How do we predict Weather and Climate?

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1 Explanation and follow on work Introduction and aim of the activity This activity involved experiencing two different but essential ways of observing the weather. Weather balloons (also known as radiosondes) provide details of temperature, humidity, pressure plus wind direction and speed up to about 20km, ideally four times a day, spread six hours apart from 00 UTC (GMT). The data are relayed in real-time to a ground receiver and when the ascent is complete the whole data set is relayed to the national forecast centre and around the world as part of the incessant exchange of data. The global weather balloon network plays a critical role in providing details of the vertical structure of the atmosphere to feed into operational forecast models. The aim here is to plot a selection of the temperature and humidity data on a graph to illustrate where, for example, the air cools most rapidly with height and where very damp and very dry layers might occur. Observations of the weather at the surface are important too as input for weather forecast models and to define, over a long period of time, the climate of a particular site. Plotting such observations on a map is an essential step in determining where important weather features like fronts are located. The aim is to experience making a range of observations carefully and accurately at an officially recognised observatory. The data will include temperature, humidity (wet-bulb temperature), wind speed and direction, and cloud type and cloud amount. Description of what was done and achieved The students witness the balloon launch and will plot the ascent data in school. They can produce two graphs. One on which a temperature line is drawn as a function of height (or pressure) and the other on which a relative humidity line is drawn. The relative humidity will tie in with the cloud observations taken during the visit, while the total change in temperature from the surface to the lowest value aloft divided by the distance between them indicates the lapse rate of temperature. This is a measure of quite how rapidly or slowly the air temperature changes with height. The Department of Meteorology The Department of Meteorology was established in 1965 and is internationally renowned for its excellent teaching and research in atmospheric, oceanic and climate science. Walker Institute for Climate System Research The Walker Institute for Climate System Research brings together the University of Reading s unique breadth and depth of climate expertise. The Institute aims to improve understanding of our future climate and its impacts for the benefit of society.

2 They also will have carried out a range of typical weather observations and will have logged them. These will include drybulb temperature, wetbulb temperature, wind speed and direction plus cloud amount and cloud type. They will have to be as precise as possible with their readings, and be aware that some are more precise than others. For example, the temperature readings can quite easily be made to the nearest 0.1 C while cloud amount is gauged to the nearest okta or eighth of the sky covered. This amount is often difficult to judge with scattered cloud because the cover can be assessed reasonably well overhead but becomes difficult when looking towards the horizon. Background to the observations Weather balloons and surface observations You will watch the launch of a weather balloon (also called a radiosonde ) from the University s Atmospheric Observatory and take some observations of the weather at the surface. Weather balloons Every day, around the world, meteorologists launch weather balloons from about 650 places to give us very important data, from an instrument package attached to the balloon, on how pressure, temperature, humidity and wind change with height. The balloon is filled with helium gas and gets bigger and bigger as it ascends because it moves into lower and lower pressure. At some point the balloon s skin is so stretched it bursts and the instrument package falls back to Earth on a parachute. So from one launch we collect very detailed information both going up and coming down. It normally takes about an hour to reach burst height with observations coming every few seconds, so there is a lot of data. The top of the ascent usually is up to 20km or so above the ground, well into the stratosphere. We can t do proper weather forecasts without knowing quite how the atmosphere changes upwards. In today s forecast centres, data from balloons are fed extremely quickly into the computer model as a critically important form of data. Making observations at the surface are useful of course, but the upper air data are vital to map the 3-D pattern of the atmosphere. Where the cold, or the dry, or the warm, or the moist, cloudy and windy conditions are across the world, through the depth of the atmosphere, rely in part on the network of radiosondes. Busy upper-air stations launch four balloons a day at 00, 06, 12 and18 UTC but some might only do one normally either the 00 or 12 UTC observation. This may be because they can t afford lots of ascents at over 100 each.

3 Sample data from an ascent The table below has just a few observations taken from an ascent from a place near Paris around midnight on a winter s day. The left column shows how high the balloon is by listing the air pressure as the instruments go up, the pressure drops and meteorologists use pressure to tell how high up the observation is. They also however use the more usual height above the surface too, so the second column lists the location in metres up in the air. The third column list the air temperature, the fourth one the dewpoint (which although quoted as a temperature is a measure of how humid the air is it is the temperature to which the air must cool to make it saturated, so a small difference between it and the actual temperature (in column three) means it s near saturation; the larger the difference the drier the air). The final column lists the relative humidity. Note that the dewpoint temperature is calculated from the other values; it s not observed directly during the ascent. Once you have studied the data in the table to check by eye where the dry and damp layers are or where the temperature is changing most rapidly, either up or down, you can plot a graph to make the patterns clearer. It is quite common in the atmosphere to see a lot of detail in the humidity, with dry and moist layers one above the other. You may have seen this sometimes when you take off and land in an airplane. There are quite often layers of cloud separated by completely dry layers. Plotting a graph of, for example, relative humidity will illustrate where such layers might be. The damper ones will tie in with cloud layers you might have observed from the ground. The graph of relative humidity below depicts the data from the table you can see that the air is fairly humid near the ground but rapidly becomes to a very low value around 10% near 250 dm (that is 2500m). Above that is a damp layer (around 65% at 3500m) above which there are a further two dry and moist layers.

4 A sample ascent (for Trappes, France at 00 UTC, 15 February 2009) Pressure (hpa) Height (m) Temp (C) Dewpoint (C) Relative Humidity (%) Relative humidity profile Height (dm) 400 Series Relative humidity (%)

5 Surface observations There are many thousand weather observation sites around the world. Busy places like Heathrow and Gatwick Airports make observations every hour, every day, every year. Other places observe much less, like a mountain rescue station or coastguard station where it might be only once or twice a day. Official hourly reporting sites take a very wide range of measures that include temperature, humidity, pressure, cloud type and cloud amount, height of cloud above the ground, wind direction and speed, visibility and weather which means is there rain, drizzle, snow, sleet, fog etc. Like the balloon data, observations at the surface are instantly sent to the National Weather Service HQ to be plotted on a map that is then analysed either automatically or by hand. This means that special lines are drawn on them that show where the important weather features are like highs and lows at the surface or jetstreams high up in the atmosphere. Weather maps with lots of surface weather observations on them are analysed that is to say they have lines drawn on them such as isobars (lines joining places with the same mean sea level pressure). Normally nowadays this is done automatically although occasionally a forecaster will do a hand analysis.

6 Activity 1 Plotting the balloon data The instruments attached to the balloon relay observations of pressure, temperature and relative humidity every few seconds, in real time. Because pressure decreases as height above the ground increases, we can tell the height at which each value is reported. So when we draw a graph of temperature changing with height, we most often mark pressure as the vertical, Y- axis. However, you can use the height in metres or decametres (dm = 10m) as your vertical axis. On the horizontal X-axis we plot temperature or relative humidity. Plotting values of, say, temperature at regular changes in pressure (for example at 980 hpa, 960 hpa, 940 hpa etc) produces a vertical profile of temperature. We can do the same thing for relative humidity. Looking carefully at the graphs provides information on how quickly the air is getting cooler with height or even where it is actually warming with height. We can also tell where very damp layers are and therefore where the cloud is likely to be, and where dry air is located. Use the table to write down the values from today s balloon ascent Pressure (hpa) Height (metres) Temperature (C) Relative humidity (%)

7 Make two graphs Use the table of data from the ascent to make two graphs, one for temperature and one for relative humidity. Temperature (to nearest tenth of a degree Celsius) (a) Write down the surface temperature and the lowest temperature in the whole ascent. Calculate the average change of temperature per kilometre between the surface and the lowest value by dividing the temperature difference by the height difference. (b) Is there a layer within which the temperature actually increases with increasing height? If so, what height is it at and how thick is the layer? Relative humidity (to nearest 5%) (a) Where the relative humidity is 90% or more there are likely to be clouds present. Mark in any layers of cloud show how they fit with your observation of the clouds at the Observatory. (b) Very dry air, 50% relative humidity or less, is cloud-free. Mark any layers with such dry air.

8 Activity 2 Making and logging surface observations The Department s observer takes a large range of weather data, once a day at 0900 UTC (this is at 10 o clock during British Summer Time). They log (write down in an official log book ) every value, and after the end of each month send the data to the UK Met Office in Exeter. The observations include: Maximum temperature (over the last 24 hours) Minimum temperature ( ) Temperature at 0900 UTC Wetbulb temperature at 0900 UTC Rainfall total (over last 24 hours) Wind direction and speed at 0900 UTC Evaporation (over last 24 hours) Cloud type and cloud amount You will observe temperature and wetbulb temperature (both in degrees Celsius) using the whirling psychrometer, the wind speed (knots) by using the hand-held anemometer, the wind direction (nearest 10 degrees clockwise from north) by looking at the wind vane 10 metres up at the Atmospheric Observatory, and cloud type and amount by using the illustrated cloud chart (for type). Using surface data You will be given the daily weather data for 2008 from the University s Atmospheric Observatory. With many years of daily data (usually 30) we can define the climate of the site. We have data for the University site starting in 1970, so observations from 1971 to 2000 can be used to calculate the 30-year averages of temperature etc for Reading s climate. (a) For the month of daily data you are given, calculate the average maximum temperature the average minimum temperature the total precipitation the number of ground frosts (grass minimum at or below 0C) the total hours of sunshine (b) Then, with all twelve months completed, plot a graph for the year (January to December) of monthly average temperature (mean of the

9 average maximum and minimum values) and a second graph of monthly precipitation totals. (c) Are there certain months or times of year when it was wetter? If so, what weather features might be linked to wetter periods? (d) Is it the case that the sunniest month is also the warmest. If it isn t, why would that be?