Search results

116 datasets found

Atmosphere

Aerosols

Aerosol light scattering properties at Zeppelin Observatory, Svalbard, 1999⁠ – ⁠2016, with back trajectory analysis parameters

The dataset was created to aid the investigation of long-term trends in aerosol light scattering properties using a unique data record of key aerosol optical...

Aerosol particle type fractions from the Arctic Ocean 2018 marine aerosol reference tank experiments

This dataset includes the number fractions of aerosol particles assigned to three particle types from the marine aerosol reference tank (MART) experiments...

Arctic cloud and aerosol measurements from Zeppelin Observatory, Svalbard, November 2015 to February 2018

The dataset includes cloud, aerosol and meteorological data from Zeppelin Observatory, Svalbard, for the period 27 November 2015 to 4 February 2018. This...

Cloud droplet residual and aerosol measurements from the Arctic Ocean 2018 expedition

The dataset includes cloud droplet residual and whole-air aerosol particle number size distributions from the Arctic Ocean 2018 expedition, for a selected set...

Cloud water samples obtained in summertime during the Arctic Ocean 2018 expedition

This dataset contains results from analyses of chemical composition for elevated clouds in the pristine high Arctic, obtained with a miniaturised cloud water...

Comparison of aerosol eddy covariance fluxes and chamber-simulated sea spray emissions from the Baltic Sea

The dataset comprises aerosol eddy covariance (EC) flux data measured on Östergarnsholm island in the Baltic Sea as well as data from ship-based laboratory...

Concentration of bioaerosols and ice nucleating particles at Zeppelin Observatory, Svalbard, 2017 – 2020

Using a single-particle online optical instrument we measured the coarse particle concentration (optical diameter bigger than 0.8 µm) at the Zeppelin...

Distribution of organics in aerosol particles from the Arctic Ocean 2018 marine aerosol reference tank experiments

This dataset includes a summary of the organic compounds (detected using Raman microspectroscopy) in sea spray aerosol particles. The sea spray aerosol...

Fluorescent primary biological aerosol particle concentrations in low-level Arctic cloud residuals in Svalbard 2019 – 2021

Using an optical single-particle instrument we measured the particle concentration of coarse-mode aerosol (optical diameters larger than 0.8 µm) within cloud...

Ice-nucleating particle measurements collected during the Arctic Ocean 2018 marine aerosol reference tank experiments

Size segregated samples were collected from the marine aerosol reference tank (MART) headspace using a cascade impactor for offline analysis of their...

Laboratory data on the effect of salinity and temperature on sea spray aerosol production

This dataset was created in a series of laboratory experiments to investigate the role of salinity and temperature on aerosol production and surface bubble...

Light absorbing aerosol along with air mass history measured at the Zeppelin observatory in Svalbard

The dataset includes a harmonised 22-year long time series (2002 – 2023) of light-absorbing aerosol atmospheric particles. In support of the harmonised...

Representative Raman spectra of aerosol particles from the Arctic Ocean 2018 marine aerosol reference tank experiments

This dataset includes Raman spectra of the three representative particle types identified in sea spray aerosol particles generated during the marine aerosol...

Sea salt aerosol elemental mole ratios from the Arctic Ocean 2018 marine aerosol reference tank experiments

This dataset includes the mean and 95% confidence intervals for the mole ratios C/Na, Cl/Na, S/Na, N/Na, Mg/Na, Ca/Na measured from individual submicron and...

Temperature and energy transport for idealized NorESM slab-ocean simulations for modified mid-latitude SO₂ aerosol emissions

This dataset contains temperature and horizontal heat convergence output from four different climate modelling simulations using a slab-ocean setup for the...

Vertical stratification of submicrometer aerosol particles measured during the high-Arctic ASCOS expedition 2008

The atmospheric thermodynamic structure and aerosol particle concentrations, in several size ranges, across a multitude of surface conditions over the Arctic...

Volatility of aerosol particles from NO₃ oxidation of α-pinene, isoprene and β-caryophyllene

The Nitrate Aerosol and Volatility Experiment (NArVE) aimed to study the NO₃-induced secondary organic aerosol (SOA) formation and evolution from three...

Circulation

Air parcel trajectories for the expedition ARTofMELT, Arctic Ocean, 2023 — raw data

10-day backward and forward air parcel trajectories were calculated along the ship track of the Swedish icebreaker Oden during the ARTofMELT (Atmospheric...

Backward trajectories along the ship track using the LAGRANTO model, for the expedition ARTofMELT, Arctic Ocean, 2023

7-day backward trajectories from along the ship track of the Swedish icebreaker Oden during the ARTofMELT (Atmospheric Rivers and the Onset of Arctic Melt)...

Backward trajectories within a circle around the ship using the LAGRANTO model, for the expedition ARTofMELT, Arctic Ocean, 2023

7-day backward trajectories from within a circle around the ship track of the Swedish icebreaker Oden during the ARTofMELT (Atmospheric Rivers and the Onset of...

Forward trajectories along the ship track using the LAGRANTO model, for the expedition ARTofMELT, Arctic Ocean, 2023

7-day forward trajectories from along the ship track of the Swedish icebreaker Oden during the ARTofMELT (Atmospheric Rivers and the Onset of Arctic Melt)...

Forward trajectories within a circle around the ship using the LAGRANTO model, for the expedition ARTofMELT, Arctic Ocean, 2023

7-day forward trajectories from within a circle around the ship track of the Swedish icebreaker Oden during the ARTofMELT (Atmospheric Rivers and the Onset of...

Global weather forecast data with model EC-Earth — ensemble dataset for historical and future conditions

Weather forecast uncertainty climatologies for historical (1976–2005) and future RCP8.5 scenario (2071–2100) conditions are generated with the European...

Clouds

Cloudnet remote sensing retrievals of cloud properties during the Arctic Ocean 2018 expedition

This dataset consists of a range of output products from the Cloudnet algorithm. Cloudnet combines cloud radar, ceilometer, microwave radiometer, and...

Cloudnet remote sensing retrievals of cloud properties from the SWERUS-C3 Arctic Ocean expedition in 2014

This dataset consists of a range of output products from the Cloudnet algorithm. Cloudnet combines cloud radar, ceilometer, microwave radiometer, and...

Data from a modelling study of a two-layer boundary-layer cloud system from the Arctic Ocean 2018 expedition

This dataset presents the model output from the large-eddy simulation (LES) model. The LES was employed to simulate a two-layer boundary-layer cloud structure...

Dual-channel microwave radiometer measurements from the high-Arctic ASCOS expedition 2008

Vertically integrated water vapor (PWV) and cloud liquid water (LWP) was measured using a microwave radiometer, simultaneously observing the microwave...

Meteorological observations during the SWERUS-C3 Arctic Ocean expedition 2014 and colocated regional climate model output

Observations of the atmospheric thermodynamic structure, wind profiles and surface energy budget components from the Eastern Arctic Ocean in July through early...

Multi-sensor cloud microphysics product from the high-Arctic ASCOS expedition 2008

This data set includes retrievals of cloud type, characteristic particle size, and condensed water content for both liquid and ice hydrometeors. These...

Micrometeorology

Micrometeorological data from an ice-floe site during the high-Arctic ASCOS expedition 2008

Micrometeorological observations were collected from two masts on an ice floe a few hundred meters from the Icebreaker Oden during ice drift of the ASCOS...

Micrometeorological data from icebreaker Oden’s foremast during expedition ARTofMELT, Arctic Ocean, 2023

Air-sea fluxes and other micrometeorological variables are derived from fast response (20 Hz) measurements of the 3-dimensional wind vector, temperature and...

Micrometeorological data from icebreaker Oden’s foremast during Ryder 2019 and Northwest passage 2019 expeditions

Air-sea fluxes and other micrometeorological variables derived from fast response (20 Hz) measurements of the 3-dimensional wind vector, temperature and...

Micrometeorological data from icebreaker Oden’s foremast during the Arctic Ocean 2016 expedition

Air-sea fluxes and other micrometeorological variables derived from fast response (20 Hz) measurements of the 3-dimensional wind vector, temperature and...

Micrometeorological data from icebreaker Oden’s foremast during the Arctic Ocean 2018 expedition

Air-sea fluxes and other micrometeorological variables derived from fast response (20 Hz) measurements of the 3-dimensional wind vector, temperature and...

Micrometeorological data from icebreaker Oden’s foremast during the SWERUS-C3 Arctic Ocean expedition in 2014

Air-sea fluxes and other micrometeorological variables derived from fast response (20 Hz) measurements of the 3-dimensional wind vector, temperature and...

Observations

Cloud radar data from expedition ARTofMELT, Arctic Ocean, 2023

Measurements of clouds were obtained from the RPG-Radiometer Physics RPG-FMCW-94 Doppler cloud radar deployed on the roof of a container on the 4th deck of the...

Ice station micrometeorological data from expedition ARTofMELT, Arctic Ocean, 2023

Surface fluxes of momentum, and sensible and latent heat over the sea-ice, and other micrometeorological variables are derived from fast response (20 Hz)...

Ice station surface meteorology and radiation data from expedition ARTofMELT, Arctic Ocean, 2023

Measurements of near-surface meteorology, upwelling and downwelling radiative fluxes and snow temperature data were obtained from two installations (the Met...

Meteorological data from Lihtti, northern Sweden, September 2021 – March 2023

This dataset contains meteorological data acquired by Tarfala Research Station’s automated weather station located in Lihtti, northern Sweden, between...

Micrometeorological data from an ice-floe adjacent to an open lead during the Arctic Ocean 2018 expedition

Air-sea fluxes of momentum, heat and CO₂, associated micrometeorological and mean meteorological and ocean surface variables. Fluxes are derived from fast...

Navigation, meteorological and surface seawater data from the Arctic Ocean 2016 expedition

Measurements from icebreaker Oden’s navigation, meteorological and surface seawater systems from the Arctic Ocean 2016 (AO2016) expedition to the central...

Navigation, meteorological and surface seawater data from the Arctic Ocean 2018 expedition

Measurements from icebreaker Oden’s navigation, meteorological and surface seawater systems from the Arctic Ocean 2018 (AO2018, also referred to as...

Navigation, meteorological and surface seawater data from the SWERUS-C3 Arctic Ocean expedition in 2014

Measurements from icebreaker Oden’s navigation, meteorological and surface seawater systems from the SWERUS-C3 expedition to the Eastern Arctic Ocean in July...

Ozone soundings from the high-Arctic ASCOS expedition 2008

Profiles of ozone concentration were measured by a Vaisala add-on ozone sonde, for a few of the radiosoundings launched from the helipad of the Icebreaker Oden.

Radioactivity soundings from the high-Arctic ASCOS expedition 2008

Profiles of radioactivity were measured by a Vaisala add-on radioactivity sonde for a few of the radiosoundings, measuring Geiger–Müller tube count rates...

Weather and navigation data from the high-Arctic ASCOS expedition 2008

The Icebreaker Oden's weather station was mostly located on the 7th deck about 27 m above sea level, on top of the bridge. Sensors were distributed over the...

Temperature

High resolution temperature maps for a boreal forest area in central Sweden 2015–2016

Grids of monthly minimum and maximum temperatures are presented at 25 m resolution for a boreal forest area of ~16 000 km2 in central Sweden (including parts...

High resolution topoclimate maps over Sweden

High spatial resolution (50 m grain-size) gridded maps of 10 years period (2000-2010) of monthly means, min and max temperature, and a map of Growing Degree...

Late-glacial European summer temperature reconstruction

The dataset provides a compilation of quantitative European summer temperature reconstructions for the late-glacial periods of the Bølling-Allerød...

Lateglacial plant macrofossil-based temperature reconstructions for the Baltic states from 14,000 to 11,000 years ago

The dataset contains quantitative mean air temperature reconstructions of January and July for the period of 14,000 to 11,000 years ago in the region of the...

Marine-Atmospheric Emitted Radiance Interferometer from the high-Arctic ASCOS expedition 2008

During the ASCOS expedition the temperatures of the cloud-base, ambient air, and surface were very accurately measured using the Marine-Atmospheric Emitted...

Monthly mean air temperatures from Tarfala Research Station, Kebnekaise Mountains, 1965 – 2022

This dataset contains monthly mean air temperatures from Tarfala Research Station, acquired since 1965 and updated annually. For the years 1946 – 1964,...

Monthly mean air temperatures from Tarfala Research Station, Kebnekaise Mountains, Northern Sweden

This dataset contains monthly mean air temperatures from Tarfala Research Station, acquired since 1965 and updated annually. Tarfala Research Station (TRS),...

Northern Hemisphere 2,000-year temperature reconstruction

Northern Hemisphere temperature reconstruction for the past 2,000 years by combining low-resolution proxies with tree-ring data, using a wavelet transform...

Northern Hemisphere extratropical 1000-year temperature reconstruction

1000-year temperature reconstruction for the Northern Hemisphere extratropical region (30°N-90°N) by the LOC method. The new LOC method reconstructs the...

Northern Hemisphere extratropical 2000-year and 500-year temperature reconstructions

We present two new multi-proxy reconstructions of the extra-tropical Northern Hemisphere (30–90°N) mean temperature: a two-millennia long reconstruction...

Northern Hemisphere extra-tropics 2,000-year decadal temperature reconstruction

Multi-proxy temperature reconstruction for the extra-tropical Northern Hemisphere (90–30°N) during the last two millennia based on 30 palaeotemperature...

Radiometer data from the high-Arctic ASCOS expedition 2008

High resolution (in time and space) vertical profiles of temperature were obtained from a 60GHz scanning microwave radiometer. The radiometer measures the...

Simulated seasonal temperatures 850–2005 for the seven PAGES 2k regions derived from the CESM last millennium ensemble

Simulated surface air temperature data derived from the Last Millennium Ensemble with the Community Earth System Model (CESM) version 1.1, for the period AD...

Stockholm winter/spring 500-year temperature reconstruction

Historical documentary sources, reflecting different port activities in Stockholm, are used along with the instrumental temperature record to derive a...

Warming stripes — visualisations of annual temperature series from the Nordic countries

Ongoing climate change is a complex problem and communicating it to a general audience often requires short and simple explanations. Warming stripes refer to...

Weather observations

Meteorological observations from Sherard Osborn Fjord, Northern Greenland, measured onboard icebreaker Oden, August 2019

This data set consists of air temperature, wind speed, wind direction, sea surface temperature, and precipitation recordings taken once per minute from the...

Present weather sensor visibility and precipitation data from the Arctic Ocean 2016 expedition

Measurements of visibility, precipitation type and intensity, and the corresponding World Meteorological Organization (WMO) weather codes. The dataset...

Present weather sensor visibility and precipitation data from the Arctic Ocean 2018 expedition

Measurements of visibility, precipitation type and intensity, and the corresponding the World Meteorological Organization (WMO) weather codes. The dataset...

Radiosonde data from the Arctic Ocean 2018 expedition

Atmospheric profile measurements of temperature, humidity, pressure and wind speed from radiosonde (Vaisala RS92) weather balloons. Data also include...

Radiosonde profiles from expedition ARTofMELT, Arctic Ocean, 2023

Atmospheric profile measurements of temperature, humidity, pressure and wind speed are obtained from radiosonde weather balloons, launched from the Swedish...

Radiosonde profiles from expedition ARTofMELT, Arctic Ocean, 2023 — raw data

Atmospheric profile measurements of temperature, humidity, pressure and wind speed are obtained from radiosonde weather balloons, launched from the Swedish...

Radiosoundings from the high-Arctic ASCOS expedition 2008

Radiosoundings were launched every six hours (00, 06, 12 & 18UTC), and additionally before each helicopter flight (if not at one of the normal times), using...

Stockholm Historical Weather Observations

Weather observations have been made continuously at the old astronomical observatory in Stockholm since 1754. This is one of the world's longest records of...

Stockholm Historical Weather Observations — Air pressure

Data for air pressure at the old astronomical observatory in Stockholm since 1756 are provided. Over time, different barometers have been used and measurements...

Stockholm Historical Weather Observations — Air temperature

Data for air temperatures at the old astronomical observatory in Stockholm since 1756 are provided. The data are available both with, and without, adjustments...

Stockholm Historical Weather Observations — Barometer observations in original units since 1756

Weather observations have been made continuously at the old astronomical observatory in Stockholm since 1754. The dataset provided here contains individual...

Stockholm Historical Weather Observations — Daily mean air temperatures since 1756

Data for daily mean air temperatures at the old astronomical observatory in Stockholm since 1756 are provided. Three variants of the data are available: (i)...

Stockholm Historical Weather Observations — Monthly mean air temperatures since 1756

Data for monthly mean air temperatures at the old astronomical observatory in Stockholm from 1756 until 2020 are provided. The dataset also contains annual...

Stockholm Historical Weather Observations — Observation times since 1756

This dataset provides information about the time points when weather observations were made each day (morning, noon, evening) during 1756⁠⁠ – 2018 at...

Stockholm Historical Weather Observations — Sub-daily cloud amount since 1756

Weather observations have been made continuously at the old astronomical observatory in Stockholm since 1754. The dataset provided here contains estimated...

Stockholm Historical Weather Observations — Sub-daily sea level pressure since 1756

Data for sub-daily sea level pressure (SLP) at the old astronomical observatory in Stockholm are provided for the period 1756⁠ – 2012. Three SLP...

Stockholm Historical Weather Observations — Thermometer observations since 1756

Weather observations have been made continuously at the old astronomical observatory in Stockholm since 1754. The dataset provided here contains individual air...

Temperature and humidity profiles from microwave radiometer during expedition ARTofMELT, Arctic Ocean, 2023

Profiles of temperature and humidity are obtained from an RPG HATPRO5 microwave radiometer deployed on the roof of a container on the 4th deck of Oden during...

Temperature and humidity profiles from microwave radiometer during expedition ARTofMELT, Arctic Ocean, 2023 — raw data

Profiles of temperature and humidity are obtained from an RPG HATPRO5 microwave radiometer deployed on the roof of a container on the 4th deck of Oden during...

Tethersonde mean meteorology from the high-Arctic ASCOS expedition 2008

In-situ boundary-layer meteorological observations were made from the ice using sensors hanging under a tethered balloon which was quasi-continuously winched...

Visibility, precipitation and present weather observations from expedition ARTofMELT, Arctic Ocean, 2023

Horisontal visibility, precipitation rate and air temperature data are obtained from a Present Weather observation station deployed on the 7th top deck of the...

Visibility, precipitation and present weather observations from expedition ARTofMELT, Arctic Ocean, 2023 — raw data

Horisontal visibility, precipitation rate and air temperature data are obtained from a Present Weather observation station deployed on the 7th top deck of the...

Weather data from MISU weather station during Ryder 2019 and Northwest passage 2019 expeditions

Measurements of near-surface wind speed and direction, temperature, relative humidity, air pressure, shortwave and longwave downwelling radiation, surface...

Weather data from MISU weather station during the Arctic Ocean 2018 expedition

Measurements of near-surface wind speed and direction, temperature, relative humidity, air pressure, shortwave and longwave downwelling radiation, and surface...

Weather data from the MISU weather station during the Arctic Ocean 2016 expedition

Measurements of near-surface wind speed and direction, temperature, relative humidity, air pressure, shortwave and longwave downwelling radiation, and surface...

Weather data from the MISU weather station during the SWERUS-C3 Arctic Ocean expedition in 2014

Measurements of near-surface wind speed and direction, temperature, relative humidity, air pressure, shortwave and longwave downwelling radiation, and surface...

Weather data from WeatherPak station from the high-Arctic ASCOS expedition 2008

The WeatherPak station is an integrated unit mounted approximately mid-ship on the forward railing on the Icebreaker Oden's 7th deck about 27 m above sea...

Wind, temperature, relative humidity, surface temperature and radiation from expedition ARTofMELT, Arctic Ocean, 2023

A weather station was deployed on the 7th top deck of the Swedish icebreaker Oden during the ARTofMELT (Atmospheric Rivers and the Onset of Arctic Melt)...

Wind, temperature, relative humidity, surface temperature and radiation from expedition ARTofMELT, Arctic Ocean, 2023 — raw data

A weather station was deployed on the 7th top deck of the Swedish icebreaker Oden during the ARTofMELT (Atmospheric Rivers and the Onset of Arctic Melt)...

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AO2018 ACAS ACSE AO16 AO18 AO2016 AO2018 ARTofMELT ARTofMELT 2023 expedition ASCOS Aerosol chemical properties Aerosol eddy covariance fluxes Aerosol physical properties Aerosol vertical profiling Aerosols Agroforestry Air mass history Air parcel trajectories Air pressure Air temperature Air-sea exchange Air-sea interaction Albedo Alpine Antarctic circumpolar current Arctic Arctic Ocean Arctic Ocean 2016 Arctic Ocean 2018 Arctic boundary layer Arctic clouds Arctic greening Arctic warming Atmosphere Atmospheric boundary layer Atmospheric profiles Atmospheric profiling Atmospheric stability Automated weather station Automatic bubble counter Automatic floating chamber BVOC Back trajectory Baltic Sea Baltic states Barometer Bathymetry Bioaerosols Biogeochemistry Biomarker Biomarkers Black carbon Boundary Layer Boundary layer Bronze Age CCSEM-EDX Carbon dioxide Chironomids Chukchi Sea Climate change Climate model Climate model simulation Climate modelling Climate proxy data Climate reconstructions Climate scenarios Climate sensitivity Cloud Condensation Nuclei Cloud amount Cloud and fog biology and chemistry Cloud droplet residuals Cloud radar Cloud residuals Cloudnet Cloudnetpy Clouds Coastal habitats Concentration Daily data Data visualisation Deglaciation Diffusion Drought EC-Earth ERA5 data Early instrumental data East Siberian Margin East Siberian Sea Ebullution Eddy covariance Electron microscopy Ensemble simulation Ensemble simulations Environmental forcing factors Eocene Feedback Figaro CH₄ sensor Fjord Floating chamber Flux Forest Forest understory GCM and emission scenarios GCVI GDGT Gas transfer model Girjas Global climate model Greenhouse Greenhouse gas Greenhouse gas flux Greenland Growing season degreee days H-GDGTs HCLIM HYSPLIT Herald Canyon High Arctic Historical Holocene Homogenized data Humidity Hydroclimate Ice Nucleating Particles Ice nucleating particles Ice station Ice-nucleating particles Icebreaker Oden Inorganic and organic composition Instrumental temperature data Internet of things IoT Jimma Kara Sea Kebnekaise LAGRANTO LES LPJ-GUESS Lake Land cover Land-cover change Land-use and land-cover change Laptev Sea Last millennium Lateglacial Lidar Light scattering Light-absorbing Lihtti Lipids Lomonosov Ridge Long-term trend Longwave radiation Longwave radiation spectra MART MOCCHA Marine Marine Geology Marine Heat Flow Marine aerosol reference tank Marine sediment Maximum temperature Mean temperature Meridional overturning circulation Meteorological model data Meteorology Methane Methane isotopes Microclimate Micrometeorological data Micrometeorology Microsensor Microwave profiling Mid-Holocene Minimum temperature Mixed-Phase Clouds Mixed-phase stratocumulus Model simulation Model-data comparison Monthly data Monthly maximum temperature Monthly mean air temperature Monthly minimum temperature Multi-method Multibeam NorESM Northern Greenland Northern Hemisphere Northern hemisphere Northwest Passage Ocean Oceanography Oden Organic carbon Organic compound Organo nitrates Paleoclimate Paleoclimate reconstruction Particle type number fraction Plant macrofossils Pleistocene Position and speed Precipitation Precipitation intensity Precipitation type Predictions Preindustrial Present weather Proxy RCA4 RCP Radar Doppler velocity Radar backsactter profiles Radiosonde Radiosoundings Raman Raman microspectroscopy Reconstruction Regional climate model Regional climate models Relative humidity Remote sensing Remote sensing of clouds and fogs Ryder 2019 expedition SWERUS-C3 SWERUS-C3 expedition Salinity Sea Ice Sea ice Sea salt aerosol Sea spray aerosol Sea spray aerosols Sea spray production Sea surface temperature Sea-ice loss Seawater temperature Secondary organic aerosols Senseair CO₂ sensor Sensitivity experiments Shallow inshore areas Ship log Ship log and weather station Ship navigation Short wave radiation Shortwave radiation Size-segrated aerosol Skin temperature Snow depth Snow temperature Southern Ocean Southwestern Ethiopia Statistical downscaling Stockholm Subarctic Summer Summer temperature Surface bubble spectra Surface energy budget Surface fluxes Surface temperature Surface water Svalbard Sweden Tarfala Tarfala Research Station Temperature Temperature reconstruction Tethered sonde profiles Topoclimate Topography Trace gas Tropical atmosphere Tropical circulation Tropical precipitation Turbulence Turbulent drag Turbulent fluxes Vegetation feedback Vertical profiles Visibility Volatility Weather Weather forecast Weather observations Weather station Weather stations Wind Wind direction Wind profilers Wind speed Younger Dryas Zeppelin brGDGTs iButton isoGDGTs Ångström exponent