HUMIDITY AND PRECIPITATION LABORATORY

HUMIDITY AND PRECIPITATION LABORATORY1. ObjectivesThe objectives of this lab are to familiarize you with a few of the ways in which two commonmeteorological variables (humidity and precipitation amount) are measured, and to assess the errorsimplicit in the measurements.2. Humidity2.1: Home weather stations have relatively accurate capacitance humidity sensors. However, to ameteorologist these are of limited utility because they can only give the indoor relative humidity (RH).Nevertheless, by assuming that the mixing ratio is the same both inside and outside of the building, andby measuring the RH inside and the temperatures (T) outside and inside, we can use the inside RH toestimate the outside RH. Part of the lab will be to assess the validity of this assumption.2.2: Specific tasks• Compare the RHs of room air from the Vaisala Humicaps and the psychrometers. Take repeatreadings to assess reproducibility.• Calibrate the Vaisala against the four saturated salt solutions.• Test the hypothesis of conservation of water vapor by measuring the RH and T inside, with theVaisala, and then measuring the outside RH and T with the Vaisala.3. PrecipitationThe Davis gauge is designed to measure increments of 0.01 inches of rainfall. It would therefore be wiseto work in inches in this case and convert to millimeters in your analysis because the digitization worksbetter on the Davis using inches.3.1: Calibrate a Davis tipping bucket rain gauge by determining the volume that each bucket of the gaugeholds. You can do this by using a pipette apparatus and slowly pouring a certain amount of water intoeach bucket until it tips. Try at least 3 trials in each of the two buckets so that you can calculate simplestatistics. Also measure the diameter of the collection funnel for use in later calculations.3.2 For the Davis tipping bucket gauge, determine the relationship between the true and measured (Davis)rain rate, and to determine whether the measurement error changes with rain rate. Do this by pouringmeasured volumes of water per unit time, using the buret, into the gauge. Use at least four differentpouring rates. Make sure to record the measured volume and the amount indicated on the Davis. Thelowest rate should be equivalent to ~10 mm hr-1 and the rates should be separated by at least a factor oftwo. Without making a mess, try pouring in a measured amount of rain using a graduated beaker tosimulate rain at a very high rate to determine if the Davis gauge would see a tropical downpour.1Humidity and Precipitation Lab WorksheetComplete the following activities and turn your results as though you would a typical problemset (i.e. no formal write-up required).1) Capacitance Humidity (Vaisala) vs Psychrometer: Provide the mean and standarddeviation of your RH measurements using the Vaisala and the psychrometer. Are RHvalues measured by each significantly different?2) Calibration of the Vaisala RH Probes vs Salt Solutions: Produce a well-labeledcalibration figure, a linear regression, and total uncertainty in the Vaisala RHmeasurements based on the results of that regression of calibration data. Comment on thetime response of the sensor.3) Conservation of Water Vapor Test: Compare the measured outside RH with thatestimated from the inside measurements assuming conservation of mixing ratio. Is there abias? Give some reasoning for a bias if one is observed. In other words, what might causethe assumption of constant water vapor concentration inside and outside to be invalid?4) Tipping Bucket Volume Determinations: From 3.1 of the lab procedure, you cancalculate the mean and standard deviation of your bucket volumes. Assuming a normaldistribution, a value within three standard deviations has almost 100% probability ofoccurrence. Using this information, deduce the lowest amount of rain (in millimeters)measurable for each bucket of the Davis rain gauge.5) Error in the Davis Tipping Bucket Gauge as a Function of Rain Rate: Convert thevolume of water that you measured in a given time into equivalent millimeters per hourusing knowledge of the collection area of the funnel. Use a linear regression to determinethe functional relationship between rain rate and observed rate from the Davis. Produce awell-labeled calibration figure along with the linear regression and uncertainty analysisresults.