th	Day	13% 2 10% 20% 33% 21%	490
	21st to end	Day	13% 3 11% 29% 33% 12%	490
October	1st to 10th	Day	7% 2 22% 36% 22% 10%	348
	11th to 20th	Day	8% 6% 24% 36% 21% 5%	300
	21st to end	Day	18% 10% 23% 25% 18% 6%	330
November	1st to 10th	Day	17% 7% 25% 32% 15% 4	300
	11th to 20th	Day	14% 13% 36% 25% 11% 2	300
	21st to end	Day	15% 15% 35% 20% 11% 3	291
December	1st to 10th	Day	21% 16% 34% 20% 7% 1	270
	11th to 20th	Day	20% 12% 37% 21% 9% 1	270
	21st to end	Day	20% 15% 38% 20% 6% 1	297
January	1st to 10th	Night	21% 11% 33% 29% 5%	950
	11th to 20th	Night	19% 13% 32% 28% 7%	950
	21st to end	Night	21% 12% 36% 29% 3	1028
February	1st to 10th	Night	20% 11% 34% 30% 5%	760
	11th to 20th	Night	22% 13% 31% 28% 5%	743
	21st to end	Night	24% 15% 37% 23% 1	628
March	1st to 10th	Night	18% 12% 31% 34% 6%	684
	11th to 20th	Night	15% 9% 36% 32% 6% 1	570
	21st to end	Night	16% 8% 37% 30% 9%	627
April	1st to 10th	Night	13% 8% 36% 31% 10% 1	570
	11th to 20th	Night	14% 8% 33% 31% 12% 1	563
	21st to end	Night	8% 4 37% 36% 15%	570
May	1st to 10th	Night	9% 5% 26% 42% 16% 2	570
	11th to 20th	Night	10% 3 21% 41% 22% 3	561
	21st to end	Night	6% 2 17% 36% 32% 7%	617
June	1st to 10th	Night	7% 2 10% 39% 37% 5%	570
	11th to 20th	Night	4 2 6% 31% 48% 8%	570
	21st to end	Night	5% 2 8% 29% 45% 12%	570
July	1st to 10th	Night	5% 1 6% 29% 50% 9%	555
	11th to 20th	Night	5% 1 4 25% 48% 16%	552
	21st to end	Night	5% 1 4 28% 50% 12%	624
August	1st to 10th	Night	3 4 26% 50% 17%	567
	11th to 20th	Night	5% 1 5% 23% 52% 13%	564
	21st to end	Night	9% 2 6% 26% 42% 16%	627
September	1st to 10th	Night	8% 2 7% 32% 44% 7%	570
	11th to 20th	Night	10% 2 9% 29% 43% 7%	585
	21st to end	Night	10% 3 10% 36% 37% 5%	760
October	1st to 10th	Night	8% 2 14% 38% 33% 6%	760
	11th to 20th	Night	13% 5% 21% 37% 23% 2	760
	21st to end	Night	15% 4 20% 35% 22% 3	836
November	1st to 10th	Night	14% 5% 21% 38% 19% 3	780
	11th to 20th	Night	14% 7% 27% 37% 14% 1	800
	21st to end	Night	15% 6% 30% 33% 14% 2	789
December	1st to 10th	Night	19% 8% 32% 32% 10%	760
	11th to 20th	Night	20% 7% 31% 33% 9%	836
	21st to end	Night	23% 11% 30% 30% 6%	1045

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