ATOC 1050 Summer Homework 1. Due: June 8, 2011

Transcription

1 ATOC 1050 Summer 2011 Homework 1 Due: June 8, 2011 Make sure your answers are legible, grammatically correct, and have units associated with them if applicable. Each question can be answered in a few sentences. Total possible points: One May afternoon, the temperature at your house in St. Louis is 74 F, while the dew point is 65 F. You call your friend in Las Vegas, where the temperature is 88 F and the dew point is 31 F. After your phone call, you both decide to go for a swim in your swimming pools. Which one of you will feel colder when you step out of the pool you, or your friend in Las Vegas? Your answer should be a qualitative discussion regarding relative humidity and how the body responds to evaporation (10 points). Your friend in Las Vegas will feel colder than you will at your home. The reason is due to the relative humidity and how our body responds to moisture in the air. At your home in St. Louis, the temperature and dew point are much closer together than in Las Vegas, indicating the relative humidity is higher at your house than in Las Vegas. Thus, even though the temperature is higher in Las Vegas, the air is much drier, and will cause evaporation to occur more quickly from your skin than from your friend s. Because evaporation causes your skin to cool, a faster evaporation rate causes more cooling. 2. Antarctica, the coldest continent on the Earth, is also one of the largest deserts in the world, where the precipitation per year is less than 2 cm in many locations. The relative humidity, however, is generally quite high, especially during the winter, where values can regularly reach 70% or higher. Explain how the relative humidity can be so high in an area with so little moisture in the air (10 points). The relative humidity describes how much moisture is in the air relative to the total amount possible at that given temperature and pressure. It is not an absolute measure of moisture content. Colder air cannot hold as much moisture as warmer air, and as such its capacity for moisture is low. Therefore, even though there is little moisture in the air, the low temperature means that saturation can occur more easily (and as such the relative humidity is higher). Another way to state this would be that because the moisture content is low, the vapor pressure must be low. Also, because the temperature is low, the saturation vapor pressure is low. Therefore, these two values do not differ greatly, and the relative humidity must be high.

2 3. Using the Skew-T diagram given below for a Denver rawinsonde launched on 29 May 2011 at 12Z, answer the following questions (25 points). a) What is the temperature at the surface (2 points)? 10 C b) What is the dew point temperature at the surface (2 points)? 8 C c) Are there any cloud layers? If so, at what level? How do you know that there is a cloud at this level (3 points)? Yes, there is a cloud at 740 mb. We know there is a cloud there because the temperature and dew point are equal, which means the vapor pressure and saturation vapor pressures are equal. d) What is the relative humidity at the cloud layer level? How do you know what the relative humidity is? Hint you do not need to do any calculations for this question. If there is no cloud, say not applicable (5 points). The relative humidity at the cloud layer is 100%. We can see this on the diagram because our temperature and dew point temperatures are the same, but we also know this because for a cloud to occur, saturation must take place, which occurs when the temperature cools to the dew point temperature at a constant pressure. When the temperature and dew point temperatures are equal, the vapor pressure and saturation vapor pressures are equal, and the relative humidity is 100%. e) Are there any temperature inversions on this sounding? If so, at what level (2 points)? Yes, there is an inversion between 740 and 700 mb. There is also a small one just around 575 mb (you do not need to give this inversion to receive full credit). f) What is the temperature at 500 mb (2 points)? -9 C. g) What is the dew point temperature at 500 mb (2 points)? -35 C.

3 h) At what level is the tropopause? How do you know this (3 points)? The tropopause is at 150 mb. We know this because the temperature is increasing with height in the upper troposphere. i) What is the wind speed at 400 mb? What is the wind direction at 400 mb? Give your answer for wind direction in degrees (3 points). The wind speed is 50 knots. The wind direction is approximately 225. j) From what direction (in cardinal directions north, south, south-easterly, etc) is the wind at 400 mb (1 point)? The wind direction is southwesterly.

4 4. Using the surface station plots below, answer the following questions (20 points). At station MCK (McCook, Nebraska): a) What is the temperature (2 points)? 55 F b) What is the dew point temperature (2 points)? 53 F c) What is the pressure (2 points)? mb d) What is the wind speed and direction (4 points)? Northeasterly wind at 10 knots. e) What is the cloud cover (2 points)? Overcast (or 8 octets) f) What, if any, is the significant weather occurring at this time (2 points)? Light rain. g) In Kansas City, Missouri (station MCI), what is the significant weather occurring (2 points)? Thunderstorm h) What is the cloud cover at Rapid City, South Dakota (station RAP)? Give some reasons why the cloud is what is being reported (4 points). Obscured. Might be due to fog, smoke, or maybe the station simply cannot report it. 5. Discuss the collection of observations versus using NWP models to understand the weather. What are some limitations of both? What are some benefits? What do you think is a better method for understanding the weather? (For this last part, there is not necessarily a right answer here credit will be given in your justification) (15 points). Observations collected by instruments at the surface and in the upper atmosphere provide us with direct measurements of the atmospheric state. NWP models need to infer the atmospheric state by initializing a set of values from observations or previous forecasts, and then calculating how the atmosphere changes with time. Some limitations of observations are that they are limited in how often they are taken. For example, rawinsondes are only launched twice per day. All data is limited to sampling only the

5 location where the observations are being collected observing the weather over the horizon is not possible. Problems with the instruments can also lead to incorrect data. Also, if the observations are human made, they can be subjective. Some limitations to models are that your forecasts will be incorrect if your initial conditions are wrong, if you do not have a high enough resolution to observe the weather in that region, or if you make improper assumptions within the equations that are driving the model. The benefits to observations are that they provide us with direct measurements of the weather without having to infer the atmospheric state from inexact equations. Also the collection of measurements are input into NWP models to improve their forecasts, so they are necessary. The benefits to NWP models are that they can fill in gaps where data is lacking, and their development has led to a greater ability to forecast the weather in both short and long time ranges.