Hazard
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hazard clear sky chart from 2005-11-06 to 2026-04-02.

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	14% 9% 3 8% 5% 5% 5% 4 6% 7% 35%	131501
	Night	15% 9% 3 8% 4 4 4 3 5% 7% 37%	130343
Transparency	Day	46% 5% 10% 22% 15% 2	135225
	Night	46% 4 10% 22% 15% 3	134689
Seeing	Day	21% 13% 24% 21% 14% 6%	16708
	Night	20% 11% 22% 28% 17% 2	26852

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% 17% 6% 13% 4 3 3 2 4 6% 25%	5258
	11th to 20th	Day	13% 13% 8% 12% 5% 4 3 3 3 3 34%	5280
	21st to end	Day	18% 13% 4 8% 6% 3 3 3 4 5% 33%	5780
February	1st to 10th	Day	18% 14% 4 9% 4 3 4 3 5% 6% 30%	1974
	11th to 20th	Day	22% 11% 4 10% 4 4 4 3 5% 5% 28%	2048
	21st to end	Day	19% 12% 4 9% 5% 4 4 3 4 5% 32%	1730
March	1st to 10th	Day	18% 15% 3 7% 5% 4 4 3 4 5% 33%	4810
	11th to 20th	Day	13% 8% 3 7% 4 4 4 3 5% 7% 42%	5090
	21st to end	Day	16% 8% 3 7% 4 5% 4 3 6% 7% 37%	5586
April	1st to 10th	Day	15% 7% 3 7% 5% 4 4 4 6% 8% 36%	2228
	11th to 20th	Day	16% 7% 2 7% 4 4 3 3 5% 6% 42%	2465
	21st to end	Day	17% 7% 3 7% 5% 5% 4 4 6% 7% 37%	2600
May	1st to 10th	Day	16% 10% 3 9% 6% 6% 6% 5% 7% 9% 25%	5862
	11th to 20th	Day	15% 9% 4 9% 6% 5% 6% 4 6% 7% 28%	5861
	21st to end	Day	14% 7% 2 7% 5% 5% 6% 5% 8% 10% 30%	6433
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 5% 6% 9% 38%	2600
	11th to 20th	Day	8% 6% 2 7% 6% 5% 6% 5% 8% 9% 38%	2600
	21st to end	Day	8% 6% 3 6% 6% 5% 5% 5% 8% 10% 38%	2600
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 7% 5% 7% 9% 40%	5900
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 5% 8% 11% 33%	5900
	21st to end	Day	15% 8% 3 9% 7% 6% 6% 5% 7% 9% 24%	6424
August	1st to 10th	Day	10% 6% 3 8% 5% 6% 5% 5% 8% 10% 36%	4234
	11th to 20th	Day	11% 5% 2 7% 5% 5% 7% 5% 7% 10% 36%	4174
	21st to end	Day	6% 3 1 4 3 4 5% 3 6% 8% 56%	4399
September	1st to 10th	Day	14% 6% 2 5% 5% 5% 5% 4 6% 7% 43%	2200
	11th to 20th	Day	13% 5% 2 7% 4 4 5% 3 5% 7% 44%	2200
	21st to end	Day	17% 7% 2 6% 4 3 4 3 6% 6% 42%	2200
October	1st to 10th	Day	13% 4 2 5% 4 3 3 3 5% 6% 50%	2185
	11th to 20th	Day	14% 5% 3 5% 3 3 3 3 5% 6% 50%	2000
	21st to end	Day	19% 8% 2 5% 4 3 3 3 5% 7% 44%	2200
November	1st to 10th	Day	12% 6% 2 5% 3 4 4 3 5% 6% 49%	1852
	11th to 20th	Day	14% 10% 2 6% 4 3 4 4 4 6% 43%	1890
	21st to end	Day	16% 11% 4 6% 3 3 3 3 5% 5% 39%	1890
December	1st to 10th	Day	14% 11% 3 11% 5% 5% 5% 3 3 5% 35%	3020
	11th to 20th	Day	20% 15% 4 9% 4 4 4 3 5% 5% 26%	3463
	21st to end	Day	14% 15% 5% 10% 5% 6% 6% 4 6% 6% 23%	4565
January	1st to 10th	Night	15% 17% 7% 13% 5% 3 3 3 3 5% 27%	7635
	11th to 20th	Night	17% 12% 4 11% 5% 4 3 3 3 4 34%	7680
	21st to end	Night	18% 11% 4 10% 5% 4 4 3 3 4 34%	8401
February	1st to 10th	Night	18% 12% 4 9% 5% 5% 3 3 5% 6% 31%	2641
	11th to 20th	Night	24% 11% 5% 8% 4 4 3 3 4 5% 28%	2458
	21st to end	Night	21% 13% 4 8% 4 4 4 4 5% 5% 28%	2076
March	1st to 10th	Night	18% 12% 2 7% 4 4 4 3 4 6% 36%	5262
	11th to 20th	Night	12% 8% 3 4 3 3 3 3 6% 7% 48%	5090
	21st to end	Night	12% 9% 2 8% 6% 5% 6% 4 5% 7% 35%	5588
April	1st to 10th	Night	14% 8% 2 7% 4 4 4 4 5% 6% 42%	2211
	11th to 20th	Night	15% 8% 2 5% 5% 4 4 4 5% 7% 42%	1882
	21st to end	Night	15% 8% 2 5% 5% 4 5% 3 6% 7% 39%	1800
May	1st to 10th	Night	16% 8% 3 8% 6% 5% 6% 4 6% 9% 29%	4033
	11th to 20th	Night	16% 8% 3 7% 6% 5% 6% 4 7% 8% 30%	4032
	21st to end	Night	15% 7% 3 8% 5% 5% 5% 4 7% 8% 33%	4421
June	1st to 10th	Night	9% 5% 2 6% 4 4 5% 5% 8% 9% 42%	1800
	11th to 20th	Night	5% 4 2 6% 6% 5% 6% 6% 7% 10% 45%	1800
	21st to end	Night	8% 5% 2 7% 4 4 5% 5% 8% 11% 41%	1800
July	1st to 10th	Night	9% 6% 2 6% 4 5% 6% 5% 8% 11% 37%	4058
	11th to 20th	Night	9% 5% 2 7% 4 5% 6% 5% 7% 12% 38%	4057
	21st to end	Night	13% 8% 3 10% 6% 6% 7% 4 6% 9% 28%	4427
August	1st to 10th	Night	10% 6% 2 7% 5% 4 5% 4 6% 10% 41%	2915
	11th to 20th	Night	11% 6% 2 7% 4 4 4 4 7% 10% 40%	2875
	21st to end	Night	6% 2 2 4 3 3 3 3 6% 9% 60%	3468
September	1st to 10th	Night	11% 5% 3 6% 4 4 4 3 6% 7% 47%	2192
	11th to 20th	Night	12% 6% 2 6% 5% 3 4 3 5% 8% 45%	2200
	21st to end	Night	13% 5% 2 5% 4 3 4 4 5% 6% 47%	2200
October	1st to 10th	Night	12% 4 2 5% 3 3 3 3 5% 7% 53%	2215
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 6% 49%	2400
	21st to end	Night	17% 7% 2 4 4 4 4 3 4 6% 45%	2640
November	1st to 10th	Night	14% 8% 3 5% 3 3 2 2 5% 6% 49%	2661
	11th to 20th	Night	15% 8% 3 5% 4 3 4 3 4 7% 44%	2730
	21st to end	Night	15% 11% 3 6% 3 3 4 3 5% 5% 42%	2730
December	1st to 10th	Night	16% 14% 5% 10% 4 4 3 3 4 5% 33%	4380
	11th to 20th	Night	21% 14% 5% 8% 4 3 3 2 4 5% 30%	4952
	21st to end	Night	17% 15% 4 9% 5% 5% 5% 4 6% 8% 23%	6633

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	63% 2 6% 19% 9% 1	5259
	11th to 20th	Day	57% 2 4 18% 17% 2	5280
	21st to end	Day	54% 2 7% 24% 12% 1	5780
February	1st to 10th	Day	56% 4 7% 19% 14% 1	1974
	11th to 20th	Day	59% 3 6% 18% 12% 2	2058
	21st to end	Day	56% 3 7% 18% 13% 4	1730
March	1st to 10th	Day	54% 3 6% 14% 17% 6%	4810
	11th to 20th	Day	42% 3 6% 20% 23% 5%	5090
	21st to end	Day	46% 3 8% 27% 15% 1	5586
April	1st to 10th	Day	45% 4 11% 22% 15% 3	2228
	11th to 20th	Day	43% 3 8% 22% 19% 5%	2465
	21st to end	Day	47% 4 10% 25% 12% 2	2600
May	1st to 10th	Day	55% 5% 12% 18% 10% 1	5897
	11th to 20th	Day	54% 6% 11% 20% 9% 1	5865
	21st to end	Day	45% 6% 14% 18% 13% 3	6447
June	1st to 10th	Day	41% 7% 12% 23% 15% 1	2600
	11th to 20th	Day	40% 7% 14% 19% 18% 2	2600
	21st to end	Day	38% 8% 14% 20% 17% 3	2600
July	1st to 10th	Day	38% 8% 17% 22% 15% 1	5835
	11th to 20th	Day	41% 10% 18% 24% 6% 1	5831
	21st to end	Day	54% 8% 12% 18% 7% 1	6240
August	1st to 10th	Day	40% 8% 16% 25% 10%	4221
	11th to 20th	Day	39% 7% 15% 26% 11% 1	4213
	21st to end	Day	26% 6% 13% 38% 15% 1	4399
September	1st to 10th	Day	39% 6% 12% 20% 17% 5%	2200
	11th to 20th	Day	40% 5% 11% 25% 18% 1	2200
	21st to end	Day	43% 4 9% 22% 20% 2	2200
October	1st to 10th	Day	27% 4 8% 21% 36% 3	3562
	11th to 20th	Day	27% 2 10% 26% 29% 6%	3240
	21st to end	Day	31% 2 8% 30% 19% 9%	3538
November	1st to 10th	Day	36% 3 8% 24% 24% 4	1852
	11th to 20th	Day	43% 3 5% 24% 20% 5%	1890
	21st to end	Day	47% 3 8% 22% 17% 3	1890
December	1st to 10th	Day	53% 4 6% 19% 17% 1	3020
	11th to 20th	Day	61% 2 7% 17% 12% 2	3460
	21st to end	Day	60% 5% 9% 20% 6%	4565
January	1st to 10th	Night	62% 2 10% 18% 7% 1	7636
	11th to 20th	Night	55% 3 5% 16% 19% 3	7680
	21st to end	Night	55% 2 7% 19% 15% 1	8401
February	1st to 10th	Night	55% 3 10% 18% 12% 2	2641
	11th to 20th	Night	60% 3 6% 18% 10% 3	2459
	21st to end	Night	58% 3 8% 16% 12% 3	2076
March	1st to 10th	Night	52% 2 6% 20% 16% 4	5262
	11th to 20th	Night	36% 2 7% 28% 21% 6%	5090
	21st to end	Night	48% 4 10% 25% 13% 1	5588
April	1st to 10th	Night	43% 4 10% 22% 17% 3	2211
	11th to 20th	Night	42% 3 9% 22% 17% 6%	1882
	21st to end	Night	44% 4 12% 26% 13% 1	1800
May	1st to 10th	Night	51% 6% 13% 20% 9% 1	4060
	11th to 20th	Night	51% 7% 13% 22% 7% 1	4033
	21st to end	Night	47% 7% 13% 20% 12% 1	4429
June	1st to 10th	Night	35% 8% 17% 22% 16% 3	1800
	11th to 20th	Night	31% 8% 19% 24% 16% 2	1800
	21st to end	Night	35% 8% 15% 22% 16% 4	1800
July	1st to 10th	Night	37% 10% 17% 23% 11% 1	4015
	11th to 20th	Night	36% 9% 21% 25% 7% 2	4006
	21st to end	Night	51% 8% 13% 19% 8%	4297
August	1st to 10th	Night	38% 7% 17% 26% 12% 1	2904
	11th to 20th	Night	38% 6% 15% 28% 13% 1	2902
	21st to end	Night	21% 5% 14% 38% 20% 2	3468
September	1st to 10th	Night	35% 5% 14% 22% 18% 6%	2192
	11th to 20th	Night	38% 6% 9% 24% 22% 2	2200
	21st to end	Night	37% 6% 9% 24% 22% 3	2200
October	1st to 10th	Night	30% 5% 7% 22% 33% 4	3618
	11th to 20th	Night	29% 3 6% 24% 28% 10%	3899
	21st to end	Night	28% 2 6% 26% 27% 10%	4252
November	1st to 10th	Night	38% 3 7% 22% 24% 6%	2661
	11th to 20th	Night	42% 3 7% 21% 22% 6%	2730
	21st to end	Night	45% 3 7% 26% 17% 3	2730
December	1st to 10th	Night	54% 3 9% 21% 12% 1	4380
	11th to 20th	Night	58% 3 8% 19% 11% 1	4954
	21st to end	Night	60% 4 9% 16% 10%	6633

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	28% 28% 27% 12% 4 1	330
	11th to 20th	Day	39% 28% 20% 11% 2	330
	21st to end	Day	33% 29% 26% 9% 3	363
February	1st to 10th	Day	30% 24% 29% 13% 4 1	330
	11th to 20th	Day	43% 27% 19% 9% 2	330
	21st to end	Day	35% 26% 30% 7% 1	273
March	1st to 10th	Day	31% 20% 30% 13% 4	430
	11th to 20th	Day	28% 23% 29% 16% 3 1	540
	21st to end	Day	30% 19% 30% 15% 6%	594
April	1st to 10th	Day	29% 17% 28% 17% 8% 1	504
	11th to 20th	Day	23% 18% 31% 18% 8% 2	492
	21st to end	Day	24% 18% 28% 19% 9% 3	500
May	1st to 10th	Day	27% 17% 27% 17% 8% 4	503
	11th to 20th	Day	19% 14% 25% 24% 13% 5%	527
	21st to end	Day	16% 9% 25% 23% 17% 10%	573
June	1st to 10th	Day	12% 8% 26% 26% 18% 9%	530
	11th to 20th	Day	10% 8% 23% 26% 24% 9%	530
	21st to end	Day	8% 5% 29% 25% 22% 11%	530
July	1st to 10th	Day	13% 5% 18% 28% 21% 15%	525
	11th to 20th	Day	9% 6% 17% 32% 22% 15%	524
	21st to end	Day	14% 5% 15% 30% 24% 12%	576
August	1st to 10th	Day	15% 5% 17% 27% 24% 12%	526
	11th to 20th	Day	14% 4 18% 26% 27% 9%	519
	21st to end	Day	11% 3 13% 30% 29% 14%	583
September	1st to 10th	Day	15% 5% 14% 25% 30% 11%	530
	11th to 20th	Day	15% 2 12% 29% 28% 14%	530
	21st to end	Day	20% 9% 19% 27% 18% 7%	530
October	1st to 10th	Day	16% 7% 18% 27% 23% 9%	530
	11th to 20th	Day	17% 11% 26% 26% 16% 4	530
	21st to end	Day	27% 11% 25% 23% 11% 3	583
November	1st to 10th	Day	21% 14% 31% 22% 9% 4	330
	11th to 20th	Day	27% 15% 34% 15% 9% 1	330
	21st to end	Day	28% 18% 28% 18% 6% 1	330
December	1st to 10th	Day	32% 21% 30% 13% 3 1	330
	11th to 20th	Day	30% 18% 27% 19% 5% 1	330
	21st to end	Day	33% 20% 28% 14% 5%	363
January	1st to 10th	Night	28% 21% 28% 19% 4	840
	11th to 20th	Night	33% 20% 25% 19% 4	840
	21st to end	Night	25% 19% 33% 21% 2	924
February	1st to 10th	Night	25% 18% 30% 25% 2	840
	11th to 20th	Night	34% 19% 30% 15% 1	823
	21st to end	Night	34% 17% 30% 17% 2	692
March	1st to 10th	Night	26% 16% 30% 23% 4	840
	11th to 20th	Night	25% 17% 30% 21% 6% 1	840
	21st to end	Night	22% 17% 30% 24% 7%	924
April	1st to 10th	Night	22% 16% 27% 27% 8%	804
	11th to 20th	Night	21% 12% 28% 27% 11% 1	593
	21st to end	Night	19% 8% 32% 31% 9% 1	600
May	1st to 10th	Night	18% 9% 27% 31% 14% 2	600
	11th to 20th	Night	18% 6% 20% 35% 19% 1	591
	21st to end	Night	11% 4 19% 33% 25% 7%	650
June	1st to 10th	Night	12% 4 15% 39% 27% 3	600
	11th to 20th	Night	6% 2 17% 38% 32% 5%	600
	21st to end	Night	10% 2 16% 36% 31% 5%	600
July	1st to 10th	Night	14% 2 10% 30% 37% 8%	585
	11th to 20th	Night	7% 2 11% 30% 44% 6%	582
	21st to end	Night	13% 2 8% 32% 37% 7%	657
August	1st to 10th	Night	12% 3 8% 31% 40% 6%	597
	11th to 20th	Night	13% 2 8% 36% 35% 7%	594
	21st to end	Night	10% 2 10% 30% 40% 9%	660
September	1st to 10th	Night	12% 3 11% 32% 37% 5%	792
	11th to 20th	Night	15% 2 12% 31% 36% 5%	800
	21st to end	Night	15% 4 16% 36% 25% 4	800
October	1st to 10th	Night	14% 6% 16% 31% 30% 4	800
	11th to 20th	Night	21% 8% 19% 34% 17% 2	800
	21st to end	Night	21% 10% 23% 32% 13% 1	880
November	1st to 10th	Night	17% 10% 23% 32% 15% 2	820
	11th to 20th	Night	20% 15% 25% 30% 9% 1	840
	21st to end	Night	24% 13% 22% 30% 10%	840
December	1st to 10th	Night	30% 16% 25% 24% 5%	840
	11th to 20th	Night	25% 17% 26% 26% 5% 1	840
	21st to end	Night	29% 17% 24% 27% 4	924

Page generated at 2026-03-30 02:18:43. Version ac07

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