This process is known as evaporational cooling and the magnitude of its effects is determined by how dry the low levels are. This, of course, can be measured using the dew point! Low dew points and large temperature dew point depressions the difference between the temperature and the dew point indicate the potential for a dramatic drop in temperature once precipitation begins falling.
Jack Sillin is an Atmospheric Science student Cornell '22 and weather forecaster who regularly writes for weather.
Follow me on twitter JackSillin. Save my name, email, and website in this browser for the next time I comment. Notice: It seems you have Javascript disabled in your Browser. In order to submit a comment to this post, please write this code along with your comment: 13abdef5cd93f0a3a69a Weather encyclopedia. Dew forms on surfaces when the temperature cools to the dew point.
About the author Jack Sillin Jack Sillin is an Atmospheric Science student Cornell '22 and weather forecaster who regularly writes for weather. Related posts. Method : Single-anonymous Revisions: 1 Screened for originality? Buy this article in print. However, as surface temperatures may not be a good indicator of atmospheric moisture, an alternative is to use atmospheric temperatures, but the use of atmospheric temperatures lacks precedent.
Using radiosonde atmospheric temperature data at a range of geopotential heights from 34 weather stations across Australia and its territories, we examine whether atmospheric temperature can improve our understanding of rainfall-temperature sensitivities. There is considerable variability in the calculated sensitivity when using atmospheric air temperature, while atmospheric dew point temperature showed robust positive sensitivities, similar to when surface dew point temperature measurements were used.
We conclude atmospheric dew point temperature may be a promising candidate for future investigations of empirically calculated sensitivities of rainfall to temperature but does not appear superior to the use of surface dew point temperature measurements.
Original content from this work may be used under the terms of the Creative Commons Attribution 3. Any further distribution of this work must maintain attribution to the author s and the title of the work, journal citation and DOI. Climate change represents one of the most pressing issues facing society due to its effect on meteorological and hydrological events.
Rising temperatures can directly impact rainfall patterns by increasing the atmospheric moisture holding capacity leading to increased rainfall extremes Trenberth et al , O'Gorman Hence, understanding the relationship between temperature and extreme rainfall is a key step towards understanding the effects of climate change on rainfall.
If the saturation vapour pressure increases at this rate it is plausible to suggest the maximum rainfall intensity should increase at a similar rate Trenberth , Westra et al This scaling relationship is based on two core assumptions; that relative humidity will remain approximately constant in the future Soden and Held , and that extreme rainfall events precipitate all available moisture Lenderink and van Meijgaard However, the possible violation of these assumptions, alongside artefacts introduced due to the use of surface temperatures in the calculations results in deviations from the C—C relationship being observed.
Scaling of rainfall extremes much higher than C—C super C—C has been found across a variety of climates and regions Lenderink and van Meijgaard , Liu et al , Mishra et al , particularly for short rainfall durations Hardwick Jones et al , Lenderink et al , Panthou et al , Busuioc et al , Wasko et al , Wibig and Piotrowski Super C—C scaling has generally been attributed to the convective nature of short duration storms Berg and Haerter , Berg et al , Park and Min During a storm the natural release of latent heat could cause additional moisture convergence in the system, invigorating the storm and increasing the rainfall intensity beyond the theoretical C—C value Trenberth et al , Westra et al Using lightning Molnar et al or cloud type Berg and Haerter as a proxy for identifying convective events, super C—C scaling rates have been found for convective storms.
When convective storms are removed from the storm sample, the scaling reduces to close or less than C—C. Hence changes in atmospheric circulations which are associated with convection also affect the scaling calculated Blenkinsop et al , Chan et al Negative scaling has also been identified in many regions across the world Utsumi et al , Wasko et al and is particularly associated with the higher temperatures experienced in tropical regions Hardwick Jones et al , Maeda et al Negative scaling contradicts observed historical increases in tropical rainfall extremes Donat et al , Guerreiro et al This has led to the explanation that, at higher temperatures, the relative humidity decreases, as there is no more moisture to be sourced Hardwick Jones et al and evaporation limitations Priestley , Roderick et al result in a reversal from positive to negative scaling Drobinski et al , , Gao et al Indeed the most negative scaling is found in regions with the largest humidity limitations Wasko et al However, other explanations are also possible.
Warmer surface temperatures generally occur on less cloudy days with less rainfall, resulting in negative scaling Trenberth and Shea , which would account for the observation that at higher temperatures the proportion of the day experiencing rainfall is also smaller Utsumi et al It has also been shown that storms of shorter duration generally occur when temperatures are higher.
As these storms also have lower rainfall intensities due to their shorter duration there is a resulting negative bias in the scaling relationship calculated Wasko et al Negative scaling has also been explained by considering the timing of measured temperatures in relation to the storm arrival.
Temperatures can decrease during a rainfall event for many reasons, including evaporative cooling, or movement of cold air associated with the rainfall event Bao et al As a result the temperature coincident with the rainfall may not truly reflect the temperature that occurred when the rainfall was actually generated Ali and Mishra , Bao et al Using the temperature three days prior to rainfall events has returned positive scaling in some locations where previously negatively scaling was calculated Ali and Mishra However, as it has been refuted that local cooling is the reason for negative scaling Barbero et al , there continues to be an assertion that moisture limitations are the most likely physical explanation for negative scaling Lenderink et al To overcome moisture limitations at higher temperatures, two alternatives in the literature have been proposed for calculating rainfall-temperature sensitivities; surface dew point temperature and atmospheric air temperature.
It has been strongly advocated that dew point temperature e. A multitude of studies have investigated the sensitivity of extreme rainfall with dew point temperature Lenderink and van Meijgaard , Lenderink et al , Panthou et al , Ali and Mishra , Park and Min , Ali et al , Wasko et al Most recently a global study Ali et al showed that, for the majority of tropical areas, positive scaling closer to the C—C relationship is obtained when surface dew point temperature is used instead of surface air temperature.
However, one of the main arguments, for not using surface temperatures air or dew point is that they are impacted by the rainfall from the storm event. An alternative proposed is to use an upper troposphere temperature at a height sufficient enough to avoid fluctuations due to the storm event Mishra et al and prevent the dominance of solar surface heating Chan et al Using atmospheric temperature is physically more consistent with rainfall causing processes.
Rainfall totals are constrained by the amount of the moisture in the atmosphere and hence exhibit a very strong correlation to the integrated water vapour Roderick et al The integrated water vapour is a function of the mean temperature in the water vapour column Hagemann et al , and rainfall resulting from convection is dependent on the atmospheric temperature Neelin et al Hence a change in atmospheric air temperature as opposed to surface temperature should better capture a change in the saturation vapour pressure that is linked to extreme rainfall.
To that extent climate modelled hPA air temperatures have also resulted in positive scaling across the United Kingdom where surface temperatures did not Chan et al Atmospheric air temperature has also been used for developing non-stationary IDF curves Ali and Mishra , Golroudbary et al The notion that surface temperature may a not be physically well linked to atmospheric moisture, and b subject to many statistical artefacts, gives credence to the use of atmospheric temperatures.
However, there is currently a lack of precedent for using atmospheric temperatures for rainfall-temperature scaling calculations. Our aim is to investigate the relationship of rainfall with atmospheric air temperature and atmospheric dew point temperature at various geopotential heights to see whether there is merit in using atmospheric temperatures over surface temperatures for calculating the scaling of rainfall with temperature. We use data sourced from the Australian Bureau of Meteorology at 34 locations around Australia and its surrounding territories.
Atmospheric air temperature and atmospheric dew point temperature is measured using balloon-borne radiosonde instruments up to an altitude of 25 km. The frequency of measurements varies but generally occurs every 6 h. The locations of these sites are summarised in table S1 and are available online at stacks. Daily rainfall is reported at each of the radiosonde sites at the ground at 9 am local time as a daily accumulation and is measured through either a manually read rain gauge or automatic weather station.
Daily surface air and dew point temperatures are also sourced from the Australian Bureau of Meteorology at each of the radiosonde measurements locations. The stations are spread evenly across the continent of Australia covering a range of atmospheric and climatic conditions Linacre and Geerts , Peel et al This includes tropical regions in the north where rainfall is summer dominant, and temperate regions in the south-east and south-west where rainfall is winter dominant.
In central Australia the climate is arid desert. Hence, this data set represents a diverse range of climates. For each station, the atmospheric air temperatures and atmospheric dew point temperatures were averaged for the 24 h concurrent with the period of any non-zero rainfall observation.
A state of saturation exists when the air is holding the maximum amount of water vapor possible at the existing temperature and pressure. When the dew point temperature and air temperature are equal, the air is said to be saturated.
Therefore, if the air cools, moisture must be removed from the air and this is accomplished through condensation. This process results in the formation of tiny water droplets that can lead to the development of fog, frost, clouds, or even precipitation.
Relative Humidity can be inferred from dew point values.
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