ALBERT

Description: Thermal convection in the Antarctic and Greenland ice sheets has been dismissed on the grounds that radio-echo stratigraphy is undisturbed for long distances. However, the undisturbed stratigraphy lies, for the most part, above the density inversion in polar ice sheets and therefore does not disprove convection. An echo-free zone is widespread below the density inversion, yet nobody has cited this as a strong indication that convection is indeed present at depth. A generalized Rayleigh criterion for thermal convection in elastic-viscoplastic polycrystalline solids heated from below is developed and applied to ice-sheet convection. An infinite Rayleigh number at the onset of primary creep decreases with time and becomes constant when secondary creep dominates, suggesting that any thermal buoyancy stress can initiate convection but convection cannot be sustained below a buoyancy stress of about 3 kPa. An analysis of the temperature profile down the Byrd Station core hole suggests that about 1000 m of ice below the density inversion will sustain convection. Creep along the Byrd Station strain network, radar sounding in East Antarctica, and seismic sounding in West Antarctica are examined for evidence of convective creep superimposed on advective creep. It is concluded that the evidence for convection is there, if we look for it with the intention of finding it.

Description: SummaryThe effects of sowing date on leaf appearance, spikelet initiation and mainstem apical development in spring barley cv. Triumph were investigated in field experiments involving 13 sowings made during the period 22 February to 1 July over the years 1982–1985. Delaying sowing was associated with faster rates and shorter durations of leaf appearance and spikelet initiation and earlier attainment of all stages of apex development. The rates of leaf appearance and spikelet initiation were correlated with daylength at crop emergence although there was evidence that the latter process was also influenced by temperature. The derivation of three models relating the duration of developmental phases to temperature and/or daylength is described. In these models the attainment of successive stages of apical development was assumed to require the perception by the crop of a ‘threshold amount’ (THR) of accumulated temperature (THR(ΣT)) and/or daylength (THR(ΣPT), THR(ΣP)) above certain base values (TbPb). The base values of temperature and/or daylength for each phase were derived as those values which minimized the coefficient of variation of the amounts of accumulated temperature and/or daylength experienced by all 13 sowings. For various developmental phases the model based on temperature gave base values between 0 and 3 °C. The model based on daylength gave base values between 11 and 13 h. In both these models there was a highly significant correlation between observed and expected dates of attainment of various stages of apex development. It is suggested that the derived relationships between temperature, daylength and apical development could be used as an aid in the forward planning of crop management. The model based on temperature and daylength (photothermal time) gave no useful base values and it is concluded that more precise methods of relating development to these factors must be sought.

Description: SummaryAn analysis of chickpea experiments carried out in northern Syria during the 1980–1 and 1981–2 growing seasons showed that both intercepted solar radiation and its rate of conversion to dry matter were variable components of dry-matter production. Among the sources of variation in the experiments, the most important factor affecting both interception and utilization of solar radiation was site. Winter planting also led to increased solar radiation interception and utilization. Used in conjunction with chickpea lines resistant to blight, winter planting seems likely to lead to increased productivity. In higher rainfall areas, where the crop is usually grown, such an increase would be of commercial significance. In drier areas, winter planting would enable the cultivation of chickpea as a subsistence crop.

Description: Thermal convection in the Antarctic and Greenland ice sheets has been dismissed on the grounds that radio-echo stratigraphy is undisturbed for long distances. However, the undisturbed stratigraphy lies, for the most part, above the density inversion in polar ice sheets and therefore does not disprove convection. An echo-free zone is widespread below the density inversion, yet nobody has cited this as a strong indication that convection is indeed present at depth. A generalized Rayleigh criterion for thermal convection in elastic-viscoplastic polycrystalline solids heated from below is developed and applied to ice-sheet convection. An infinite Rayleigh number at the onset of primary creep decreases with time and becomes constant when secondary creep dominates, suggesting that any thermal buoyancy stress can initiate convection but convection cannot be sustained below a buoyancy stress of about 3 kPa. An analysis of the temperature profile down the Byrd Station core hole suggests that about 1000 m of ice below the density inversion will sustain convection. Creep along the Byrd Station strain network, radar sounding in East Antarctica, and seismic sounding in West Antarctica are examined for evidence of convective creep superimposed on advective creep. It is concluded that the evidence for convection is there, if we look for it with the intention of finding it.