Whatt Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Whatt Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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	12% 10% 4 9% 6% 5% 5% 4 5% 6% 34%	109013
	Night	12% 11% 4 10% 6% 5% 5% 4 5% 6% 32%	110200
Transparency	Day	49% 6% 13% 22% 9% 1	112512
	Night	52% 6% 12% 20% 9% 1	114863
Seeing	Day	17% 14% 33% 24% 10% 2	16485
	Night	18% 12% 32% 27% 10% 1	26585

By Month

Percentage Cloud Cover NE (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	5% 14% 6% 18% 11% 8% 5% 3 4 5% 20%	3617
	11th to 20th	Day	6% 14% 5% 13% 10% 12% 8% 9% 6% 5% 12%	3589
	21st to end	Day	7% 14% 7% 17% 7% 6% 5% 4 5% 5% 22%	4070
February	1st to 10th	Day	7% 14% 6% 12% 6% 6% 5% 5% 5% 5% 29%	1800
	11th to 20th	Day	8% 13% 5% 12% 8% 5% 5% 4 5% 4 33%	1754
	21st to end	Day	9% 13% 5% 12% 7% 6% 5% 3 5% 6% 29%	1629
March	1st to 10th	Day	11% 10% 3 8% 5% 4 4 4 5% 7% 38%	3218
	11th to 20th	Day	8% 11% 6% 12% 7% 4 4 4 5% 6% 33%	3486
	21st to end	Day	18% 13% 4 13% 5% 4 4 3 5% 6% 28%	4027
April	1st to 10th	Day	15% 13% 4 10% 6% 5% 4 4 5% 5% 30%	2280
	11th to 20th	Day	19% 12% 4 7% 5% 5% 4 3 5% 5% 32%	2254
	21st to end	Day	20% 11% 3 7% 5% 4 3 3 4 5% 34%	2464
May	1st to 10th	Day	19% 8% 3 5% 4 4 4 3 4 6% 41%	4428
	11th to 20th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 7% 40%	4399
	21st to end	Day	17% 8% 3 8% 5% 5% 4 4 5% 6% 35%	4837
June	1st to 10th	Day	11% 6% 2 6% 5% 4 4 4 7% 8% 43%	2660
	11th to 20th	Day	12% 7% 3 6% 5% 5% 5% 4 6% 7% 40%	2660
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 9% 39%	2660
July	1st to 10th	Day	10% 5% 2 5% 4 4 4 4 6% 9% 45%	4440
	11th to 20th	Day	6% 4 2 7% 4 4 4 4 7% 10% 49%	4440
	21st to end	Day	8% 5% 2 4 3 3 4 4 6% 8% 53%	4814
August	1st to 10th	Day	10% 6% 2 8% 5% 5% 6% 4 7% 8% 40%	4423
	11th to 20th	Day	15% 4 1 5% 3 3 3 3 5% 7% 51%	4041
	21st to end	Day	13% 6% 3 6% 4 4 4 3 6% 8% 43%	4213
September	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 6% 8% 45%	2280
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 4 5% 9% 42%	2280
	21st to end	Day	17% 8% 3 7% 4 4 4 3 5% 6% 40%	2020
October	1st to 10th	Day	17% 8% 2 8% 5% 4 4 4 5% 5% 37%	1900
	11th to 20th	Day	17% 14% 4 8% 5% 4 5% 3 4 7% 29%	1900
	21st to end	Day	17% 14% 4 9% 5% 5% 4 3 5% 5% 28%	1917
November	1st to 10th	Day	12% 14% 4 9% 5% 5% 5% 3 5% 7% 30%	1717
	11th to 20th	Day	12% 17% 7% 13% 7% 6% 6% 3 4 5% 21%	1600
	21st to end	Day	14% 13% 5% 14% 8% 5% 5% 4 6% 4 22%	1600
December	1st to 10th	Day	9% 15% 7% 26% 10% 6% 4 4 4 5% 12%	2610
	11th to 20th	Day	9% 17% 7% 15% 10% 6% 7% 3 4 4 19%	3004
	21st to end	Day	20% 19% 7% 14% 6% 5% 4 3 3 3 16%	3982
January	1st to 10th	Night	8% 13% 6% 18% 8% 7% 5% 3 4 5% 23%	6348
	11th to 20th	Night	7% 13% 7% 15% 9% 9% 8% 7% 5% 5% 15%	6316
	21st to end	Night	7% 16% 7% 14% 7% 6% 5% 4 5% 5% 24%	6904
February	1st to 10th	Night	7% 15% 5% 12% 7% 5% 6% 4 5% 6% 29%	2600
	11th to 20th	Night	9% 10% 3 9% 9% 7% 6% 5% 5% 7% 30%	2533
	21st to end	Night	11% 14% 4 9% 6% 5% 4 4 5% 7% 31%	2000
March	1st to 10th	Night	8% 9% 3 7% 5% 4 4 4 5% 7% 43%	3842
	11th to 20th	Night	10% 10% 5% 12% 6% 4 4 4 4 6% 34%	3530
	21st to end	Night	20% 13% 5% 11% 6% 4 4 2 3 4 28%	3604
April	1st to 10th	Night	18% 12% 4 7% 5% 5% 4 3 5% 6% 31%	1900
	11th to 20th	Night	18% 10% 4 8% 4 3 4 3 6% 5% 33%	1875
	21st to end	Night	20% 11% 2 6% 4 4 4 3 5% 5% 35%	1712
May	1st to 10th	Night	16% 7% 3 7% 4 4 5% 3 6% 7% 37%	2523
	11th to 20th	Night	13% 7% 2 7% 4 5% 4 4 4 5% 46%	2506
	21st to end	Night	14% 8% 3 8% 6% 5% 5% 4 6% 7% 35%	2750
June	1st to 10th	Night	13% 6% 3 5% 5% 5% 5% 4 6% 7% 42%	1520
	11th to 20th	Night	11% 6% 2 6% 4 5% 6% 5% 7% 8% 40%	1520
	21st to end	Night	13% 7% 2 6% 4 5% 4 3 7% 8% 42%	1520
July	1st to 10th	Night	9% 5% 2 5% 4 4 5% 4 6% 8% 49%	2530
	11th to 20th	Night	5% 4 2 5% 4 3 4 4 7% 8% 53%	2530
	21st to end	Night	9% 6% 2 4 3 3 4 3 5% 7% 55%	2750
August	1st to 10th	Night	11% 5% 2 6% 4 4 5% 3 7% 8% 45%	2517
	11th to 20th	Night	14% 4 2 5% 2 3 3 3 5% 8% 51%	2794
	21st to end	Night	17% 6% 2 6% 4 4 3 3 5% 6% 42%	3443
September	1st to 10th	Night	10% 4 2 6% 4 4 5% 3 5% 7% 49%	1900
	11th to 20th	Night	10% 4 2 6% 4 4 5% 4 5% 7% 48%	1900
	21st to end	Night	16% 6% 3 6% 3 5% 3 2 5% 8% 41%	2136
October	1st to 10th	Night	17% 8% 2 6% 4 3 4 3 5% 6% 43%	2280
	11th to 20th	Night	17% 11% 3 7% 5% 5% 4 4 5% 6% 32%	2280
	21st to end	Night	20% 12% 3 8% 6% 4 4 4 5% 6% 28%	2680
November	1st to 10th	Night	14% 13% 3 8% 7% 5% 4 4 5% 7% 30%	2578
	11th to 20th	Night	12% 15% 6% 11% 7% 5% 6% 4 5% 6% 24%	2800
	21st to end	Night	13% 12% 5% 13% 8% 5% 5% 3 6% 6% 24%	2800
December	1st to 10th	Night	10% 14% 7% 22% 9% 6% 5% 3 4 5% 16%	4580
	11th to 20th	Night	10% 16% 6% 15% 8% 7% 6% 3 5% 6% 17%	5203
	21st to end	Night	23% 18% 5% 11% 5% 4 4 3 4 5% 18%	6996

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	68% 6% 7% 13% 5%	3618
	11th to 20th	Day	71% 8% 8% 11% 2	3620
	21st to end	Day	63% 5% 10% 17% 4	4070
February	1st to 10th	Day	57% 7% 11% 15% 10%	1800
	11th to 20th	Day	56% 7% 9% 20% 8%	1763
	21st to end	Day	57% 6% 10% 15% 11% 1	1629
March	1st to 10th	Day	45% 7% 16% 22% 9% 1	3220
	11th to 20th	Day	51% 6% 15% 19% 10%	3486
	21st to end	Day	59% 5% 11% 19% 6% 1	4027
April	1st to 10th	Day	57% 5% 10% 17% 11% 1	2280
	11th to 20th	Day	54% 5% 9% 19% 10% 2	2254
	21st to end	Day	54% 4 10% 21% 10% 1	2464
May	1st to 10th	Day	45% 6% 10% 23% 15% 1	4440
	11th to 20th	Day	42% 6% 15% 26% 9% 1	4403
	21st to end	Day	49% 7% 11% 18% 15% 1	4837
June	1st to 10th	Day	38% 7% 15% 25% 14% 1	2660
	11th to 20th	Day	43% 7% 13% 21% 16% 1	2660
	21st to end	Day	40% 8% 16% 26% 9% 1	2660
July	1st to 10th	Day	34% 7% 19% 25% 14% 1	4371
	11th to 20th	Day	28% 8% 16% 33% 13% 1	4365
	21st to end	Day	27% 6% 16% 33% 17% 1	4662
August	1st to 10th	Day	40% 9% 15% 24% 11% 1	4408
	11th to 20th	Day	33% 5% 20% 32% 10%	4080
	21st to end	Day	37% 7% 17% 27% 10% 1	4212
September	1st to 10th	Day	37% 6% 16% 30% 10%	2280
	11th to 20th	Day	39% 8% 18% 27% 7%	2280
	21st to end	Day	45% 7% 13% 26% 9%	2020
October	1st to 10th	Day	37% 4 16% 30% 12% 1	3160
	11th to 20th	Day	49% 5% 10% 25% 11%	3140
	21st to end	Day	53% 6% 14% 22% 6%	3129
November	1st to 10th	Day	55% 5% 14% 18% 8% 1	1717
	11th to 20th	Day	67% 4 11% 13% 5%	1600
	21st to end	Day	65% 7% 12% 13% 3 1	1600
December	1st to 10th	Day	77% 5% 8% 8% 1	2610
	11th to 20th	Day	68% 6% 9% 11% 6%	3005
	21st to end	Day	74% 5% 7% 13% 1	3982
January	1st to 10th	Night	65% 6% 10% 14% 5%	6348
	11th to 20th	Night	71% 8% 11% 9% 2	6360
	21st to end	Night	61% 7% 12% 15% 5%	6904
February	1st to 10th	Night	58% 6% 10% 17% 8% 1	2600
	11th to 20th	Night	53% 8% 11% 19% 8% 1	2534
	21st to end	Night	54% 6% 12% 20% 8% 1	2000
March	1st to 10th	Night	40% 6% 16% 23% 14% 1	3853
	11th to 20th	Night	51% 7% 12% 22% 7% 1	3531
	21st to end	Night	62% 4 7% 16% 9% 1	3604
April	1st to 10th	Night	55% 6% 8% 17% 13% 1	1900
	11th to 20th	Night	51% 5% 10% 21% 11% 2	1875
	21st to end	Night	51% 4 8% 24% 13% 1	1712
May	1st to 10th	Night	46% 7% 13% 22% 12% 1	2530
	11th to 20th	Night	40% 5% 10% 28% 15% 3	2507
	21st to end	Night	46% 8% 13% 20% 11% 1	2751
June	1st to 10th	Night	39% 7% 15% 26% 12% 2	1520
	11th to 20th	Night	39% 8% 13% 20% 18% 2	1520
	21st to end	Night	39% 7% 13% 27% 12% 1	1520
July	1st to 10th	Night	32% 8% 18% 26% 12% 3	2490
	11th to 20th	Night	25% 8% 17% 35% 13% 2	2482
	21st to end	Night	28% 6% 13% 30% 21% 2	2662
August	1st to 10th	Night	37% 6% 14% 27% 14% 2	2508
	11th to 20th	Night	32% 6% 18% 31% 11% 1	2821
	21st to end	Night	41% 7% 16% 25% 10% 1	3443
September	1st to 10th	Night	33% 6% 15% 33% 13% 1	1900
	11th to 20th	Night	34% 7% 17% 34% 8%	1900
	21st to end	Night	42% 7% 15% 27% 9%	2136
October	1st to 10th	Night	31% 4 11% 33% 20% 1	3800
	11th to 20th	Night	48% 5% 12% 21% 13% 1	3779
	21st to end	Night	55% 5% 10% 23% 7%	4418
November	1st to 10th	Night	55% 5% 13% 20% 7%	2578
	11th to 20th	Night	63% 5% 13% 12% 6%	2800
	21st to end	Night	62% 6% 15% 12% 4 1	2800
December	1st to 10th	Night	73% 5% 11% 9% 1	4580
	11th to 20th	Night	69% 6% 10% 12% 3	5201
	21st to end	Night	70% 6% 9% 13% 3	6996

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	14% 17% 39% 21% 6% 2	300
	11th to 20th	Day	16% 18% 35% 25% 5%	300
	21st to end	Day	17% 15% 40% 24% 4 1	330
February	1st to 10th	Day	14% 20% 44% 17% 4 1	300
	11th to 20th	Day	14% 26% 40% 14% 6% 1	300
	21st to end	Day	18% 21% 47% 13% 1	249
March	1st to 10th	Day	18% 16% 39% 21% 5%	300
	11th to 20th	Day	15% 16% 48% 12% 8%	273
	21st to end	Day	22% 16% 34% 19% 6% 2	420
April	1st to 10th	Day	19% 20% 33% 21% 5% 1	460
	11th to 20th	Day	24% 17% 31% 19% 9%	452
	21st to end	Day	25% 13% 30% 23% 8% 1	566
May	1st to 10th	Day	23% 14% 33% 23% 6% 2	653
	11th to 20th	Day	16% 15% 33% 24% 9% 3	675
	21st to end	Day	19% 11% 31% 27% 10% 3	734
June	1st to 10th	Day	11% 13% 35% 29% 10% 2	680
	11th to 20th	Day	15% 12% 36% 28% 7% 2	680
	21st to end	Day	15% 12% 31% 27% 14% 2	680
July	1st to 10th	Day	8% 5% 30% 34% 18% 5%	671
	11th to 20th	Day	9% 8% 32% 30% 17% 4	668
	21st to end	Day	9% 6% 33% 31% 16% 6%	739
August	1st to 10th	Day	13% 8% 28% 34% 15% 3	676
	11th to 20th	Day	14% 6% 24% 34% 19% 4	665
	21st to end	Day	17% 9% 26% 28% 16% 3	559
September	1st to 10th	Day	12% 13% 30% 26% 17% 3	490
	11th to 20th	Day	12% 10% 31% 31% 12% 3	490
	21st to end	Day	21% 14% 27% 23% 13% 3	415
October	1st to 10th	Day	23% 13% 28% 25% 9% 2	300
	11th to 20th	Day	23% 20% 36% 17% 3	300
	21st to end	Day	25% 22% 37% 13% 3	330
November	1st to 10th	Day	20% 20% 40% 14% 5%	300
	11th to 20th	Day	21% 26% 37% 11% 4	300
	21st to end	Day	22% 20% 39% 15% 2 1	300
December	1st to 10th	Day	16% 20% 40% 19% 5% 1	300
	11th to 20th	Day	21% 21% 38% 16% 3	300
	21st to end	Day	25% 18% 38% 15% 3 1	330
January	1st to 10th	Night	15% 15% 41% 24% 6%	1000
	11th to 20th	Night	19% 11% 42% 22% 6% 1	1000
	21st to end	Night	17% 14% 40% 24% 5%	1100
February	1st to 10th	Night	15% 16% 40% 22% 6% 1	1000
	11th to 20th	Night	13% 17% 38% 23% 8% 1	979
	21st to end	Night	20% 12% 37% 26% 6%	680
March	1st to 10th	Night	15% 13% 34% 31% 6% 1	800
	11th to 20th	Night	18% 15% 34% 24% 8% 1	596
	21st to end	Night	19% 9% 32% 26% 11% 4	627
April	1st to 10th	Night	25% 10% 30% 27% 7% 1	570
	11th to 20th	Night	26% 10% 25% 29% 9% 1	563
	21st to end	Night	25% 8% 23% 30% 12% 1	570
May	1st to 10th	Night	20% 10% 31% 30% 8% 2	570
	11th to 20th	Night	17% 9% 28% 34% 11% 2	561
	21st to end	Night	18% 6% 24% 35% 15% 1	617
June	1st to 10th	Night	15% 5% 28% 37% 14% 1	570
	11th to 20th	Night	16% 6% 25% 39% 13% 1	570
	21st to end	Night	15% 6% 23% 37% 16% 2	570
July	1st to 10th	Night	6% 5% 23% 40% 25% 2	555
	11th to 20th	Night	10% 4 28% 39% 19% 1	552
	21st to end	Night	10% 3 25% 39% 20% 3	624
August	1st to 10th	Night	11% 4 22% 42% 20% 1	567
	11th to 20th	Night	13% 5% 21% 35% 22% 4	564
	21st to end	Night	14% 7% 24% 32% 21% 2	627
September	1st to 10th	Night	11% 7% 25% 34% 20% 2	570
	11th to 20th	Night	11% 8% 28% 35% 16% 2	570
	21st to end	Night	19% 9% 30% 27% 13% 1	735
October	1st to 10th	Night	22% 11% 28% 25% 13% 1	760
	11th to 20th	Night	24% 18% 31% 21% 6%	760
	21st to end	Night	27% 12% 30% 25% 6%	836
November	1st to 10th	Night	22% 13% 35% 24% 5% 1	822
	11th to 20th	Night	20% 19% 35% 18% 7%	1000
	21st to end	Night	21% 17% 38% 20% 4	1000
December	1st to 10th	Night	19% 16% 39% 20% 5% 1	1000
	11th to 20th	Night	20% 19% 33% 22% 5% 1	1000
	21st to end	Night	28% 15% 35% 18% 4	1100

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