Lab 04: Weather Forecasting

Terence Day

Weather forecasts… do you trust them? If not, then can you do better? In this lab you will prepare your own weather forecast for tomorrow. You can do the forecast for anywhere in Canada that you’re located. If you’re not presently in Canada, then you can do it for a Canadian place of your choosing.

Learning Objectives

After completion of this lab, you will be able to:

  • Use a climate forecast to determine “normal” weather for a particular place;
  • Describe current weather conditions;
  • Convert UTC to local time;
  • Interpret cloud cover from satellite imagery;
  • Read a weather map; and
  • Construct a weather forecast.

Pre-Readings

  1. Weather radar is an application of Doppler radar. Most textbooks have a section on weather radar, usually in the chapter that deals with atmospheric moisture and precipitation.
  2. Coordinated Universal Time (UTC), also known as UTC, or Zulu time, is used to state the time of data collection. Note that the concept of UTC is different from GMT (Greenwich Mean Time), but the times are actually the same.
  3. Satellite imagery is a type of remote sensing. Your textbook has a brief introduction to how remote sensing works with different wavelengths. Most textbooks also include some material on different types of satellite orbit. Weather satellites use geostationary orbits.
  4. Different types of clouds can be identified on satellite images. All textbooks include material on cloud classification.
  5. Reading a weather map requires some skill. Read the section in your textbook on weather systems (especially fronts), and be familiar with the way that weather station data is shown on surface analyses.

Lab Exercises

EX1: Climate and Weather

“Climate is what you expect, and weather is what you get” (Heinlein, 1973, p. 352). So, based on climate information, what should we expect tomorrow? The Environment Canada website has data on the climate of places, but the information is easier to obtain from WeatherStats. Go to list view (at the top of the page). There will be a list of major Canadian stations, but you will find more places listed by province (tabs at the top of the page). Choose the place you wish to produce a forecast for, and click on it.

Towards the bottom of the page is the Daily almanac. That will give you the normal expected weather for today’s date. Ideally this will be based on 30 years of data, but sometimes the data are for a shorter period. You want the data for tomorrow, so click on Time warp (top right) and change the date to tomorrow.

Your forecast will focus mostly on temperature and precipitation, so take a look at the normal maximum and minimum values of temperature and precipitation for tomorrow. Also look at the extreme values.

  1. State the average and range of temperature and precipitation for tomorrows’ date. This is a climate forecast for tomorrow, based on what’s happened on tomorrow’s date over the past 30 years. It’s possible that these values will be exceeded, but it’s also quite likely that temperature and precipitation tomorrow will fall in the range that you’ve just determined.

EX2: What’s Been Happening in the Past 24 Hours?

Before we can predict the future weather, we need to know the current and recent weather. This data is available on the same WeatherStats page. You will have to change the “time warp” and return to today’s date.

You will find current conditions listed, plus the temperature changes over the past 24 hours.

  1. Write a short paragraph on the current weather and changes over the past 24 hours. If you are or were at or close to that location then please add your own observations, e.g. “Although no rain was recorded, I felt a few drops as I waited for the bus outside City Hall at 10am.” Be sure to note where the wind is coming from.

EX3: Weather Radar

Weather radar is also on the same page. Weather radar is used to show regional precipitation patterns, and originates from ground stations. For example, there is one located at Silver Star. There are (or were) 31 stations in Canada, all located along the US/Canada border where most of the Canadian population is located.

The radar is shown as an animation and the times are at the top of the radar panel. These times are in UTC (see the pre-readings).

Answer the following questions:

  1. When does the animation start and end (in UTC)?
  2. When does the animation start and end in local time?
  3. In a few sentences, describe where precipitation is occurring, where it came in from, and where it appears to be headed. Does it look like any precipitation is headed towards the place you’re interested in?

EX4: Satellite Imagery

All of the data on the weatherstats.ca website comes from Environment Canada. However, to get deeper into what’s happening we will need to use the real Environment Canada website.

Click on Satellite. Depending on your location, use either the GOES-East or GOES-West imagery.

Click on Visible. This gives you visible light imagery, which is what you would see if you were looking down from the satellite. (Trick question: How many people are on board a weather satellite?)

At the top of the image there is some information. The last bit will read something like, “13.20 UTC”. This is the UTC time of the image. In other words, this is data captured in the past few hours, but is not quite real time data. Now try the animation feature.

Answer the following questions:

  1. What is your local time of the latest image (converted from UTC)?
  2. What is the time duration between the earliest and latest images in the animation, based on the UTC times of the images?
  3. What does GOES stand for?
  4. Explain the “G” in GOES.

EX5: Interpretation of Satellite Imagery

The satellite imagery we’ve looked at so far is based on visible light. Now look at the infrared (IR,10.7 μm) and the visible & topography images. The IR image gives the temperature of the clouds and surface. Note that the scale is counterintuitive, with red as the coldest.

From satellite imagery, cloud type can be identified by shape and height. Remember that it gets colder as you get higher. If a cloud is very low (where the temperature similar to ground temperature) and featureless it is probably stratus. If it’s fairly low and lumpy in texture it’s probably cumulus, and if it’s high and streaky it is cirrus. Cumulonimbus can be identified by the range of different temperatures within the cloud, and sometimes the anvil shape of those clouds is apparent.

Answer the following questions:

  1. What cloud types can you identify near your forecast location using the visible and infrared images?
  2. Looking at the animation of satellite images, where are the clouds moving?
  3. Identify locations near your forecast site where clouds are dissipating and areas where clouds seem to be growing.
  4. Can you see any evidence of low or high pressure systems? (Remember in the northern hemisphere a low pressure system rotates counter-clockwise.)
  5. Can you see any weather systems moving towards or away from your forecast location?

EX6: Surface Analyses

Go back to the Environment Canada website. Halfway down the page is a link to analyses and modelling. Click on it. This is where you’ll find links to professional weather maps and the models that forecasters use.

Look at the most recent MSLP (mean sea-level pressure) chart first. Use the one for the Northern hemisphere, since that has the fronts marked on it. The Canadian (preliminary) one doesn’t have the fronts marked.  There are four versions of the charts: 00, 06, 12, and 18. These are the UTC times. Looking at each of these, you will see a date, as well as the time. Use the one that is most recent. You can enlarge the map so that you can read the numbers more clearly.

  1. Where are areas of high and low pressure? Where are areas of warm and cold air? Can you identify different air masses on the map? Can you identify any fronts near your forecast area?

EX7: Upper Air Analysis

From the analyses and modelling page scroll down to Upper air analyses.

  1. Look at the most recent 500 hPa map. Where are troughs and ridges located relative to your forecast site?
  2. Look at the 250 hPa map. Where is the jet stream located relative to your forecast site?

EX8: Your Forecast

Summarize your findings for your forecast location by answering the following questions:

  1. Where are weather systems coming from?
  2. What are the temperatures and dew point temperatures like upstream of your forecast location?
  3. Based on the satellite images, what do you think the cloud cover will be like overnight and tomorrow? How will cloud cover influence the temperature?
  4. Why is tomorrow’s weather going to be different from today’s weather? Describe the major processes that will determine tomorrow’s weather.
  5. Write a script for your weather forecast that could be read on the radio.
  6. Read your weather forecast and submit it as an mp3 file.

References

Heinlein, Robert A. (Robert Anson), 1907-1988. (1973). Time enough for love, the lives of Lazarus Long; a novel. New York :Putnam