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NASA’s Perseverance rover’s Mars Environmental Dynamics Analyzer is collecting data at Jezero crater, characterizing the physical processes in the lowest layer of the Martian atmosphere. Here we present measurements from the instrument’s first 250 sols of operation, revealing a spatially and temporally variable meteorology at Jezero. We find that temperature measurements at four heights capture the response of the atmospheric surface layer to multiple phenomena. We observe the transition from a stable night-time thermal inversion to a daytime, highly turbulent convective regime, with large vertical thermal gradients. Measurement of multiple daily optical depths suggests aerosol concentrations are higher in the morning than in the afternoon. Measured wind patterns are driven mainly by local topography, with a small contribution from regional winds. Daily and seasonal variability of relative humidity shows a complex hydrologic cycle. These observations suggest that changes in some local surface properties, such as surface albedo and thermal inertia, play an influential role. On a larger scale, surface pressure measurements show typical signatures of gravity waves and baroclinic eddies in a part of the seasonal cycle previously characterized as low wave activity. These observations, both combined and simultaneous, unveil the diversity of processes driving change on today’s Martian surface at Jezero crater.

The Curiosity Rover has been exploring the Gale Crater using the on‐board Navigation Camera to monitor the line‐of‐sight (LOS) dust extinction within the crater. Previous studies of the line‐of‐sight extinction have shown an annual trend where a minimum in extinction occurs around Ls= 100° and a maximum occurs around Ls= 300°. However, past studies have been constrained to images acquired only between 10:00 and 14:00 local time, limiting our understanding of the variation of dust extinction in Gale Crater throughout the day. Here, using a method that corrects for variable lighting geometry, we reanalyze the line‐of‐sight images captured throughout the mission to include images acquired in the early morning and late afternoon. Additionally, we update the line‐of‐sight record to include over 1,000 additional sols of the mission through sol 3,663, which updates our record of extinction in Gale to the end of Mars Year 36—a period of almost 5.5 Mars Years. Using images taken throughout the sol, we examine the diurnal trend in dust extinction, where a maximum is seen around solar noon. This diurnal trend is observed throughout the year, with a larger diurnal variation observed during the southern summer. Additionally, the geographic homogeneity in dust loading is quantified, with higher dust loading observed in the western portion of the line‐of‐sight images, corresponding with the north‐western portion of the crater rim. These observations suggest that dust lifting from the surface and vertical mixing are factors in the diurnal, seasonal, and geographic dynamics observed in the extinctions. The Curiosity Rover has used its Navigation Camera to image the northern rim of Gale Crater for over 3,000 martian days. These images are used to calculate the amount of airborne dust between the rover and the crater rim. Past studies using these images found that the amount of dust was lowest during southern‐hemisphere winter and highest during southern‐hemisphere summer. In this work, a new method to find the amount of dust was used, allowing us to use images taken throughout the day for the first time. The entire set of crater rim images was reanlayzed using the new method, including new images captured up to sol 3,663. We find a daily cycle, where the amount of dust in the air is lowest in the morning and evening, and highest around noon. In addition, there is more airborne dust between the rover and the crater rim on the western side of the image compared with the center and eastern side. This suggests that dust is moving within the Gale Crater on short timescales, where dust lifting from the surface likely plays a key role in the diurnal trend, in combination with air outside the crater mixing with air within the crater. The line‐of‐sight dust extinction is calculated through 3,663 sols using the Mars Science Laboratory's Navigation CameraDiurnally, the dust extinction peaks at solar noon, with lower values in the morning and eveningThe diurnal variation is higher during the dustier southern summer and lower during the southern winter when dust extinction is at a minimum The line‐of‐sight dust extinction is calculated through 3,663 sols using the Mars Science Laboratory's Navigation Camera Diurnally, the dust extinction peaks at solar noon, with lower values in the morning and evening The diurnal variation is higher during the dustier southern summer and lower during the southern winter when dust extinction is at a minimum

Mars' atmosphere typically supports dust aerosol with an effective radius near 1.5 μm, varying from ~1 μm during low dust times near northern summer solstice to ~2 μm during higher dust times in southern spring and summer. After global dust events, size variations outside this range have not previously been observed. We report on imaging and spectral observations by the Curiosity rover through the 2018 global dust event. These observations show that the dust effective radius was seasonally normal prior to the local onset of increased opacity, increased rapidly above 4 μm with increasing opacity, remained above 3 μm over a period of ~50 Martian solar days, then returned to seasonal values before the opacity did so. This demonstrates lifting and regional‐scale transport of a dust population ~3 times the size of typical dust aerosol. During the global dust storm of 2018, the Curiosity rover measured the variation of atmospheric dust over time. During the onset of the dust storm, typical Martian dust was enhanced by much larger particles that were freshly lifted off the surface in distant storms and then carried to the rover site at Gale crater. The larger dust particles persisted for weeks, but fell out of the atmosphere faster than the typical dust as normal conditions were restored. The Curiosity rover observed dust aerosol size variations through the 2018 global dust eventThe average dust radius increased above 4 μm, more than double the largest sizes previously seen with Curiosity's instrumentsThe observations demonstrate the lifting and regional‐scale transport of dust significantly larger than typical dust aerosol

The Rover Environmental Monitoring Station (REMS) instrument is on board NASA's Mars Science Laboratory (MSL) Curiosity rover. REMS has been measuring surface pressure, air, and ground brightness temperature, relative humidity, and ultraviolet (UV) irradiance since MSL's landing in 2012. In Mars Year (MY) 34 (2018) a global dust storm reached Gale Crater at Ls~ 190°. REMS offers a unique opportunity to better understand the impact of a global dust storm on local environmental conditions, which complements previous observations by the Viking landers and Mars Exploration Rovers. All atmospheric variables measured by REMS are strongly affected albeit at different times. During the onset phase, the daily maximum UV radiation decreased by 90% between sols 2075 (opacity ~1) and 2085 (opacity ~8.5). The diurnal range in ground and air temperatures decreased by 35 and 56 K, respectively, with also a diurnal‐average decrease of ~2 and 4 K respectively. The maximum relative humidity, which occurs right before sunrise, decreased to below 5%, compared with prestorm values of up to 29%, due to the warmer air temperatures at night, while the inferred water vapor abundance suggests an increase during the storm. Between sols 2085 and 2130, the typical nighttime stable inversion layer was absent near the surface as ground temperatures remained warmer than near‐surface air temperatures. Finally, the frequency domain behavior of the diurnal pressure cycle shows a strong increase in the strength of the semidiurnal and terdiurnal modes peaking after the local opacity maximum, also suggesting differences in the dust abundance inside and outside Gale. Atmospheric opacity over Gale Crater was increased by more than 8 times and disturbed all the atmospheric variables measured by REMSREMS data suggest that the nighttime near‐surface atmosphere stability was reduced and its water abundance increased during the GDSThe semidiurnal mode peaked after the local opacity maximum, suggesting different dust abundance inside and outside Gale

A distinctive feature of the Martian atmosphere is that the winter polar vortices exhibit annuli of high potential vorticity (PV) with a local minimum near the pole. These annuli are seen in observations, reanalyses, and free‐running general circulation model simulations of Mars, but are not generally a feature of Earth's polar vortices, where there is a monotonic increase in magnitude of PV with latitude. The creation and maintenance of the annular polar vortices on Mars are not well understood. Here we use simulations with a Martian general circulation model to the show that annular vortices are related to another distinctive, and possibly unique in the solar system, feature of the Martian atmosphere: the condensation of the predominant atmospheric gas species (CO2) in polar winter regions. The latent heat associated with CO2condensation leads to destruction of PV in the polar lower atmosphere, inducing the formation of an annular PV structure. The winter polar vortices in the Martian atmosphere exhibit annuli of high potential vorticity with a local minimum near the poleLatent heating associated with CO2condensation leads to the formation of an annular potential vorticity structureThe existence of annular vortices on other planetary bodies may be related to condensation of atmospheric gas species in polar regions

The structure and evolution of the Martian polar vortices is examined using two recently available reanalysis systems: version 1.0 of the Mars Analysis Correction Data Assimilation (MACDA) and a preliminary version of the Ensemble Mars Atmosphere Reanalysis System (EMARS). There is quantitative agreement between the reanalyses in the lower atmosphere, where Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) data are assimilated, but there are differences at higher altitudes reflecting differences in the free‐running general circulation model simulations used in the two reanalyses. The reanalyses show similar potential vorticity (PV) structure of the vortices: There is near‐uniform small PV equatorward of the core of the westerly jet, steep meridional PV gradients on the polar side of the jet core, and a maximum of PV located off of the pole. In maps of 30 sol mean PV, there is a near‐continuous elliptical ring of high PV with roughly constant shape and longitudinal orientation from fall to spring. However, the shape and orientation of the vortex varies on daily time scales, and there is not a continuous ring of PV but rather a series of smaller scale coherent regions of high PV. The PV structure of the Martian polar vortices is, as has been reported before, very different from that of Earth's stratospheric polar vortices, but there are similarities with Earth's tropospheric vortices which also occur at the edge of the Hadley Cell, and have near‐uniform small PV equatorward of the jet, and a large increase of PV poleward of the jet due to increased stratification. There is quantitative agreement in polar vortex structure in two Mars reanalysesThere is near‐zero PV equatorward of jet core, steepest meridional gradients poleward of jet, and maximum PV is offset from the poleOn monthly timescale there is an elliptical annulus of potential vorticity but shape and orientation is much more variable at shorter times

The pressure sensors on Mars rover Perseverance measure the pressure field in the Jezero crater on regular hourly basis starting in sol 15 after landing. The present study extends up to sol 460 encompassing the range of solar longitudes from Ls∼ 13°–241° (Martian Year (MY) 36). The data show the changing daily pressure cycle, the sol‐to‐sol seasonal evolution of the mean pressure field driven by the CO2sublimation and deposition cycle at the poles, the characterization of up to six components of the atmospheric tides and their relationship to dust content in the atmosphere. They also show the presence of wave disturbances with periods 2–5 sols, exploring their baroclinic nature, short period oscillations (mainly at night‐time) in the range 8–24 min that we interpret as internal gravity waves, transient pressure drops with duration ∼1–150 s produced by vortices, and rapid turbulent fluctuations. We also analyze the effects on pressure measurements produced by a regional dust storm over Jezero at Ls∼ 155°. Mars rover Perseverance landed on 18 February 2021 on Jezero crater. It carries a weather station that has measured, among other quantities, surface atmospheric pressure. This study covers the first 460 sols or Martian days, a period that comprises a large part of the Martian year, including spring, summer and a part of autumn. Each sol, the pressure has significant changes, and those can be understood as a result of the so‐called thermal tides, oscillations of pressure with periods that are fractions of one sol. The mean value of pressure each sols changes with the season, driven by the CO2sublimation in summer and condensation in winter at both poles. We report oscillations of the mean daily pressure with periods of a few sols, related to waves at distant parts of the planet. Within single sols, we find oscillations of night pressure with periods of tens of minutes, caused by gravity waves. Looking at shorter time intervals, we find the signature of the close passage of vortices such as dust devils, and very rapid daytime turbulent fluctuations. We finally analyze the effects on all these phenomena produced by a regional dust storm that evolved over Jezero in early January 2022. We study the pressure measurements performed on the first 460 sols by the rover Perseverance M2020The daily and seasonal cycles and the evolution of six tidal components and their relationship to dust content are presentedWe characterize long‐period waves (sols), short‐period gravity waves (min.), rapid pressure fluctuations and a regional dust storm impact We study the pressure measurements performed on the first 460 sols by the rover Perseverance M2020 The daily and seasonal cycles and the evolution of six tidal components and their relationship to dust content are presented We characterize long‐period waves (sols), short‐period gravity waves (min.), rapid pressure fluctuations and a regional dust storm impact

Wind speeds measured by the Mars 2020 Perseverance rover in Jezero crater were fitted as a Weibull distribution. InSight wind data acquired in Elysium Planitia were also used to contextualize observations. Jezero winds were found to be much calmer on average than in previous landing sites, despite the intense aeolian activity observed. However, a great influence of turbulence and wave activity was observed in the wind speed variations, thus driving the probability of reaching the highest wind speeds at Jezero, instead of sustained winds driven by local, regional, or large‐scale circulation. The power spectral density of wind speed fluctuations follows a power‐law, whose slope deviates depending on the time of day from that predicted considering homogeneous and isotropic turbulence. Daytime wave activity is related to convection cells and smaller eddies in the boundary layer, advected over the crater. The signature of convection cells was also found during dust storm conditions, when prevailing winds were consistent with a tidal drive. Nighttime fluctuations were also intense, suggesting strong mechanical turbulence. Convective vortices were usually involved in rapid wind fluctuations and extreme winds, with variations peaking at 9.2 times the background winds. Transient high wind events by vortex‐passages, turbulence, and wave activity could be driving aeolian activity at Jezero. We report the detection of a strong dust cloud of 0.75–1.5 km in length passing over the rover. The observed aeolian activity had major implications for instrumentation, with the wind sensor suffering damage throughout the mission, probably due to flying debris advected by winds. Jezero winds as measured in the crater floor by Perseverance were found to be much calmer on average than in previous landing sites. Turbulence and wave activity provoked rapid fluctuations that changed wind speed from calm conditions to more than 10–15 ms−1in the timescale of seconds to minutes. Daytime wave activity is related to convection cells and smaller eddies in the boundary layer, advected over the crater. These convection cells are produced under strong thermal gradients typically present during daytime. Pressure drops, associated with convective vortices, were usually involved in rapid wind fluctuations and, in some cases, in extreme winds as measured by Perseverance. An intense aeolian activity was observed at Jezero crater produced by transient high wind events. This aeolian activity had major implications for instrumentation, with the Perseverance wind sensor suffering damage probably due to flying debris advected by winds. Also, we report the detection of a strong dust cloud of 0.75–1.5 km passing over the rover. This paper has a companion paper (part 1) in the same issue, which is focused on wind patterns and analyzed the mechanisms driving atmospheric circulation at Jezero. Jezero winds are found to be much calmer on average than in previous landing sites, despite the intense aeolian activity observedTurbulence, wave activity, and convective vortices drive the peak wind speeds observed at JezeroWe report the detection of a dust cloud of 0.75–1.5 km in length passing over the rover Jezero winds are found to be much calmer on average than in previous landing sites, despite the intense aeolian activity observed Turbulence, wave activity, and convective vortices drive the peak wind speeds observed at Jezero We report the detection of a dust cloud of 0.75–1.5 km in length passing over the rover

This is the first part of a two‐part paper. NASA's Mars 2020 Perseverance rover measured winds on the Jezero crater floor close to the delta of an ancient river. A mostly repeatable diurnal cycle was observed and presented two regimes: (a) a convective regime, from dawn to sunset, with average easterly to southeasterly winds, during which maximum wind speeds were measured, and (b) a nighttime regime with westerly‐northwesterly winds followed by a relatively calm period with highly variable wind directions as a function of sol and time of night. The timing and magnitude of the observed regimes are consistent with primary control by regional and local slope flows. Data suggest that the surface circulation at Jezero region in northern spring and summer is highly unaffected by large‐scale circulation except during particular periods in the diurnal cycle or generally during dust storms, which is supported by MarsWRF model simulations. Consequently, the observed seasonal variability was weak. However, sol‐to‐sol and seasonal variability were measured, most of it during certain nighttime periods. Traveling waves consistent with baroclinic instability were clearly observed in surface winds at Ls∼ 75°. The early MY36/2022A regional dust storm at Ls∼ 153° disturbed the wind patterns with changes suggesting enhanced tidal flows. After sunset, the dust storm also produced detectable gravity wave activity, increasing the mixing in the nighttime planetary boundary layer during storm conditions. Inferred wind directions from dust devil movies strongly suggest that prevailing winds continued to be slope‐driven during the late summer, fall and early winter seasons. This is the first part of a two‐part paper. NASA's Mars 2020 Perseverance rover landed close to the western rim of Jezero crater (18.44°N, 77.45°E) on 18 February 2021. The wind data acquired by the rover measured a mostly repeatable diurnal cycle with two regimes: (a) a convective period, from dawn to sunset with average easterly to southeasterly winds, in which maximum wind speeds were measured, and (b) a nighttime regime with westerly‐northwesterly winds followed by a relatively calm period with highly variable wind directions as a function of sol and time of night. The timing and magnitude of the observed regimes are consistent with primary control by regional and local slope flows, as have been observed to varying degrees at other landing sites on Mars. Data suggest that the surface circulation at Jezero is highly unaffected by large‐scale circulation, except during particular periods. An early regional dust storm prior to fall equinox also disturbed wind patterns with changes suggesting the strengthening of flows linked to large‐scale atmospheric oscillations called tides. The latter are primarily driven by the daily pattern of solar heating around Mars and its interaction with topography. A mostly repeatable diurnal cycle of winds is consistent with primary control by regional and local slope flowsGreat sub‐diurnal and sol‐to‐sol variability in winds was observed during the calm period at nightTraveling and gravity waves were observed, along with the probable effect of enhanced tidal flows A mostly repeatable diurnal cycle of winds is consistent with primary control by regional and local slope flows Great sub‐diurnal and sol‐to‐sol variability in winds was observed during the calm period at night Traveling and gravity waves were observed, along with the probable effect of enhanced tidal flows

Observations by several cameras on the Perseverance rover showed a 22° scattering halo around the Sun over several hours during northern midsummer (solar longitude 142°). Such a halo has not previously been seen beyond Earth. The halo occurred during the aphelion cloud belt season and the cloudiest time yet observed from the Perseverance site. The halo required crystalline water‐ice cloud particles in the form of hexagonal columns large enough for refraction to be significant, at least 11 μm in diameter and length. From a possible 40–50 km altitude, and over the 3.3 hr duration of the halo, particles could have fallen 3–12 km, causing downward transport of water and dust. Halo‐forming clouds are likely rare due to the high supersaturation of water that is required but may be more common in northern subtropical regions during northern midsummer. A scattering halo, or bright ring around the Sun, was seen in pictures taken by cameras on the Perseverance rover. Such halos are commonly seen in ice clouds on Earth but have never before been seen on Mars. When the halo was seen, on 15 December 2021, the rover was within a period of unusually cloudy weather near the end of a season known for water‐ice clouds in northern tropical areas such as the rover's site in Jezero crater. The appearance and size of the halo showed that the clouds were made of water‐ice crystals shaped like hexagonal columns. The crystals were likely larger than those in most water‐ice clouds on Mars, which allowed the halo to form. A 22°‐radius halo around the Sun was imaged for over 3 hr by cameras on the Perseverance rover on MarsSuch a halo is diagnostic of scattering by large, hexagonal water‐ice crystals and has not previously been seen beyond EarthThe halo implies that high supersaturation of water may be more common in late summer in the northern subtropics than elsewhere A 22°‐radius halo around the Sun was imaged for over 3 hr by cameras on the Perseverance rover on Mars Such a halo is diagnostic of scattering by large, hexagonal water‐ice crystals and has not previously been seen beyond Earth The halo implies that high supersaturation of water may be more common in late summer in the northern subtropics than elsewhere

We use in situ environmental measurements by the Mars Science Laboratory (MSL) mission to obtain the surface energy budget (SEB) across Curiosity's traverse during the first 2500 sols of the mission. This includes values of the downwelling shortwave solar radiation, the upwelling solar radiation reflected by the surface, the downwelling longwave radiation from the atmosphere, the upwelling longwave radiation emitted by the surface, the sensible heat flux associated with turbulent motions, and the latent heat flux associated with water phase changes. We then analyze their temporal variation on different timescales and relate this to the mechanisms causing these variations. Through its Rover Environmental Monitoring Station, MSL allows for a more accurate determination of the SEB than its predecessors on Mars. Moreover, the unprecedented duration, cadence, and frequency of MSL environmental observations allow for analyses of the SEB from diurnal to interannual timescales. The results presented in this article can be used to evaluate the consistency with predictions from atmospheric numerical models, to validate aerosol radiative properties under a range of dust conditions, to understand the energy available for solar‐powered missions, and to enable comparisons with measurements of the SEB by the Perseverance rover at Jezero crater. The primary energy input at the Martian surface is the solar radiation, which depends on the time of the day and season, geographical location (latitude and altitude), and atmospheric dust and gas abundances. Another energy input is the thermal atmospheric forcing, which depends on the vertical distribution of dust and water ice aerosols as well as CO2and H2O molecules. Together with the reflected solar radiation and the thermal radiation emitted by the surface, these four terms make up the net radiative forcing of the surface. In response to it, energy outputs as turbulent motions and water phase changes emerge to cool down/warm up the ground. The remaining energy is available to control the thermal environment in the surface and shallow subsurface through conduction into the soil. By using first‐of‐their‐kind measurements from the Mars Science Laboratory mission, we calculate the energy inputs and outputs across Curiosity's traverse over the first 2500 Martian days of the mission. We then analyze their temporal variations and relate this to the mechanisms causing such variations. An accurate determination of the surface energy budget is key to preparing for the human exploration of Mars because it contributes to improvements in the predictive capabilities of numerical models. We have calculated each term of the surface energy budget during the first 2500 sols of the Mars Science Laboratory missionWe have analyzed the variation of each term from diurnal to interannual timescales in relation to the mechanisms causing the variationsOur results are important in preparation for the human exploration of Mars to evaluate the consistency with predictions from models We have calculated each term of the surface energy budget during the first 2500 sols of the Mars Science Laboratory mission We have analyzed the variation of each term from diurnal to interannual timescales in relation to the mechanisms causing the variations Our results are important in preparation for the human exploration of Mars to evaluate the consistency with predictions from models

Mars’ planet‐encircling or global dust storms (GDSs) are an iconic and enigmatic feature of the Red Planet. Occurring every few Mars Years (MYs), on average, they are a stochastic process in the otherwise largely repeatable annual cycle of martian weather. In 2018 (MY 34 in the calendar of Clancy et al. [2000], https://doi.org/10.1029/1999je001089), an international fleet of spacecraft–six orbiters and two rovers–observed the most recent GDS. This introduction and the articles of this special collection describe the evolution and impacts of the storm from the surface to the exosphere, compare this storm to previous GDSs, identify new phenomena never previously seen in such storms, and attempt to determine how and when GDSs develop. Mars’ planet‐encircling or global dust storms are an iconic and enigmatic feature of the Red Planet. Occurring every few Mars Years, on average, they are a seemingly random process in the otherwise largely repeatable annual cycle of martian weather. In 2018, an international fleet of spacecraft, six orbiters and two rovers, observed the most recent global dust storm. This introduction and the articles of this special collection describe the evolution and impacts of the storm from the surface to the top of the atmosphere and beyond, compare this storm to previous global dust storms, identify new phenomena never‐before seen in such storms, and attempt to determine how and when global dust storms develop. Six orbiters, two rovers, and Earth‐based telescopes observed the 2018 Mars global dust stormThe 27 papers of this special collection study the storm with observations, modeling, and theoryThe 2018 storm is the most comprehensively studied Mars global dust storm to date Six orbiters, two rovers, and Earth‐based telescopes observed the 2018 Mars global dust storm The 27 papers of this special collection study the storm with observations, modeling, and theory The 2018 storm is the most comprehensively studied Mars global dust storm to date

Recurring slope lineae (RSL) are seasonal flows on steep slopes on Mars. Their formation mechanism is unknown, but dry granular flows are a likely explanation. Any proposed trigger for these flows must be consistent with the observed temperature dependence of RSL: more active in warmer months or when sun‐facing. Here, we use atmospheric modeling and laboratory experiments to explore a potential mechanism that involves both wet and dry processes at Hale Crater, a known RSL location. We propose that dry flows are triggered by changes in soil cohesion due to the loss of water. When surface temperature and humidity were experimentally simulated, salts likely found in the soil only completely dehydrated during the active season for RSL. We propose that the loss of water from soil in warmer months (or when illuminated) lowers soil cohesion and maximum stability angle. Slope failure may occur, exposing darker underlying material and creating RSL. Some steep hillsides on Mars have dark streaks that appear in warm months and disappear in cold months, year after year. These “recurring slope lineae,” or RSL, could be flowing liquid but are most likely dust avalanching. RSL appear in warmer months, which could be a clue as to what causes the features. In this study, we use an atmospheric model to predict relative humidity and temperature at the surface at one RSL location, Hale Crater. We then simulate these conditions in the laboratory. We find that a type of salt found on Mars can take up and release water from the atmosphere every day, but the salt only loses all of its water when the RSL active season was simulated. We propose that the dry grain flows are triggered in the summer when the soil loses water and becomes less “sticky.” When this happens, dust and sand may slide downhill, exposing the darker material underneath, and create RSL. Temperature and relative humidity conditions at Martian recurring slope lineae locations were modeled and experimentally simulatedWhen surface conditions at an RSL location were simulated, Mars‐relevant salts only completely dehydrated during the RSL active seasonRSL on Mars may be a dry grain flow triggered by seasonal changes in water content and resulting changes in soil cohesion