Upper-air Water Vapour

	Water vapour is the predominant gaseous source of infrared opacity in the atmosphere, accounting for about 60% of the natural greenhouse effect for clear skies. Water vapour condenses to produce clouds, changing radiative properties and releasing latent heat that drives or modifies atmospheric circulation systems. Water vapour in the lower stratosphere, though present only in small amounts, is radiatively significant. Consequently, there is potential for additional climate change feedbacks through perturbations of the processes that control the amount of water vapour in the stratosphere, such as the Brewer-Dobson circulation that strongly influences tropical tropopause temperatures and the amount of CH4 oxidized in the stratosphere.
	Domain:	Atmosphere
	Subdomain:	Upper Atmosphere
	Scientific Area:	Hydrosphere
	ECV Steward:
	Products:	Water Vapour Mixing Ratio in the Upper Troposphere and Lower Stratosphere, Water Vapour Mixing Ratio in the Middle and Upper Stratosphere, Water Vapour Mixing Ratio in the Mesosphere, Relative Humidity in the Boundary Layer, Relative Humidity in the Free Troposphere, Relative Humidity in the Upper Troposphere and Lower Stratosphere, Specific Humidity in the Boundary Layer, Specific Humidity in the Free Troposphere, Integrated Water Vapour

ECV Products and Requirements

These products and requirements reflect the Implementation Plan 2022 (GCOS-244).

The requirements are found in the complete 2022 ECVs Requirements document as well: ECV Upper-air Water Vapour.

Products	Water Vapour Mixing Ratio in the
Products			Upper Troposphere and Lower Stratosphere	Middle and Upper Stratosphere	Mesosphere
	(*)	Unit	Values	Values	Values
Horizontal Resolution	G	km	15	50	50
	B		100	500	500
	T		500	1500	1500
Vertical Resolution	G	km	0.01	0.5	0.5
	B		0.1	1	1
	T		0.25	3	3
Temporal Resolution	G	h	3	3	3
	B		6	6	6
	T		24	72	72
Timeliness	G	h	1	1	1
	B		120	168	168
	T		720	720	720
Required Measurement Uncertainty (2-sigma)	G	ppmv	0.1	0.1	0.1
	B		0.25	0.25	0.25
	T		0.5	0.5	0.5
Stability	G	ppmv/decade	<0.1	<0.2	<0.2
	B		0.1	0.2	0.2
	T		0.25	0.5	0.5

Products	Relative Humiditiy in the
Products			Boundary Layer		Free Troposphere	Upper Troposphere and Lower Stratosphere
	(*)	Unit	Values	Unit	Values	Values
Horizontal Resolution	G	km	15	km	15	15
	B		100		100	100
	T		500		1000	500
Vertical Resolution	G	m	1	km	0.01	0.01
	B		10		0.1	0.1
	T		100		1	0.25
Temporal Resolution	G	h	<1	h	<1	3
	B		6		6	6
	T		12		12	24
Timeliness	G	h	1	h	1	1
	B		120		120	120
	T		720		720	720
Required Measurement Uncertainty (2-sigma)	G	%RH	0.1	%RH	0.1	0.5
	B		0.5		0.5	1
	T		1		1	2
Stability	G	%RH/ decade	0.1	%RH/ decade	0.1	<0.5
	B		0.5		0.5	0.5
	T		1		1	2

Products	Specific Humidity in the						Integrated Water Vapour
Products			Boundary Layer		Free Troposphere		Integrated Water Vapour
	(*)	Unit	Values	Unit	Values	Unit	Values
Horizontal Resolution	G	km	15	km	15	km	25
	B		100		100		250
	T		500		1000		1000
Vertical Resolution	G	m	1	km	0.01		-
	B		10		0.1		-
	T		100		1		-
Temporal Resolution	G	h	<1	h	<1	h	0.2
	B		1		1		1
	T		3		3		24
Timeliness	G	h	1	h	1	h	24
	B		120		120		120
	T		720		720		720
Required Measurement Uncertainty (2-sigma)	G	g Kg-1	0.1	g Kg-1	0.1	Kg m-3	0.1
	B		0.5		0.5		0.5
	T		1		1		1
Stability	G	g Kg-1/ decade	0.01	g Kg-1/ decade	0.01	Kg m-3/ decade	0.1
	B		0.05		0.05		0.2
	T		0.1		0.1		0.5

(*) Goal (G): an ideal requirement above which further improvements are not necessary. Breakthrough (B): an intermediate level between threshold and goal which, if achieved, would result in a significant improvement for the targeted application. The breakthrough value may also indicate the level at which specified uses within climate monitoring become possible. It may be appropriate to have different breakthrough values for different uses. Threshold (T): the minimum requirement to be met to ensure that data are useful

Data Sources

This list provides sources for openly accessible data sets with worldwide coverage for which metadata is available. It is curated by the respective GCOS ECV Steward(s). The list does not claim to be complete. Anyone with a suitable dataset who wishes it to be added to this list should contact the GCOS Secretariat.